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The Astrophysical Journal, 767:160 (23pp), 2013 April 20
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doi:10.1088/0004-637X/767/2/160

2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

SPECTRAL STATE EVOLUTION OF 4U 1820-30: THE STABILITY OF THE SPECTRAL INDEX OF THE COMPTONIZATION TAIL
` Dipartimento di Fisica, Universita di Ferrara, Via Saragat 1, I-44122 Ferrara, Italy; titarchuk@fe.infn.it, lev@milkyway.gsfc.nasa.gov, frontera@fe.infn.it 2 George Mason University, Fairfax, VA 22030, USA 3 Goddard Space Flight Center, NASA, Code 663, Greenbelt, MD 20770, USA Moscow M.V. Lomonosov State University/Sternberg Astronomical Institute, Universitetsky Prospect 13, Moscow 119992, Russia; seif@sai.msu.ru Received 2012 July 27; accepted 2013 February 8; published 2013 April 8
1

Lev Titarchuk1

,2 ,3

, Elena Seifina4 , and Filippo Frontera1

4

ABSTRACT We analyze the X-ray spectra and their timing properties of the compact X-ray binary 4U 1820-30. We establish spectral transitions in this source seen with BeppoSAX and the Rossi X-ray Timing Explorer (RXTE). During the RXTE observations (1996­2009), the source was in the soft state approximately 75% of the time making the lower banana and upper banana transitions combined with long-term low­high state transitions. We reveal that all of the X-ray spectra of 4U 1820-30 are fit by a combination of a thermal (Blackbody) component, a Comptonization component (COMPTB), and a Gaussian-line component. Thus, using this spectral analysis, we find that the photon power-law index of the Comptonization component is almost unchangeable ( 2), while the electron temperature kTe changes from 2.9 to 21 keV during these spectral events. We also establish that for these spectral events the normalization of the COMPTB component (which is proportional to the mass accretion rate M ) increases by a factor of eight when kTe decreases from 21 keV to 2.9 keV. Previously, this index stability effect was also found analyzing X-ray data for the Z-source GX 340+0 and for the atolls 4U 1728-34 and GX 3+1. Thus, we can suggest that this spectral stability property is a spectral signature of an accreting neutron star source. On the other hand, in a black hole binary monotonically increases with M and ultimately its value saturates at large M . Key words: accretion, accretion disks ­ black hole physics ­ radiation mechanisms: non-thermal ­ stars: individual (4U 1820-30) Online-only material: color figures 1. INTRODUCTION Accreting neutron stars (NSs) can be observationally classified into two distinct categories using a color­color diagram (CCD). These two categories, atolls and Z sources, are based on their different CCD forms when the source undergoes spectral and luminosity changes. Along with this phenomenological difference between atolls and Z-sources in terms of the CCD, there are important X-ray spectral and timing characteristics that are essentially different for these types of NS low-mass X-ray binaries (LMXBs). The main observational difference between these types is the specific range of luminosity changes. The atolls are observed when their luminosity changes from 0.01 up to 0.5 of the Eddington limit LEdd (see Christian & Swank 1997; Ford et al. 2000), while Z-sources are seen when the resulting luminosity is near the Eddington regime (e.g., Seifina et al. 2013). In this paper, we present our analysis of the peculiar atoll 4U 1820-30, which is a bright atoll source in terms of its luminosity and, at the same time, is a typical atoll source in terms of its timing evolution. On the other hand, during the bright phase, 4U 1820-30 is as bright as a subclass of persistent bright atolls (GX 13+1, GX 9+1, GX 9+9, and GX 3+1), but 4U 1820-30 shows a larger range of luminosity and demonstrates all states in terms of CCD (from the island to banana states). The aforementioned bright atolls are only seen in the banana state (e.g., Hasinger & van der Klis 1989). Furthermore, 4U 1820-30 has a maximal luminosity of about 0.5 LEdd and thus it adjoins the luminosity range of Z-sources. Also, 4U 1820-30 shows Type-I X-ray bursts and characteristic timing features of a typical atoll demonstrating 1 an evolution of band-limited noise (BLN), very low frequency noise (VLFN) components, and low-frequency quasi-periodic oscillations (LFQPOs, in the 20­40 Hz range) when it evolves from the banana state to the island state. However, unlike the other atolls, 4U 1820-30 exhibits LFQPOs with frequencies near 6 Hz during the banana state (Wijnands et al. 1999; also see Section 6.2 in this paper), which are usually seen in Zsources during the normal branch. Thus, 4U 1820-30 combines properties of ordinary and bright atolls and Z-sources in terms of timing and spectral evolution, luminosity, and detection of 6 Hz QPOs. 4U 1820-30 is an LMXB observed at 0. 66 from the center of the NGC 6624 cluster. Grindlay et al. (1976) were the first to identify this source as a Type-I X-ray burst. Kuulkers et al. (2003) estimated the distance d = 7.6 ± 0.4 kpc to 4U 1820-30, assuming that the peak luminosity equals LEdd for a He burst atmosphere. Vacca et al. (1986) estimate the distance to 4U 1820-30 as 6.4 ± 0.6 kpc using the analysis of UV diagrams for NGC 6624. Rappaport et al. (1987) find that the binary system comprises a He white dwarf of mass of 0.06 ± 0.08 M and NS (with a mass later evaluated by Shaposhnikov & Titarchuk 2004 as 1.3 M ), orbiting with a period of 11.4 minutes (Stella et al. 1987). Hasinger & van der Klis (1989) classify 4U 1820-30 as an atoll source. Priedhorsky & Terrell (1984), Simon (2003), and Wen et al. (2006) find that the flux varies between the soft and the hard states (banana and island states, respectively) quasiperiodically with a period of 170 days. These flux variations have been suggested to be related to the tidal effects of a remote third star (Chou & Grindlay 2001;Zdziarskietal. 2007).


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Rossi X-ray Timing Explorer (RXTE) observations revealed 4U 1820-30 as a prominent source of kilohertz quasi-periodic oscillations (kHz QPO; Smale et al.1997). X-ray bursts are only observed at low fluxes (e.g., Clark et al. 1977). Furthermore, Cornelisse et al. (2003) and Zhang et al. (1998) find that the observed kHz QPOs correlate with the flux, which probably suggests that these variations are due to a luminosity change caused by a change in the mass accretion rate. Strohmayer & Bildsten (2004) established that the short (10­15 s) Type-I outbursts are due to the unstable thermonuclear burning of a mixture of hydrogen and helium at the bottom of the NS atmosphere. SAS-3 observations showed strong evidence that X-ray bursts can only occur in its low-intensity state (Clark et al. 1977). All bursts observed from 4U 1820-30 indicate that the photosphere expands in radius by a factor of 20. Such an expansion leads to strong softening of the resulting spectrum (see, e.g., Strohmayer & Bildsten 2004). Moreover, a several hour long "superburst" was observed from 4U 1820-30 on 1999 September 9. It is now understood that superbursts can be caused by burning in the carbon ashes produced by Type-I bursts (Strohmayer & Brown 2002). Einstein, EXOSAT, Ginga, ASCA, and BeppoSAX also observed 4U 1820-30. Many spectral models have been applied to fit the observed X-ray spectra. For example, models that are a sum of a blackbody (BB) with thermal bremsstrahlung or a BB with a power law combined with exponential cutoff (CPL) have been employed. A more detailed model was developed using a Comptonization spectrum by Sunyaev & Titarchuk (1980; see the CompST model in XSPEC) combined with a BB. Note that White et al. (1986) and Christian & Swank (1997) show that models based on the thermal bremsstrahlung mechanism are unphysical. The emission measures found using this model have to be of order 1060 cm-3 and they are too large for the plasma cloud near the NS whose radius is only of order 106 cm. Therefore, the CPL and CompST components combined with a BB were applied to fit the X-ray spectra of 4U 1820-30 (see, e.g., Bloser et al. 2000). The BB temperature kTBB , the photon index , and the exponential cutoff energy EC are the CPL+BB model parameters. Bloser et al. (2000) also included photoelectric absorption at low energies using the cross section of Morrison & McCamman (1983). White et al. (1986) and Hirano et al. (1987) show that a Gaussian at 6.7 keV is often needed to take into account a blend of K iron lines. Parsignault & Grindlay (1978) applied a power-law fit to the 4U 1820-30 ANS (Astronomische Nederlandse Satelliet) data and found X-ray spectral changes due to intensity variations. These authors found that the photon index changes from 2 at high count rates to 1.4 when the count rate is low. In other words, they found that the spectrum became harder when the luminosity decreased. Stella et al. (1987) used the CPL+BB model to fit the data of the EXOSAT ME instrument in the energy range from 1 to 30 keV. These particular EXOSAT data were obtained during 1984­1985. The source was found at a high-luminosity state (6.0 â 1037 erg s-1 ) and a low-luminosity state (2.0 â 1037 erg s-1 ). The best-fit parameters of these EXOSAT spectra (, EC , and kTBB ) are 1.7, 12 keV, and 2 keV, respectively, in the high state and 2.5, EC >30 keV, and kTBB = 2.3 keV in the low state. Smale et al. (1994) analyzed the ASCA/GIS data for 4U 1820-30, which was observed in the low state in 1993. They fit the 0.6­11 keV spectrum using the CompST + BB model. The best-fit CompST parameters were around 3.6, 13.5, and 0.76 keV for the plasma temperature, optical, and a BB tem2

perature, respectively. Christian & Swank (1997) reported on the Einstein (SSS + MPC) 1978 observation in the 0.5­20 keV energy range. They found that the source was in the high state characterized by a luminosity of 5.5 â 1037 erg s-1 and the bestfit parameters of the CompST+BB model were very similar to those obtained using the ASCA data. Piraino et al. (1999) and Kaaret et al. (1999) analyzed the first observations of 4U 1820-30 extending above 30 keV. For this analysis, they used the observations obtained by the Narrow Field Instruments (NFIs, 0.1­200 keV) of BeppoSAX in 1998. The best-fit parameters of the observed spectrum in the 0.3­40 keV energy range are kTBB = 0.47 keV, = 0.55, and EC = 4.5 keV for the CPL+BB model, whereas the best-fit parameters of the CompST+BB model are kTBB = 0.46­0.66 keV, kTe = 2.83 keV, and = 13.7. It is worth noting that the instruments with a response below 1 keV provide low values of kTBB . Although the range of luminosities and the bestfit parameters inferred using BeppoSAX and the CompST + BB model gave very similar values with respect to those obtained using the other instruments (see above). The BeppoSAX Phoswich Detection System (PDS) could not detect the emission from 4U 1820-30 above 40 keV. Note, BATSE (see Bloser et al. 1996) also failed to find this source in the 20­100 keV range during the first four years of the CGRO. For the first time, a high energy tail above 50 keV has been found by INTEGRAL in the hard state of 4U 1820-30 (Tarana et al. 2006), which put this source in the list of X-ray bursters that exhibit high-energy emission. It is interesting to also note that other atolls can be described by similar spectral models. Lin et al. (2007), hereafter LRH07, pointed out that at the low-LX end of the soft-state track a weak Comptonization component is needed. LRH07 modified the BPL and COMPTT models, applying their modifications to atolls Aql X-1 and 4U 1608-52. They were interested to find out how much energy is directly visible as a pure thermal radiation and thus one can obtain the remaining fraction for the Comptonized radiation f. In this sense, this approach is similar to that using the COMPTB model (see Section 3 below). In this way, to account for specific spectra, LRH07 describe the hard state by means of a BB+BPL model and the soft state by means of a three-component model, MCD+BB+CBPL, where CBPL is a broken power law with the high energy cutoff taking into account the Comptonization effect. In this paper, we show a thorough X-ray spectral­timing analysis of the data for 4U 1820-30 using the BeppoSAX and RXTE/PCA/HEXTE available observations that were made during 1998­1999 and 1996­2009. Unlike the past spectral analysis, we adopt a unified model, capable of describing the spectra observed during both the soft and hard states. The full list of observations used in our data analysis is presented in Section 2 and Tables 1 and 2. We describe in detail our spectral model and spectral analysis using this model in Section 3. We interpret X-ray spectral­timing evolution when the source undergoes the spectral state transition in Sections 4­6. We explain our results in detail and come to the final conclusions in Sections 7 and 8. 2. DATA SELECTION We obtained broadband energy spectra of the source using data from three BeppoSAX NFIs, namely the Low Energy Concentrator Spectrometer (LECS) with the 0.3­4 keV energy band, the Medium Energy Concentrator Spectrometer (MECS) with the 1.8­10 keV band, and the PDS with the 15­200 keV


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Figure 1. Evolution of the ASM/RXTE count rate during the 1996­2009 observations of 4U 1820-30. Blue vertical strips (at top of the panel) indicate the temporal distribution of the RXTE data of pointed observations used in our analysis, while the bright blue rectangles indicate the RXTE data sets listed in Table 2, and the green triangles show BeppoSAX NFI data listed in Table 1. (A color version of this figure is available in the online journal.) Table 1 BeppoSAX Observations of 4U 1820-30 Used in the Analysis Observational ID 20105004 20537004 20537005 Start Time (UT) 1997 Oct 2 06:45:23 1998 Apr 17 04:31:23 1998 Sept 23 12:44:56 End Time (UT) 1997 Oct 2 19:09:36 1998 Apr 18 02:55:28 1998 Sept 24 15:30:05 MJD Interval 50723.2­50723.8 50920.1­50921.11 51079.5­51080.61

Reference. (1) Kaaret et al. 1999. Table 2 Groups of RXTE Observations of 4U 1820-30 Number of Set R1 Dates, MJD 50204­50207 50151, 50235 50371­50386 50488­50701 50920­51467 51206­51956 51996­51999 52355, 52429 52439 52482­52808 52894 53258­53591 53692­53693 53959­54028 53921­54306 54126­54129 54947­55002 54947­55116 RXTE Proposal ID 10074 10076 10075 20075 30053, 30057 40017, 40019 60030 40017 70031 70030 80105 90027 91435 91151 92030 70030 94090 92030 1996 1996 1996 1997 1998 1999 2001 2002 2002 2002 2003 2004 2005 2006 2006 2007 2009 2009 Dates UT May 1­4 Mar 9 and Jun 1 Oct 15­30 Feb 9­Sep 10 Apr 4­1999 Oct 16 Jan 28­2003 Jun 4 Mar 28­31 Mar 22; 2003 Jun 6 Jun 14 13:21:52­14:14:40 Jul 23­2003 Jun 18 Sep 12 05:41:04­09:36:32 Sep 10­2005 Aug 9 Nov 18­19 Aug 12 and Oct 9 Jul 5­2007 Jul 25 Jan 26­29 Apr 26­Jun 20 Sep 8­Oct 12 Reference 1 1 1, 2, 3, 4 1, 3, 4, 5, 6, 7 3

R2 R3 R4

8 8

R5 R6

R7

9

References. (1) Chou & Grindlay 2001; (2) Smale et al. 1997; (3) Kaaret et al. 1999; (4) Zhang et al. 1998; ´ (5) Bloser et al. 2000; (6) Kusmierek et al. 2011; (7) Shaposhnikov & Titarchuk 2004; (8) Migliari et al. 2004; (9) Krimm et al. 2009.

band (see Parmar et al. 1997; Boella et al. 1997; Frontera et al. 1997, respectively). We used the SAXDAS data analysis package for data processing. We renormalized the LECS data based on the MECS 3

data. We treated relative normalizations of the NFIs as free parameters when we proceeded with model fitting, but we fixed the MECS normalization at 1. Each of these normalizations is controlled if they are in a standard range for a given


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Table 3 Best-fit Parameters of Spectral Analysis of BeppoSAX Observations of 4U 1820-30 in the 0.3­200 keV Energy Rangea Observational ID 20105004 20537004 20537003 MJD (day) 50723.28 50920.19 51079.53 TBB , (keV) 0.63(3) 0.58(1) 0.69(2) NBB
b

Ts (keV) 1.35(2) 1.24(5) 1.27(4)

= -1 1.00(4) 1.01(5) 0.99(8)

Te (keV) 3.25(2) 3.42(2) 3.37(1)

log(A) -0.11(5) -0.10(6) -0.12(4)

NCOMPTB 3.45(2) 4.42(6) 4.65(1)

Eline , (keV) 6.7(1) 6.6(5) 6.5(2)

Nline

b



2 red

(dof)

2.33(2) 2.74(3) 2.68(3)

0.45(2) 1.38(7) 0.41(5)

1.11(364) 0.89(364) 1.2(234)

Notes. Parameter errors correspond to a 90% confidence level. a The spectral model is wabs (Blackbody + CompTB + Gaussian) smedge, where N is fixed at a value 3.00 â 1021 cm-2 (Bloser et al. 2000). H b Normalization parameters of Blackbody and CompTB components are in units of 10-2 â L /d 2 erg s-1 kpc-2 , where L 39 10 39 is the source luminosity in units of 2 1039 erg s-1 , d10 is the distance to the source in units of 10 kpc and the Gaussian component is in units of 10-2 â total photons cm-2 s-1 in line, wherein line of the Gaussian component is fixed at a value 0.8 keV (see comments in the text); smeared edge included at 7.7 keV.

Figure 2. Top: the best-fit spectrum of 4U 1820-30 during banana branch events in E F (E ) units using BeppoSAX observation 20105004 carried out on 1997 October 2. The data are presented by crosses and the best-fit spectral model wabs (Blackbody+CompTB+Gaussian) is shown with a green line. The model components are shown by blue, red, and crimson lines for Blackbody, CompTB, and Gaussian components, respectively. Bottom panel: (reduced 2 = 1.11 for 364 dof). The best-fit model parameters are = 2.00 ± 0.04, kTe = 3.25 ± 0.02 keV, and Eline = 6.7 ± 0.1 keV (see more details in Table 3). (A color version of this figure is available in the online journal.)

instrument.5 Furthermore, we rebinned the spectra to obtain significant data points. The LECS spectra were rebinned using a binning factor that varies with energy (Section 3.1.6 of Cookbook for the BeppoSAX NFI spectral analysis), imple5

menting the rebinning template files in GRPPHA of XSPEC.6 The PDS spectra were rebinned with a linear binning factor of two; we group two bins together leading to the bin width of 1 keV. For all of these spectra, we use a systematic error of 1%. The BeppoSAX observations used in our analysis are shown in Table 1. We also use publicly available RXTE data sets (Bradt et al. 1993) that were obtained from 1997 April to 2009 March. In total, they include 92 observations taken at different states of the source. The LHEASOFT/FTOOLS 5.3 software package was applied to process the data. Also for our spectral analysis, we applied PCA Standard 2 mode data, collected in the 3­20 keV energy range. We also used the most recent release of PCA response calibration (ftool pcarmf v11.1) and the standard dead time correction to the data. A background subtraction in off-source observations is performed on the data. We use only data from 20 to 150 keV energy in order to avoid the problems related to the HEXTE response and background determination. We apply the GSFC public archive to analyze all available data sets (see http://heasarc.gsfc.nasa.gov). We present a full list of observations covering the source evolution during different spectralstate events in Table 2. We implement an analysis of 13 years of RXTE observations of 4U 1820-30 for seven intervals (see blue rectangles in Figure 1 (top)). We fitted the RXTE energy spectra using XSPEC astrophysical fitting software. For our data analysis, we have also applied the publicly available 4U 1820-30 data in the energy range from 2 to 10 keV from the All-Sky Monitor (ASM/RXTE) for all observation scans. According to the ASM monitoring system, 4U 1820-30 shows long-term variations with a possible period of 176 days of the 2­10 keV flux (see Figure 1 and Priedhorsky & Terrell 1984;Simon 2003; Wen et al. 2006). The count rate changes in the interval of 5­35 counts s-1 throughout each cycle (Figure 1). Our RXTE spectral studies are directed to investigate: (1) the continuum spectrum, in particular, the hard X-ray tail and its evolution during long-term flux variations, (2) the variation ( 10 s) of the best-fit spectral parameters for short- and longterm phases, and (3) the dependence of the spectral index and the electron temperature on the total flux and accretion rate. Data from the PCA and HEXTE detectors as well as BeppoSAX detectors have been used to constrain spectral fits, while ASM data provided long-term intensity state monitoring. Results of our long-term study of 4U 1820-30 are presented in detail in the next sections and compared with our previous results for 4U 1728-34 and GX 3+1.
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http://heasarc.nasa.gov/docs/sax/abc/saxabc/saxabc.html

http://heasarc.gsfc.nasa.gov/FTP/sax/cal/responses/grouping

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The Astrophysical Journal, 767:160 (23pp), 2013 April 20

Titarchuk, Seifina, & Frontera

Table 4 Best-fit Parameters of Spectral Analysis of PCA and HEXTE/RXTE Observations of 4U 1820-30 in the 3­200 keV Energy Rangea Observational ID 10076-01-01-00 10074-01-01-00 10074-01-01-01 10074-01-01-02 10074-01-02-02 10074-01-02-00 10074-01-02-01 10076-01-02-00 10075-01-01-000 10075-01-01-010 10075-01-01-020 10075-01-01-031 20075-01-01-00 20075-01-02-00 20075-01-02-01 20075-01-03-01 20075-01-03-00 20075-01-04-00 20075-01-05-00 20075-01-05-01 20075-01-06-00 20075-01-06-01 20075-01-07-01 20075-01-07-00 20075-01-08-00 20075-01-08-01 20075-01-09-00 20075-01-10-00 20075-01-10-01 30057-01-01-01 30057-01-01-00 30057-01-01-02 30057-01-01-03 30057-01-01-04 30053-03-01-000 30053-03-02-00 30053-03-02-01 30053-03-02-04 30053-03-02-05 30053-03-02-02 30053-03-02-03 30057-01-02-00 30057-01-02-01 30057-01-02-02 30057-01-02-03 30057-01-02-05 30057-01-03-01 30057-01-03-02 30057-01-03-000 30057-01-04-12 30057-01-04-00 30057-01-04-01 30057-01-04-02 30057-01-04-03 30057-01-04-04 30057-01-04-05 30057-01-04-06 30057-01-04-07G 30057-01-04-08G 30057-01-05-00 30057-01-04-09 30057-01-06-02 30057-01-06-03 30057-01-06-00 30057-01-06-04 MJD (day) 50151.937 50204.375 50204.511 50204.785 50207.381 50207.591 50207.927 50235.532 50371.943 50382.566 50384.633 50386.830 50488.426 50513.955 50514.001 50528.638 50530.705 50548.703 50570.585 50578.549 50595.327 50596.432 50622.760 50622.546 50645.216 50645.485 50675.454 50701.149 50701.251 50907.412 50908.660 50909.662 50910.589 50910.790 50920.197 51079.552 51079.719 51079.812 51079.879 51080.229 51080.413 51320.532 51321.713 51323.729 51324.378 51324.511 51410.501 51411.499 51412.315 51418.427 51418.557 51419.289 51419.427 51424.416 51424.483 51432.379 51425.348 51428.354 51430.007 51435.001 51436.225 51466.231 51466.440 51466.499 51466.965 = -1 1.00(2) 1.0(1) 0.99(2) 1.00(2) 1.00(5) 0.99(2) 0.99(3) 0.99(2) 1.00(2) 0.99(2) 0.96(9) 0.99(2) 1.01(2) 0.99(2) 1.00(1) 0.97(6) 0.95(1) 0.96(3) 0.99(2) 1.00(2) 0.99(3) 1.00(2) 1.01(2) 1.00(1) 1.00(2) 1.00(1) 0.99(1) 1.00(1) 1.00(2) 1.02(5) 1.00(1) 1.01(3) 1.00(1) 0.99(2) 0.99(4) 1.00(3) 1.00(2) 0.99(1) 0.9(1) 0.99(4) 1.00(3) 0.98(3) 1.00(1) 1.01(3) 1.00(1) 1.02(3) 1.00(3) 0.99(2) 1.02(4) 1.00(2) 0.98(3) 1.00(2) 1.00(1) 1.02(3) 0.99(3) 1.00(1) 0.98(3) 1.03(3) 1.00(1) 0.99(1) 0.98(3) 1.00(2) 1.00(1) 0.97(3) 1.01(3) Te (keV) 2.87(2) 2.93(4) 2.91(1) 2.95(2) 2.84(2) 2.86(1) 2.89(3) 2.89(2) 3.02(1) 3.09(2) 2.84(1) 2.89(2) 2.90(2) 2.85(1) 2.92(1) 2.92(5) 2.91(2) 2.95(1) 3.93(2) 2.84(4) 2.88(2) 2.83(3) 2.86(2) 2.86(2) 2.93(1) 2.91(1) 2.87(3) 2.92(2) 2.91(1) 3.00(2) 2.83(1) 2.95(1) 2.94(2) 2.87(2) 2.93(2) 2.91(1) 2.89(4) 2.86(2) 2.86(5) 2.99(1) 2.93(3) 2.96(3) 2.99(1) 2.93(1) 2.95(2) 2.94(1) 2.91(2) 2.94(1) 2.87(4) 2.83(3) 2.87(1) 2.93(1) 2.95(2) 2.91(1) 2.93(1) 2.92(2) 2.93(2) 2.89(1) 2.89(1) 2.89(2) 2.93(1) 2.93(1) 2.91(1) 2.93(2) 2.91(1) log(A)b 2.00b 2.00b 2.00b 0.92(8) 0.9(1) 2.00b 2.00b 1.09(7) 2.00b 2.00b 2.00b 0.98(4) 2.00b 0.90(5) 2.00b 2.00b 0.65(5) 0.7(1) 0.8(1) 2.00b 1.03(4) 0.96(1) 0.97(4) 0.97(3) 2.00b 2.00b 0.67(2) 2.00b 2.00b 0.70(3) 1.02(1) 0.72(2) 0.66(2) 0.77(2) 2.00b 0.75(1) 0.73(1) 0.99(5) 0.41(3) 0.74(1) 2.00b 0.85(1) 0.72(3) 2.00b 0.92(3) 0.91(4) 0.84(3) 0.96(4) 0.91(4) 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 0.88(4) 0.86(3) 2.00b 2.00b 0.61(4) 0.60(4) 1.02(7) 1.02(8) 0.97(7) 0.96(9) NCOMPTB 7.13(1) 5.5(2) 5.3(1) 5.25(9) 7.5(1) 7.74(2) 7.42(8) 4.91(1) 4.0(2) 3.66(3) 4.73(1) 5.43(4) 6.92(3) 7.99(4) 7.86(3) 5.05(2) 6.47(7) 5.53(5) 3.22(2) 6.42(4) 7.96(1) 8.49(1) 8.37(4) 8.31(3) 7.86(1) 6.37(4) 7.81(2) 4.90(3) 5.17(3) 4.51(1) 6.18(2) 5.06(3) 5.43(1) 6.20(3) 4.74(5) 6.42(1) 6.45(3) 5.97(1) 5.91(2) 4.59(1) 4.09(3) 5.28(4) 6.02(4) 4.92(3) 5.12(3) 5.03(2) 4.93(3) 4.63(4) 5.82(3) 3.84(7) 3.89(3) 3.54(1) 3.60(2) 3.77(2) 3.75(3) 3.31(2) 4.53(3) 2.91(2) 2.91(2) 4.86(5) 4.77(4) 5.55(9) 5.83(9) 5.62(6) 6.18(8)
c

Ts (keV) 1.30(5) 1.0(1) 1.08(2) 1.05(3) 1.06(5) 1.13(1) 1.06(4) 1.31(5) 1.07(2) 1.30(3) 1.31(5) 1.31(2) 1.30(8) 1.32(6) 1.11(2) 1.10(5) 1.11(3) 1.23(8) 1.10(2) 1.12(4) 1.11(6) 1.10(5) 1.11(7) 1.10(5) 1.10(9) 1.10(7) 1.12(8) 1.11(5) 1.10(6) 1.12(5) 1.11(2) 1.10(4) 1.12(5) 1.12(5) 1.10(2) 1.11(1) 1.10(5) 1.12(6) 1.11(4) 1.10(5) 1.11(2) 1.12(3) 1.10(4) 1.11(5) 1.12(2) 1.12(3) 1.13(2) 1.12(5) 1.12(4) 1.15(5) 1.14(2) 1.11(3) 1.12(2) 1.12(2) 1.16(3) 1.11(4) 1.11(2) 1.12(1) 1.12(2) 1.14(5) 1.13(4) 1.12(2) 1.11(2) 1.11(5) 1.10(3)

NBbody

c

Eline , (keV) 6.27(8) 6.52(6) 6.5(1) 6.40(4) 6.58(4) 6.48(3) 6.38(5) 6.57(4) 6.49(3) 6.62(7) 6.53(4) 6.64(3) 6.56(2) 6.57(4) 6.51(3) 6.44(8) 6.60(3) 6.56(3) 6.90(6) 6.40(5) 6.54(3) 6.40(6) 6.50(3) 6.48(2) 6.44(3) 6.50(4) 6.52(5) 6.37(3) 6.37(3) 6.5(1) 6.41(9) 6.51(8) 6.6(1) 6.5(1) 6.54(2) 6.52(3) 6.51(4) 6.45(9) 6.65(6) 6.51(4) 6.42(5) 6.51(6) 6.5(1) 6.51(4) 6.42(6) 6.51(9) 6.57(8) 6.54(9) 6.51(5) 6.49(8) 6.48(9) 6.5(1) 6.43(4) 6.51(6) 6.56(5) 6.51(1) 6.52(9) 6.58(5) 6.51(2) 6.56(3) 6.51(6) 6.47(3) 6.50(7) 6.55(6) 6.51(4)

Nline

c



2 red

(dof)

F1 /F

2

d

3.42(1) 2.31(3) 2.57(4) 2.48(6) 2.15(4) 2.46(7) 2.16(3) 1.96(1) 2.27(1) 2.03(2) 2.18(1) 2.18(2) 2.65(3) 2.58(5) 2.60(3) 2.73(4) 2.55(3) 2.49(2) 2.27(1) 2.29(2) 2.83(5) 2.73(3) 3.26(7) 3.24(6) 2.83(2) 2.73(7) 2.74(5) 2.91(1) 2.90(5) 2.40(1) 2.35(7) 2.58(5) 2.53(6) 2.64(7) 2.62(4) 2.84(2) 2.77(2) 2.84(2) 2.86(2) 2.63(1) 2.58(3) 3.43(7) 3.36(1) 3.47(1) 3.29(6) 3.33(5) 3.01(6) 3.01(1) 2.94(1) 2.83(7) 2.93(6) 2.48(2) 2.43(5) 2.49(5) 2.50(5) 2.19(4) 2.39(5) 1.87(3) 1.87(3) 2.93(1) 2.81(3) 3.65(8) 3.71(9) 3.67(5) 3.95(9)

1.1(1) 0.15(3) 0.23(3) 0.14(2) 0.23(3) 0.32(5) 0.20(2) 0.22(2) 0.12(1) 0.14(1) 0.15(2) 0.23(3) 0.24(2) 0.29(4) 0.26(4) 0.17(5) 0.20(2) 0.18(2) 0.14(3) 0.38(4) 0.47(3) 0.45(3) 0.61(4) 0.63(4) 0.30(3) 0.40(5) 0.46(7) 0.41(4) 0.41(4) 0.31(1) 0.37(2) 0.29(2) 0.33(2) 0.32(1) 0.34(2) 0.36(3) 0.34(3) 0.77(1) 0.15(3) 0.19(2) 0.29(1) 0.31(3) 0.47(5) 0.33(1) 0.32(2) 0.33(1) 0.32(2) 0.35(2) 0.46(3) 0.30(4) 0.27(1) 0.29(2) 0.29(1) 0.27(2) 0.28(3) 0.31(4) 0.36(1) 0.24(4) 0.24(5) 0.41(1) 0.35(1) 0.57(8) 0.53(7) 0.60(8) 0.75(9)

1.54(78) 0.92(78) 0.93(78) 1.06(78) 1.09(78) 1.58(78) 0.87(78) 0.74(78) 1.63(78) 1.08(78) 1.24(78) 1.23(78) 0.74(78) 0.94(78) 0.94(78) 1.03(78) 0.94(78) 1.01(78) 1.01(78) 1.01(78) 1.01(78) 1.01(78) 1.01(78) 1.01(78) 1.01(78) 1.01(78) 1.01(78) 1.09(78) 1.09(78) 1.76(78) 1.20(78) 1.12(78) 0.79(78) 0.79(78) 0.57(78) 1.17(78) 1.19(78) 1.22(78) 0.90(78) 1.13(78) 1.17(78) 0.86(78) 0.84(78) 0.82(78) 1.24(78) 0.97(78) 0.98(78) 0.98(78) 1.50(78) 0.76(78) 1.10(78) 1.50(78) 0.86(78) 0.78(78) 0.85(78) 0.75(78) 1.08(78) 1.08(78) 1.04(78) 1.18(78) 0.98(78) 1.46(78) 0.79(78) 1.18(78) 1.12(78)

6.13/2.24 4.58/1.61 4.52/1.58 4.33/1.53 5.90/2.06 6.29/2.37 6.05/2.33 4.05/1.43 3.47/1.36 3.16/1.30 3.91/1.43 4.43/1.58 5.34/1.89 6.29/2.19 6.51/2.53 4.27/1.54 5.03/1.80 3.90/1.44 2.49/1.42 5.23/1.86 6.46/2.30 6.75/2.36 6.90/2.38 6.86/2.36 6.48/2.56 5.33/1.89 5.97/2.04 4.35/1.56 4.56/1.63 3.63/1.32 4.95/1.74 4.05/1.45 4.29/1.52 4.74/1.68 4.07/1.45 5.06/1.83 5.05/1.80 4.97/1.76 4.37/1.54 3.73/1.35 3.50/1.28 4.71/1.59 4.89/1.76 4.20/1.53 4.28/1.55 4.23/1.52 4.08/1.43 3.89/1.41 4.71/1.68 3.42/1.21 3.38/1.21 3.06/1.17 3.09/1.18 3.19/1.17 3.18/1.18 2.78/0.98 3.68/1.34 2.47/0.91 2.47/0.91 3.85/1.29 3.75/1.29 4.82/1.70 4.99/1.72 4.86/1.71 5.35/1.85

5


The Astrophysical Journal, 767:160 (23pp), 2013 April 20 Table 4 (Continued) Observational ID 30057-01-06-01 30057-01-06-05 40017-01-01-00 40017-01-01-02 40017-01-01-01 40017-01-01-03 40017-01-02-00 40017-01-03-00 40017-01-03-01 40017-01-04-00 40017-01-05-00 40017-01-06-00 40017-01-06-01 40017-01-05-01 40017-01-07-01 40017-01-07-00 40017-01-08-00 40017-01-09-00 40017-01-10-00 40017-01-10-01 40017-01-10-02 40017-01-11-01 40017-01-11-02 40017-01-11-00 40017-01-12-00 40017-01-11-03 40017-01-12-01 40017-01-13-00 40017-01-14-00 40017-01-15-00 40019-02-01-00 40019-02-01-03 40019-02-01-04 40019-02-01-10 40019-02-01-11 40019-02-01-01 40019-02-01-02G 40019-02-01-05G 40019-02-01-06 40019-02-01-07 40019-02-01-09 40017-01-16-00G 40017-01-17-00 40017-01-17-01 40017-01-18-00 40017-01-19-00 40017-01-20-00 40017-01-19-01 40017-01-19-02 40017-01-20-01 40017-01-21-00 40017-01-21-01 40017-01-21-02 40017-01-22-00 40017-01-22-01 60030-01-01-00 60030-01-01-01G 60030-01-01-02G 60030-01-01-03 60030-01-02-00 60030-01-02-01 60030-01-02-02 40017-01-23-00 40017-01-23-01 40017-01-24-00 MJD (day) 51467.035 51467.098 51206.852 51206.924 51207.002 51207.128 51222.642 51238.040 51238.242 51253.697 51268.750 51283.608 51283.768 51296.888 51300.010 51300.067 51313.655 51330.505 51343.836 51343.901 51343.968 51355.422 51355.489 51355.562 51389.381 51495.992 51496.185 51400.444 51407.503 51417.295 51421.302 51421.635 51421.707 51421.777 51421.846 51421.952 51422.083 51422.257 51422.561 51422.634 51422.706 51429.075 51440.060 51440.218 51464.368 51480.115 51495.667 51496.326 51496.395 51496.666 51941.015 51941.240 51941.303 51955.872 51956.009 51996.856 51997.785 51997.851 51997.918 51998.911 51999.642 51999.707 52355.020 52355.883 52429.708 = -1 1.00(1) 0.99(2) 1.01(3) 0.99(3) 1.00(2) 1.00(2) 0.98(3) 1.06(4) 1.02(3) 1.00(1) 1.00(2) 1.07(4) 1.04(3) 1.01(3) 1.00(2) 1.00(1) 1.01(3) 1.00(1) 1.00(2) 0.99(7) 0.99(3) 1.00(1) 1.00(2) 0.99(2) 0.99(3) 0.98(3) 1.00(3) 1.00(2) 1.02(3) 1.01(2) 1.00(1) 0.98(4) 1.00(2) 1.01(3) 1.00(2) 0.98(3) 0.99(3) 1.01(3) 1.01(3) 0.97(6) 1.00(1) 1.04(5) 0.98(3) 1.00(1) 0.99(3) 0.97(3) 1.00(1) 1.00(2) 1.01(3) 0.98(3) 1.00(1) 1.00(3) 0.99(2) 0.99(3) 1.00(2) 1.00(3) 1.01(2) 0.99(3) 1.00(2) 0.99(3) 1.02(2) 1.01(3) 1.00(1) 1.01(3) 1.01(3) Te (keV) 2.92(2) 2.96(2) 2.88(9) 2.93(5) 2.91(1) 2.94(2) 3.00(1) 2.90(3) 2.88(2) 2.97(1) 2.91(3) 2.88(2) 2.93(1) 2.91(2) 2.89(2) 2.94(1) 2.95(2) 2.90(1) 2.90(2) 2.91(2) 2.87(1) 2.95(2) 2.93(2) 2.94(1) 2.94(1) 2.95(2) 2.99(1) 2.94(1) 2.92(2) 3.00(1) 2.97(2) 3.01(1) 2.99(1) 2.97(3) 2.97(2) 3.00(2) 3.10(3) 2.97(2) 2.98(2) 2.97(1) 2.92(1) 2.95(1) 2.97(2) 2.89(2) 2.97(1) 2.90(1) 2.92(2) 3.06(2) 2.92(1) 2.98(2) 2.99(4) 3.00(1) 2.96(4) 3.03(1) 3.02(1) 2.97(1) 2.96(2) 2.88(1) 2.92(1) 2.94(2) 2.94(1) 2.94(1) 2.92(2) 2.91(1) 9.94(9) log(A)b 0.84(8) 1.00(9) 1.08(3) 0.91(3) 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 0.79(5) 2.00b 0.78(4) 0.92(6) 0.70(8) 0.87(3) 0.79(2) 1.00(3) 0.9(1)0 1.07(9) 0.95(1) 0.97(3) 0.92(3) 2.00b 0.74(2) 0.72(2) 0.79(4) 0.82(3) 0.97(2) 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 0.58(9) 0.65(8) 2.00b 0.91(3) 2.00b 0.67(2) 1.06(9) 0.65(9) 0.97(4) 0.80(4) 1.09(5) 0.93(4) 0.96(4) 0.89(4) 2.00b 2.00b 2.00b 2.00b 0.92(3) 0.84(4) 0.98(2) 2.00b 0.13(5) NCOMPTB 5.96(7) 5.54(1) 7.37(9) 7.40(8) 7.03(9) 6.96(9) 4.97(6) 3.23(7) 3.14(8) 4.62(3) 7.67(9) 9.98(6) 10.46(7) 7.71(8) 11.61(8) 10.35(9) 6.93(9) 8.12(5) 7.79(8) 7.37(9) 7.88(6) 6.13(6) 6.87(8) 6.56(7) 7.80(6) 7.31(5) 7.25(6) 7.59(4) 5.68(4) 4.69(3) 3.82(8) 3.77(4) 6.84(3) 3.87(4) 3.85(3) 3.56(3) 3.51(4) 3.89(3) 3.72(2) 3.73(4) 3.72(3) 3.19(2) 4.85(3) 5.22(3) 5.47(9) 10.98(7) 7.13(8) 6.97(4) 7.05(5) 8.16(5) 4.05(7) 4.24(3) 3.94(8) 4.73(3) 4.40(3) 10.20(8) 13.5(1) 12.54(9) 13.5(1) 11.72(9) 11.62(8) 11.31(8) 9.48(7) 10.06(6) 2.32(2)
c

Titarchuk, Seifina, & Frontera

Ts (keV) 1.12(3) 1.12(2) 1.20(4) 1.21(2) 1.13(3) 1.12(2) 1.14(5) 1.13(2) 1.12(4) 1.13(5) 1.12(2) 1.15(2) 1.13(1) 1.12(5) 1.12(2) 1.14(5) 1.13(2) 1.14(3) 1.11(2) 1.12(3) 1.14(1) 1.15(5) 1.12(2) 1.13(1) 1.13(2) 1.12(4) 1.14(5) 1.13(2) 1.12(5) 1.14(3) 1.12(2) 1.11(2) 1.11(3) 1.12(4) 1.13(5) 1.12(2) 1.12(3) 1.13(2) 1.11(5) 1.12(2) 1.12(3) 1.12(5) 1.14(4) 1.12(3) 1.13(2) 1.13(3) 1.14(5) 1.12(3) 1.12(2) 1.14(5) 1.13(4) 1.12(3) 1.11(2) 1.12(1) 1.15(5) 1.12(4) 1.14(5) 1.12(3) 1.13(5) 1.11(2) 1.11(1) 1.12(3) 1.13(5) 1.53(6) 1.69(3)

NBbody

c

Eline , (keV) 6.52(6) 6.52(8) 6.50(4) 6.51(5) 6.52(7) 6.57(3) 6.54(7) 5.59(3) 6.15(6) 6.21(5) 6.18(7) 6.5(1) 6.4(1) 6.21(6) 6.41(9) 6.27(5) 6.49(7) 6.41(4) 6.43(6) 6.51(3) 6.59(7) 6.5(1) 6.63(7) 6.53(6) 6.24(3) 6.38(6) 6.21(8) 6.58(2) 6.5(2) 6.4(1) 6.57(1) 6.61(5) 6.84(3) 6.75(2) 6.53(4) 6.67(3) 7.36(6) 6.51(4) 6.51(6) 6.59(8) 6.51(3) 6.62(5) 6.70(9) 6.52(8) 6.18(7) 6.46(9) 6.23(6) 6.85(5) 6.34(7) 6.50(8) 6.22(9) 6.63(4) 6.34(6) 6.32(9) 6.09(5) 6.32(2) 6.14(5) 6.36(3) 6.16(4) 6.38(5) 6.54(3) 6.54(7) 6.31(8) 6.46(7) 6.80(8)

Nline

c



2 red

(dof)

F1 /F

2

d

3.84(6) 3.89(9) 3.08(8) 2.98(9) 3.03(8) 3.06(7) 3.47(6) 2.31(4) 2.30(4) 2.60(5) 3.27(1) 3.93(3) 3.97(4) 2.99(2) 5.07(2) 4.58(3) 3.62(1) 3.65(1) 3.60(1) 3.76(9) 3.68(2) 3.06(1) 2.93(2) 2.98(1) 3.29(1) 4.58(3) 4.42(2) 3.13(1) 2.85(2) 2.69(2) 2.19(1) 2.19(1) 2.09(1) 2.40(3) 2.37(1) 2.20(2) 1.96(1) 2.33(3) 2.35(1) 2.30(1) 2.34(1) 1.96(1) 2.52(2) 2.53(1) 3.54(2) 4.03(1) 4.66(2) 4.22(3) 4.47(1) 4.32(1) 1.93(2) 1.77(1) 1.77(1) 2.38(2) 2.26(2) 3.68(3) 3.19(2) 4.04(5) 3.05(2) 3.26(3) 3.45(2) 3.43(2) 4.37(4) 4.90(1) 1.16(2)

0.69(4) 0.54(8) 0.50(7) 0.90(9) 0.91(9) 0.36(4) 0.90(7) 0.16(5) 0.12(6) 0.24(7) 0.44(8) 0.59(6) 0.62(7) 0.46(5) 2.19(6) 1.72(8) 1.20(1) 1.21(2) 1.52(7) 0.46(4) 1.29(5) 0.33(5) 0.35(4) 0.37(7) 0.63(2) 0.56(3) 0.59(1) 0.41(2) 0.34(2) 0.27(4) 0.14(5) 0.19(5) 0.12(3) 0.19(1) 0.13(4) 0.13(4) 0.62(3) 0.19(4) 0.15(3) 0.14(2) 0.16(3) 0.43(6) 0.69(2) 0.89(1) 1.08(3) 2.15(4) 1.93(8) 1.34(5) 1.84(7) 1.55(6) 0.67(9) 0.65(4) 0.64(3) 0.68(6) 0.92(4) 0.56(5) 0.74(6) 0.78(8) 0.69(2) 0.64(3) 0.69(4) 0.62(3) 2.13(3) 2.14(5) 0.50(1)

0.69(78) 0.72(78) 0.81(78) 1.00(78) 0.90(78) 0.88(78) 1.30(78) 1.31(78) 1.21(78) 0.86(78) 1.09(78) 1.40(78) 1.24(78) 1.03(78) 1.04(78) 1.51(78) 1.04(78) 1.03(78) 1.06(78) 1.50(78) 1.24(78) 0.88(78) 1.09(78) 1.00(78) 1.10(78) 1.00(78) 1.47(78) 1.20(78) 1.17(78) 1.39(78) 1.35(78) 0.86(78) 1.28(78) 1.15(78) 0.73(78) 0.86(78) 0.79(78) 0.91(78) 0.84(78) 0.89(78) 1.34(78) 1.28(78) 1.02(78) 1.28(78) 1.28(78) 0.96(78) 0.89(78) 0.57(78) 0.98(78) 0.91(78) 0.89(78) 1.04(78) 0.82(78) 1.07(78) 1.11(78) 1.26(78) 1.40(78) 0.95(78) 1.26(78) 1.49(78) 1.17(78) 0.77(78) 1.21(78) 1.31(78) 1.00(78)

5.09/1.74 4.83/1.72 6.10/2.24 6.01/2.22 5.91/2.18 5.83/2.15 4.58/1.72 2.82/1.05 2.71/1.02 3.98/1.49 6.41/2.53 7.89/2.81 8.39/3.22 6.02/2.21 9.52/3.39 8.31/2.93 5.79/2.09 6.51/2.32 6.62/2.42 6.13/2.17 6.60/2.36 5.13/1.90 5.64/2.11 5.37/1.99 6.48/2.63 6.41/2.19 6.29/2.20 6.00/2.23 4.71/1.71 3.92/1.47 3.31/1.27 3.29/1.29 3.23/1.26 3.39/1.29 3.37/1.28 3.12/1.21 3.03/1.26 3.40/1.29 3.27/1.24 3.26/1.24 3.49/1.29 1.03/1.47 3.81/1.35 4.13/1.42 4.84/1.79 8.79/3.25 8.67/2.22 6.02/2.21 6.38/2.18 6.73/2.34 3.43/1.32 3.35/1.04 3.22/1.00 3.94/1.55 3.77/1.49 8.17/3.03 10.84/4.49 10.07/3.71 10.71/4.29 9.29/3.58 9.19/3.41 8.90/3.25 8.21/3.26 11.91/3.55 1.50/1.62

6


The Astrophysical Journal, 767:160 (23pp), 2013 April 20 Table 4 (Continued) Observational ID 70030-03-02-00 70030-03-02-000 70030-03-02-01 70030-03-02-020 70030-03-02-03G 70030-03-02-03G 70030-03-01-01G 70030-03-01-00G 70030-03-01-02 70030-03-01-03 70030-03-03-00 70030-03-04-00 70030-03-04-01 70030-03-05-00 70030-03-05-01 70030-03-05-02 70030-03-05-03 70030-03-05-04 70030-03-07-03 70030-03-07-01 70030-03-07-00 70030-03-07-020 70031-05-01-00 80105-07-01-00 90027-01-01-00 90027-01-01-01 90027-01-01-03 90027-01-01-04 90027-01-01-05 90027-01-01-06 90027-01-02-00 90027-01-02-01 90027-01-02-07 90027-01-02-08 90027-01-02-03 90027-01-02-04 90027-01-02-05 90027-01-02-06 90027-01-03-07 90027-01-03-08 90027-01-03-00 90027-01-03-09 90027-01-03-02 90027-01-03-01 90027-01-03-10 90027-01-03-11 90027-01-03-03 90027-01-03-04 90027-01-03-05 90027-01-03-06 90027-01-04-00 90027-01-04-05 90027-01-04-01 90027-01-04-03 90027-01-04-04 90027-01-04-02 90027-01-05-00 90027-01-06-00 91435-01-01-00 91435-01-01-01 91435-01-01-02 92030-02-01-00 92030-02-02-000 92030-02-03-02 92030-02-03-00 MJD (day) 52478.468 52478.137 52479.406 52480.254 52480.067 52480.067 52480.182 52482.158 52484.157 52487.171 52489.217 52801.149 52802.070 52804.043 52805.819 52806.937 52808.057 52808.978 54126.393 54127.244 54128.226 54129.013 52439.556 52894.237 53258.915 53259.704 53261.935 53262.921 53263.249 53264.495 53265.745 53266.336 53266.727 53267.587 53268.372 53269.289 53270.860 53271.594 53272.498 53272.697 53273.491 53273.561 53274.206 53274.486 53274.759 53274.796 53275.520 53276.113 53277.419 53278.404 53279.267 53280.437 53281.354 53282.272 53283.127 53283.978 53286.080 53591.287 53692.085 53693.074 53693.986 53921.301 53921.366 53939.313 53939.439 = -1 1.02(5) 1.01(3) 1.01(2) 1.01(3) 0.99(3) 1.00(1) 0.99(4) 1.01(3) 1.00(1) 1.00(3) 1.00(2) 0.99(3) 1.00(2) 1.02(3) 1.01(2) 1.01(3) 1.03(4) 1.01(2) 1.00(2) 1.00(1) 1.02(3) 1.01(2) 1.01(3) 1.00(1) 1.00(2) 1.00(3) 0.96(4) 1.01(3) 1.00(2) 0.99(3) 1.00(1) 0.99(3) 0.99(3) 1.00(1) 1.01(2) 1.00(1) 0.98(5) 1.00(2) 1.00(2) 0.98(4) 1.01(2) 1.02(3) 1.00(2) 1.01(3) 1.04(4) 1.00(2) 0.99(3) 0.96(5) 1.00(1) 1.01(3) 1.01(2) 1.01(3) 1.02(3) 0.99(3) 1.00(2) 1.00(1) 0.99(4) 1.00(3) 0.99(4) 0.97(5) 1.00(1) 1.01(2) 1.01(3) 1.00(1) 1.00(2) Te (keV) 2.92(3) 2.93(1) 3.00(2) 2.94(1) 2.94(1) 2.94(1) 2.93(1) 2.90(3) 2.89(1) 2.93(4) 2.90(2) 5.56(5) 5.79(3) 5.75(4) 5.34(4) 4.17(3) 3.05(2) 3.06(4) 3.00(1) 2.94(1) 3.00(5) 2.96(2) 6.87(2) 3.22(4) 3.11(2) 3.20(3) 3.12(3) 3.22(2) 3.17(2) 3.16(2) 3.12(2) 3.10(1) 3.12(2) 3.08(2) 3.09(1) 3.17(2) 3.15(3) 3.12(2) 3.13(2) 3.08(3) 3.14(4) 3.09(2) 3.16(1) 3.24(2) 3.12(4) 3.12(3) 3.09(2) 3.21(3) 4.03(4) 3.35(3) 3.24(2) 3.09(4) 3.21(2) 3.27(2) 3.13(2) 3.16(1) 3.06(2) 15.06(7) 3.9(1) 3.8(1) 4.2(2) 3.03(2) 2.96(1) 3.05(3) 2.99(1) log(A)b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 0.89(4) 0.90(2) 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 0.26(2) 0.19(3) 0.68(3) 0.52(4) 0.33(4) 0.34(3) 0.27(4) 0.32(3) 0.40(3) 0.37(5) 0.40(6) 0.39(4) 0.38(4) 0.35(3) 0.32(1) 0.59(3) 0.32(4) 0.55(4) 0.46(3) 0.41(3) 0.34(6) 0.41(2) 0.51(3) 0.49(2) 0.56(4) 0.50(2) 0.53(3) 0.54(5) 0.30(4) 0.44(3) 0.42(2) 0.32(2) 0.27(1) 0.32(3) 0.9(1) 0.2(1) -0.4(1) -0.37(9) -0.6(1) 2.00b 2.00b 2.00b 2.00b NCOMPTB 6.59(4) 6.34(6) 6.00(5) 7.48(6) 6.47(4) 6.47(6) 6.63(6) 7.81(7) 8.15(8) 8.33(6) 8.30(7) 2.14(3) 2.35(2) 2.55(1) 2.99(6) 3.17(3) 4.61(2) 4.61(1) 4.43(3) 5.99(1) 6.78(4) 5.63(2) 2.77(1) 6.46(9) 3.54(4) 3.41(3) 5.81(6) 5.21(5) 6.12(6) 6.33(5) 7.15(6) 6.43(6) 7.15(7) 6.38(6) 5.65(5) 4.97(4) 6.03(5) 6.13(6) 5.85(6) 5.98(4) 5.70(6) 5.58(5) 4.62(6) 3.71(4) 3.40(3) 3.39(3) 3.42(3) 3.68(6) 3.12(4) 3.64(5) 5.59(6) 4.1(1) 4.53(4) 4.91(6) 5.82(7) 6.08(5) 4.1(1) 2.72(5) 11.33(9) 9.80(7) 9.91(8) 7.71(7) 8.38(9) 6.65(6) 6.94(5)
c

Titarchuk, Seifina, & Frontera

Ts (keV) 1.12(4) 1.14(5) 1.13(4) 1.11(2) 1.12(3) 1.11(2) 1.11(4) 1.14(5) 1.13(4) 1.15(6) 1.13(2) 1.12(1) 1.12(2) 1.11(2) 1.11(1) 1.12(3) 1.14(5) 1.13(4) 1.15(7) 1.11(2) 1.12(1) 1.12(3) 1.72(2) 1.66(3) 1.40(3) 1.41(2) 1.45(4) 1.39(3) 1.40(5) 1.42(4) 1.46(3) 1.38(2) 1.41(4) 1.43(3) 1.40(1) 1.40(2) 1.42(3) 1.37(2) 1.39(3) 1.41(2) 1.40(1) 1.45(2) 1.42(2) 1.41(1) 1.41(2) 1.44(1) 1.43(2) 1.42(2) 1.40(3) 1.41(2) 1.40(2) 1.64(5) 1.41(2) 1.40(3) 1.42(2) 1.41(2) 1.25(3) 1.65(4) 1.71(5) 1.70(2) 1.69(3) 1.72(7) 0.99(2) 0.97(3) 0.98(4)

NBbody

c

Eline , (keV) 6.27(5) 6.22(3) 6.11(3) 6.18(4) 6.17(5) 6.18(3) 6.25(4) 6.14(8) 6.31(3) 6.17(2) 6.13(5) 6.64(2) 6.65(3) 6.65(2) 6.57(1) 5.64(4) 6.08(7) 6.40(4) 6.17(2) 6.04(1) 6.30(3) 6.04(2) 6.14(3) 6.16(2) 6.40(1) 6.41(3) 6.46(2) 6.42(6) 6.49(1) 6.42(5) 6.43(7) 6.42(1) 6.40(3) 6.54(2) 6.41(3) 6.40(4) 6.40(3) 6.44(1) 6.69(5) 6.40(3) 6.69(1) 6.42(4) 6.40(4) 6.41(1) 6.57(8) 6.40(2) 6.43(1) 6.40(3) 6.41(6) 6.42(1) 6.43(4) 6.40(1) 6.40(1) 6.41(4) 6.43(7) 6.46(3) 6.24(5) 6.58(4) 6.52(4) 6.45(5) 6.46(4) 6.41(7) 6.45(4) 6.45(3) 6.47(4)

Nline

c



2 red

(dof)

F1 /F

2

d

2.81(1) 2.75(1) 3.09(3) 2.76(1) 2.39(3) 2.39(2) 2.96(1) 3.08(1) 3.07(2) 3.08(2) 3.02(3) 0.63(1) 0.95(1) 1.40(2) 1.28(1) 2.04(2) 3.01(3) 2.88(2) 3.34(2) 3.56(6) 3.28(3) 3.50(2) 2.25(2) 2.25(2) 2.67(2) 2.01(1) 2.67(4) 2.93(2) 2.74(2) 2.85(3) 2.93(2) 2.76(3) 2.93(6) 2.68(8) 2.47(2) 2.47(2) 2.47(8) 2.65(8) 2.61(6) 2.54(8) 2.45(7) 2.32(6) 2.35(4) 2.06(8) 2.23(7) 2.06(2) 2.10(8) 2.25(4) 1.59(5) 2.15(8) 2.76(3) 2.68(4) 2.77(3) 2.73(5) 2.63(2) 2.72(4) 2.42(2) 1.34(1) 6.49(7) 6.26(8) 6.32(8) 4.31(4) 3.69(3) 4.75(1) 3.17(3)

0.52(6) 0.43(4) 0.51(2) 0.34(3) 0.29(2) 0.29(3) 0.52(4) 0.65(2) 0.54(6) 0.66(4) 0.73(3) 0.19(2) 0.09(1) 0.04(2) 0.14(3) 0.53(1) 0.36(5) 0.22(4) 0.65(4) 1.14(7) 0.59(1) 1.13(6) 1.67(7) 1.86(5) 0.02(2) 0.01(1) 0.02(1) 0.06(2) 0.01(1) 0.02(2) 0.03(2) 0.01(1) 0.01(1) 0.03(1) 0.01(1) 0.02(3) 0.03(2) 0.02(1) 0.01(1) 0.02(2) 0.02(1) 0.01(1) 0.01(1) 0.02(1) 0.01(1) 0.02(2) 0.03(2) 0.01(1) 0.02(1) 0.01(1) 0.02(1) 0.09(5) 0.01(1) 0.02(2) 0.01(1) 0.03(1) 0.08(3) 1.43(4) 0.96(2) 0.81(3) 0.90(1) 0.90(2) 0.89(1) 0.97(6) 0.90(1)

1.25(78) 0.82(78) 1.13(78) 1.46(78) 1.25(78) 1.21(78) 1.30(78) 1.18(78) 1.51(78) 0.86(78) 0.77(78) 0.74(78) 1.04(78) 1.19(78) 1.17(78) 1.20(78) 1.44(78) 1.02(78) 1.33(78) 1.22(78) 0.75(78) 0.85(78) 1.26(78) 1.12(78) 0.75(78) 1.28(78) 1.11(78) 0.91(78) 1.36(78) 1.15(78) 1.04(78) 1.12(78) 0.87(78) 1.08(78) 1.04(78) 1.29(78) 0.89(78) 0.85(78) 1.04(78) 1.24(78) 0.93(78) 1.17(78) 1.05(78) 1.26(78) 1.08(78) 0.96(78) 1.13(78) 1.07(78) 1.10(78) 1.12(78) 0.97(78) 1.26(78) 0.99(78) 1.06(78) 0.98(78) 0.98(78) 1.16(78) 1.04(78) 1.20(78) 0.99(78) 0.79(78) 1.12(78) 1.14(78) 1.20(78) 0.95(78)

5.52/2.09 5.20/2.02 4.83/1.85 6.18/2.46 5.35/2.13 5.35/2.13 5.56/2.09 6.52/2.50 6.67/2.47 6.88/2.66 6.87/2.64 1.63/1.46 1.63/1.49 1.76/1.61 2.11/1.84 2.56/1.68 4.04/1.64 4.04/1.66 4.02/1.53 5.15/1.94 5.66/2.35 5.15/1.94 4.02/1.50 5.08/1.95 2.89/1.19 2.59/1.09 4.23/1.63 4.23/1.53 4.43/1.69 4.59/1.79 5.14/2.08 4.65/1.83 5.14/2.09 4.60/1.82 4.09/1.61 3.66/1.45 4.33/1.71 4.46/1.95 4.24/1.64 4.34/1.84 4.14/1.73 4.03/1.62 3.41/1.33 2.78/1.15 2.65/1.05 2.59/1.04 2.62/1.06 2.82/1.17 2.32/1.35 2.77/1.26 4.10/1.63 3.25/1.31 3.45/1.39 3.67/1.47 4.22/1.58 4.41/1.74 3.36/1.33 1.55/2.09 8.76/3.61 7.92/2.99 7.97/3.08 6.41/2.37 7.01/2.59 5.73/2.23 5.85/2.23

7


The Astrophysical Journal, 767:160 (23pp), 2013 April 20 Table 4 (Continued) Observational ID 92030-02-03-01 92030-02-11-000 91151-04-01-00 91151-04-02-00 92030-02-04-00 92030-02-05-00 92030-02-06-00 92030-02-06-01 92030-02-07-00 92030-02-10-00 92030-02-09-00 94090-01-01-00 94090-01-01-01 94090-01-01-02 94090-01-01-03 94090-01-01-04 94090-01-01-05 94090-01-02-00 94090-01-02-03 94090-01-02-04 94090-01-02-01 94090-01-02-02 94090-01-03-00 94090-01-04-00 94090-01-04-01 94090-01-04-02 94090-01-04-03 94090-01-05-01 94090-01-05-00 94090-02-01-00 94090-02-01-01 94090-02-02-00 92030-02-12-00 92030-02-12-01 92030-02-12-02 92030-02-12-03 92030-02-12-04 92030-02-13-00 92030-02-14-00 MJD (day) 53939.875 53953.255 53959.950 54028.232 54125.301 54256.402 54257.346 54257.452 54258.447 54297.188 54306.346 54947.682 54947.939 54948.728 54948.987 54949.708 54950.686 54956.589 54956.769 54957.489 54957.626 54958.664 54960.585 54978.287 54978.485 54978.749 54979.271 54980.781 54981.669 54994.514 54997.718 55002.625 55080.709 55082.884 55082.647 55082.581 55082.773 55088.379 55116.958 = -1 1.01(4) 1.00(2) 1.01(3) 1.01(4) 1.03(5) 1.00(1) 0.99(4) 1.01(2) 1.04(4) 1.02(3) 1.00(1) 1.02(3) 1.00(2) 1.03(4) 0.99(3) 1.01(2) 1.00(1) 1.02(3) 1.01(4) 1.00(2) 1.03(4) 1.00(2) 0.97(5) 1.00(2) 1.01(3) 0.99(2) 1.00(1) 1.00(3) 1.02(4) 0.97(4) 1.00(2) 0.99(4) 1.01(2) 1.00(1) 0.98(4) 1.01(2) 0.96(6) 1.01(1) 1.02(4) Te (keV) 3.02(2) 3.00(2) 3.10(2) 3.53(3) 3.12(2) 3.07(1) 3.15(8) 3.16(2) 3.06(8) 3.15(2) 3.06(8) 11.09(4) 10.28(5) 10.31(4) 10.61(6) 10.12(3) 10.01(4) 17.80(7) 16.44(7) 16.92(8) 17.31(9) 20.47(9) 20.3(1) 12.54(9) 11.32(4) 12.51(4) 14.96(5) 14.34(5) 13.25(4) 8.13(5) 3.09(2) 3.12(2) 2.98(3) 3.01(2) 3.05(2) 2.99(4) 3.07(3) 3.09(4) 3.09(2) log(A)b 2.00b 2.00b 0.27(2) 0.07(9) 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 0.35(5) 0.34(4) 0.36(6) 0.17(5) 0.05(5) 0.16(4) 0.12(1) 0.11(1) 0.27(2) 0.21(3) 0.01(2) 0.01(1) 0.47(4) 0.56(9) 0.67(8) 0.47(3) 0.54(9) 0.56(7) 0.08(4) 0.42(5) 0.41(3) 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b 2.00b NCOMPTB 6.05(7) 5.37(9) 6.78(5) 9.86(7) 4.32(3) 4.24(2) 4.25(6) 4.22(4) 5.35(5) 3.74(3) 5.35(6) 2.98(3) 2.79(2) 2.75(1) 2.66(1) 3.00(2) 2.65(1) 2.83(1) 2.75(2) 2.67(1) 2.61(1) 2.69(2) 2.56(1) 3.04(1) 3.04(5) 2.94(3) 2.87(2) 3.04(4) 3.15(5) 4.72(5) 8.48(9) 8.98(7) 7.18(9) 7.15(8) 7.50(9) 7.07(6) 7.32(6) 5.58(9) 4.52(4)
c

Titarchuk, Seifina, & Frontera

Ts (keV) 0.99(2) 0.83(4) 1.75(5) 1.86(3) 0.99(8) 0.99(4) 0.83(7) 0.82(5) 0.81(3) 0.80(4) 0.84(2) 0.83(3) 0.82(4) 0.86(3) 0.89(2) 0.81(4) 0.80(3) 1.78(5) 1.57(2) 1.62(3) 1.57(4) 1.69(3) 1.56(2) 1.43(6) 1.39(2) 1.73(9) 1.71(8) 1.40(2) 1.53(4) 1.40(5) 1.41(2) 1.40(2) 0.83(5) 0.85(3) 0.87(2) 0.82(4) 0.81(3) 0.88(2) 0.83(3)

NBbody

c

Eline , (keV) 6.46(5) 6.86(2) 6.45(4) 6.49(6) 4.85(4) 6.85(6) 5.99(7) 6.85(3) 6.81(5) 6.85(3) 6.82(4) 5.83(5) 5.65(4) 6.45(5) 6.46(4) 6.47(3) 6.45(2) 6.35(4) 6.31(4) 6.38(5) 6.37(8) 6.34(4) 6.34(5) 6.36(4) 6.33(3) 6.38(4) 6.48(6) 6.42(2) 6.39(4) 6.41(4) 6.45(3) 6.46(2) 6.86(6) 6.87(3) 5.98(4) 6.15(2) 6.84(4) 6.85(5) 6.89(8)

Nline

c



2 red

(dof)

F1 /F

2

d

-

- - - - - -

-

2.53(5) 2.81(2) 3.74(6) 5.51(8) 4.15(2) 3.37(3) 2.32(1) 2.31(1) 2.30(2) 2.31(2) 2.30(3) 1.79(2) 1.78(3) 1.79(4) 1.78(6) 1.79(5) 1.76(9) 1.43(1) 1.44(1) 1.43(3) 1.41(2) 1.40(5) 1.44(8) 1.43(2) 1.44(3) 1.45(1) 1.43(2) 0.53(1) 1.44(2) 2.54(3) 5.46(5) 6.46(7) 4.11(4) 5.44(6) 4.75(4) 4.61(9) 4.28(8) 4.45(9) 3.44(4)

0.91(2) 0.94(1) 1.03(7) 0.90(1) 0.90(2) 0.87(7) 0.90(5) 0.88(2) 0.78(2) 0.95(3) 0.91(4) 2.83(3) 2.89(1) 2.83(2) 2.82(1) 2.83(3) 2.85(2) 1.10(1) 1.12(3) 1.19(2) 1.11(2) 1.16(2) 1.10(9) 3.07(3) 3.08(2) 3.01(6) 3.07(4) 1.08(5) 3.14(9) 1.06(7) 0.90(5) 0.91(2) 0.87(3) 0.89(4) 0.90(3) 0.97(4) 0.91(2) 0.97(3) 0.94(5)

1.16(78) 1.00(78) 1.07(78) 1.12(78) 1.06(78) 1.14(78) 1.21(78) 1.10(78) 0.79(78) 1.01(78) 1.15(78) 1.21(78) 1.04(78) 1.18(78) 1.04(78) 1.09(78) 1.03(78) 1.09(78) 1.17(78) 1.09(78) 1.08(78) 1.10(78) 1.04(78) 1.10(78) 1.49(78) 0.99(78) 0.97(78) 0.96(78) 0.98(78) 0.96(78) 0.93(78) 0.96(78) 1.19(78) 1.16(78) 1.15(78) 1.10(78) 0.99(78) 1.06(78) 1.11(78)

5.26/1.98 4.68/1.82 5.32/2.06 7.65/2.96 3.89/1.54 3.81/1.50 3.79/1.47 3.91/1.52 4.79/1.80 3.58/1.44 3.45/1.36 2.01/1.63 1.94/1.62 1.89/1.66 1.76/1.77 1.89/1.73 1.69/1.79 1.58/2.14 1.59/2.08 1.52/2.05 1.50/2.00 1.49/2.08 1.41/2.00 1.78/2.60 1.80/2.63 1.72/2.68 1.67/2.56 1.96/2.60 1.84/2.76 3.18/2.96 6.78/2.47 6.99/2.65 6.13/2.38 6.14/2.36 6.38/2.60 6.09/2.33 6.20/2.62 5.07/1.91 4.06/1.57

Notes. Parameter errors correspond to a 90% confidence level. a The spectral model is wabs (Blackbody + CompTB + Gaussian), where N is fixed at a value 3.00 â 1021 cm-2 (Bloser et al. 2000); color temperature T and T H s BB are fixed at 1.3 and 0.7 keV, respectively (see comments in the text). b When parameter log(A) 1, this parameter is fixed at 2.0 (see comments in the text). c Normalization parameters of Blackbody and CompTB components are in units of 10-2 â L /d 2 erg s-1 kpc-2 , where L 39 10 39 is the source luminosity in units of 2 1039 erg s-1 , d10 is the distance to the source in units of 10 kpc and the Gaussian component is in units of 10-2 â total photons cm-2 s-1 in line. d Spectral fluxes (F /F ) in units of â10-9 erg s-1 cm-2 for (3­10) and (10­60) keV energy ranges, respectively. 1 2

We use the broadband energy spectra of BeppoSAX (Boella et al. 1997) and RXTE (Bradt et al. 1993), combined with the high-timing resolution of RXTE, to study short- and long-term spectral and timing evolution of atoll sources. 3. SPECTRAL ANALYSIS Unlike the past analyses of the source spectral data discussed in Section 1, in our study we make use of a unified model for both soft and hard states. In this way, we have an opportunity to compare the X-ray spectra of 4U 1820-30 in all states. In our spectral model, we use an assumption that the accretion material passes through the accretion disk (for example, through the standard Shakura­Sunyaev disk; Shakura & Sunyaev 1973) and the transition layer (TL; Titarchuk et al. 1998) where soft 8

photons coming from the disk and NS surface are Comptonized off hot plasma (see also Figure 2 in ST12). The ground-based observer can also observe directly some fraction of these disk and NS seed photons. Thus, our input model is a sum of the Comptonization component (COMPTB), which is the XSPEC contributed model7 (see Farinelli et al. 2008, hereafter F08) and the soft BB and line (Gaussian) components. The parameters of the COMPTB component are the seed photon temperature kTs , the electron (plasma) temperature kTe , the energy index (= - 1) of the Comptonization spectrum, the illumination fraction of the Comptonized region (TL), f [f = A/(1 + A)], and the
7

http://heasarc.gsfc.nasa.gov/docs/software/lheasoft/xanadu/xspec/models/ comptb.html


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Figure 3. Examples of typical E F (E ) spectral diagram of 4U 1820-30 during soft (left panel) and hard (right panel) state events. The best-fit RXTE spectra (top panels) using the model wabs (Blackbody + CompTB + Gaussian) with (bottom panels) for the high-luminosity (banana) state (40017-01-11-00 observation, 2 2 red = 1.00 for 78 dof, left panel) and for the low-luminosity (island) state (94090-01-04-00 observation, red = 1.10 for 78 dof, right panel). The best-fit model parameters are = 1.99 ± 0.02, kTe = 2.94 ± 0.01 keV, and EGauss = 6.53 ± 0.06 keV (for the soft state) and = 2.00 ± 0.04, kTe = 12.54 ± 0.09 keV, and EGauss = 6.35 ± 0.04 keV (for the hard state) (see more details in Table 4). Blue, red, and violet lines indicate Blackbody, CompTB, and Gauss components, respectively. (A color version of this figure is available in the online journal.)

normalization of the seed photons illuminating the Comptonized region, NCOMPTB . We include a Gaussian component in the model characterized by the parameters Eline , line , and Nline which are a centroid line energy, the line width, and the normalization, respectively. We also include a BB component and the interstellar absorption in our model characterized by the following parameters: the normalization NBB , the color temperature TBB , and a column density NH , respectively. We fix the index of the seed photon spectrum at = 2 (or = + 1 = 3). Namely, we suggest that this seed photon spectrum is BB-like. We neglect the bulk inflow effect with respect to the thermal Comptonization, assuming that a bulk parameter = 0. The parameter log(A) of the COMPTB component is fixed at 2 because the best-fit log(A) 1. Then f = A/(1 + A) as the illumination fraction parameter is approximately 1 for any log(A) 1. We use a value of NH = 3.00 â 1021 cm-2 , estimated by Bloser et al. (2000) for 4U 1820-30. We find satisfactory fits using our model for both BeppoSAX and RXTE observations of 4U 1820-30 for all available data sets. 3.1. BeppoSAX Data Analysis Table 3 shows the data analysis results for the broadband BeppoSAX spectra. On the top of Figure 2 we present an example of the BeppoSAX spectrum along with its best-fit using our model, while in the bottom panel we demonstrate (reduced 2 = 1.11 for 364 dof). The line emission is 9

clearly centered around 6.7 keV. We find that the width of this line--0.8 keV--is quite large and is much wider than the instrumental response whose width is smaller than 0.02 keV.8 This broad emission line at 6.7 keV can be a result of illumination of highly ionized iron by the X-ray continuum. Piraino et al. (2000) suggest that this broad line originates either in an ionized innermost disk region or in a hot corona above the disk. A combination of two absorption edges related to ionized iron, instead of a Gaussian line, can also describe this part of the spectrum (D'Ai et al. 2006). The La or relativistically smeared line or reflection models can also be used to describe this line feature (see Ng et al. 2010 and Egron et al. 2011, respectively). However, Seifina & Titarchuk (2011) demonstrate that the model, wabs (Blackbody+COMPTB+Gaussian), which includes a Gaussian iron line, can successfully fit the data for extensive RXTE and BeppoSAX observations of 4U 1728-34. We can interpret this broad iron line detected in 4U 1820-30 in terms of reprocessing emission by a disk. In addition, we fit a smeared absorption edge in the 7­8 keV range using the smedge XSPEC model (see Ebisawa et al. 1994). The edge energy is 7.7 ± 0.5 keV for the wabs (Bbody + CompTB + Gaussian) smedge model which indicates the presence of ionized material in the emission region. The smearing width is fixed at 10 keV.
8

See ftp://heasarc.gsfc.nasa.gov/sax/cal/responses/98_11.


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Figure 4. Six representative EFE spectral diagrams which are related to different electron temperatures of the TL [kTe = 2.9 keV (red), 3 keV (blue), 4 keV (green), 6 keV (violet), 10 keV (pink), and 12 keV (black)] using the model wa bs (Blackbody + CompTB + Gaussian) for island­banana state transitions of 4U 1820-30. The data are taken from RXTE observations 30057-01-0401 (red), 70030-03-07-020 (blue), 70030-03-05-02 (green), 70030-03-05-01 (violet), 40017-01-24-00 (pink), and 94090-01-04-00 (black). (A color version of this figure is available in the online journal.)

2 Figure 5. CompTB normalization measured in units of Lsoft /D10 vs. the 39 electron temperature kTe (in keV) obtained using the best-fit spectral model wa bs (Blackbody + CompTB + Gaussian) for atoll sources 4U 1820-30 (red), GX 3+1 (green, taken from ST12), and 4U 1728-34 (blue, taken from ST11) for RXTE data. (A color version of this figure is available in the online journal.)

We include this edge component in the fits for all BeppoSAX data (see Table 3). We obtain = 1.03 ± 0.04 (or = +1 = 2.03 ± 0.04) for all analyzed BeppoSAX data; the seed photon temperature kTs of the COMPTB component is mildly variable and its value is around 1.3 keV, whereas kTBB varies in the interval from 0.58 to 0.69 keV (see Tables 1 and 3 for details). 3.2. RXTE Data Analysis For all RXTE fits, we fix the BB temperature kTBB = 0.7 keV. This value is an upper limit in our analysis of the BeppoSAX data (see Table 3), because RXTE detectors cannot give us reliable spectra below 3 keV. In Table 4, we show the best-fit parameters of the RXTE spectra using our model. It is important to point out that for all RXTE observations of 4U 1820-30 the photon index only changes slightly around 2 ( = 1.99 ± 0.02), while the best-fit kTe varies in the 2.5­21 keV range. However, the determination of the iron line profile using the RXTE data is difficult because of the low-energy resolution of the PCA/RXTE detector. Moreover, the inclusion of a smedge component in the spectral model for RXTE data does not improve the fit quality any more. Therefore, we apply our spectral model to RXTE data using a simple Gaussian as the line component without smedge modeling. The line width line does not vary much and it is always in the interval from 0.9 to 1.3 keV during all spectral transitions. Therefore, we fix line at 1.2 keV for all spectra during the fitting procedure. The values of the best-fit seed photon temperatures, kTs = 1­1.3 keV, are consistent with those obtained using the BeppoSAX data (see Table 3). In Figure 3, we show the representative examples of the E F (E ) spectral diagrams of 4U 1820-30 during the soft 10

Figure 6. Flux ratio [10­50 keV/3­50 keV] vs. flux in the range of 3­60 keV of 4U 1820-30 for RXTE data. The spectral branches have been indicated for the island state (IS), lower banana state (LB), and upper banana (UB) state. The direction of the ISLBUB transition is indicated by the arrow. Note that kTe decreases from 21 keV to 2.9 keV along the direction of the arrow. (A color version of this figure is available in the online journal.)

(left panel) and the hard (right panel) state events. The bestfit RXTE spectra (top panels) in the model wabs (Bbody + CompTB + Gaussian) with (bottom panels) for the high2 luminosity (banana) state [40017-01-11-00 observation, red = 1.00 for 78 dof, left panel] and for the low-luminosity (island) 2 state [94090-01-04-00 observation, red = 1.10 for 78 dof, right panel] are also shown. The model best-fit parameters are = 1.99 ± 0.02, kTe = 2.94 ± 0.01 keV, and EGauss = 6.53 ± 0.06 keV for the soft state; = 2.00 ± 0.04, kTe = 12.54±0.09 keV, and EGauss = 6.35±0.04 keV for the hard state (see Table 4 for details). Violet, blue, and red lines correspond to the Gaussian, BB, and CompTB components, respectively.


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Figure 7. From top to bottom: evolution of the RXTE/ASM count rate, the model flux in 3­10 keV and 10­50 keV energy ranges (blue and crimson points, respectively), the electron temperature kTe in keV, and CompTB and Blackbody normalizations (crimson and blue, respectively) during the 1996­1997 transition set (R1­R2). The rising phases of the local (mild) transitions are marked with blue vertical strips. (A color version of this figure is available in the online journal.)

In Figure 4, to illustrate the spectral evolution 4U 1820-30, we show six representative EFE spectral diagrams for different electron temperatures of a Compton cloud [kTe = 2.9 keV (red), 3 keV (blue), 4 keV (green), 6 keV (violet), 10 keV (pink), and 12 keV (black)] applying the wabs (Blackbody + COMPTB + Gaussian) model during island­banana state transition. We show how the TL electron temperature kTe anti-correlates with the normalization NCOMPTB (proportional to M ) in Figure 5. For a comparison, we add the points for 4U 1728-34 and GX 3+1 (see Seifina & Titarchuk 2011 and Seifina & Titarchuk 2012, respectively). The electron temperature kTe decreases and saturates at about 3 keV when the mass-accretion rate increases (see an explanation of this effect in Farinelli & Titarchuk 2011; hereafter FT11). Our spectral model applied to the spectral data of BeppoSAX and RXTE is robust in all the data sets. Namely, a value of 2 reduced red = 2 /Ndof , where Ndof is a number of degrees of 2 freedom, is around 1.0 for most of the observations. red is about 1.5 for less than 3% of the spectra with high counting statistics 2 but red is never above a rejection limit of 1.6. Note that the high residuals of the poor-fit spectra (2 among 234 spectra for 11

which 2 = 1.55) occur in the iron line region. As was shown by the BeppoSAX analysis, the shape of the iron line is more complex than a simple Gaussian (see discussion in Section 3.1). Probably, the fits of this line indicate a broad line, whose shape and width are a result of scattering of the line photons in the hot plasma (TL) along with the iron smedge effect there. However, we cannot resolve this line complexity using the RXTE data. Thus using broadband BeppoSAX observations, we can obtain the best-fit parameters of our spectral model. Due to the largetime coverage of 4U 1820-30 by RXTE, we are capable of studying the source spectral transitions in the 3­200 keV energy range. 4. OVERALL PATTERN OF X-RAY PROPERTIES 4.1. Hardness­Intensity Diagram To study the properties of 4U 1820-30 during the spectral transitions when the luminosity changes, we use hard color (10­50 keV/3­50 keV) (HC) versus the 3­60 keV flux measured in units of 10-9 erg s-1 cm-2 (hardeness­intensity


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Figure 8. Similar to Figure 7 but for the RXTE 1999 transition set R3. (A color version of this figure is available in the online journal.)

diagram, HID). In Figure 6, we show the flux ratio HC versus the 3­60 keV flux using the RXTE data. As it appears from this figure, 4U 1820-30 shows a "J"-like diagonal shape in this diagram with upper short and lower elongated branches, which are joined at the lowest flux point. The spectral branches are indicated for the island state (IS), the lower banana state (LB) and the upper banana (UB) state. The direction of the ISLBUB transition is shown by an arrow. The electron temperature kTe changes from 21 keV to about 3 keV along the direction of the arrow (compare with Figure 5). The identification of HID states is made using simultaneous timing and spectral analysis, and we have revisited the previous similar RXTE data analysis made by Bloser et al. (2000) and Migliari et al. (2004). In particular, the HC drops from 0.25 to 0.1 while the 3­60 keV flux is quasi-constant when the source propagates from the IS toward the LB. On the other hand, the HC rises from 0.12 to 0.35 with a simultaneous growth of the 3­60 keV flux when the source goes further from the LB toward the UB. 12

5. EVOLUTION OF X-RAY SPECTRAL PROPERTIES DURING SPECTRAL STATE TRANSITIONS A number of X-ray spectral transitions of 4U 1820-30 with luminosity variations have been detected by RXTE during 1996­2009 (R 1­R 7 sets). We investigate common spectral­timing signatures that can be found for these spectral transition events. The source reveals different behaviors during high-luminosity and low-luminosity events. 5.1. Evolution of X-Ray Spectral Properties during High-luminosity Events In Figures 7 and 8, we show the results of our spectral analysis of the RXTE observations applying the wabs (Blackbody + COMPTB + Gaussian) model. In the top panels, we present the RXTE/ASM count rate and the model fluxes from 3 to 10 keV and from 10 to 50 keV (see blue and crimson points, respectively). The TL electron temperature kTe as a function of time is shown in the third panel from the top. The temperature kTe changes in the 2.9­4 keV interval during the


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Figure 9. Similar to Figure 7 but for the RXTE 2009 transition set R7. The quasi-plateau phases of the low-luminosity state of 4U 1820-30 are marked using orange vertical strips. (A color version of this figure is available in the online journal.)

time period MJD 50490­MJD 50700 and only slightly varies around 3 keV during the MJD 51200­MJD 51500 time interval. The COMPTB normalization NCOMPTB and the normalization of the low-temperature BB component NBB (crimson and blue points, respectively) are presented in the second-to-bottom panels of Figures 7 and 8. One can clearly see that the COMPTB normalization NCOMPTB correlates with variations of the ASM count rate and the 3­10 keV model flux. On the other hand, the BB normalization NBB only slightly varies. 5.2. Evolution of X-Ray Spectral Properties during the Low-luminosity Events Since late April of 2009, 4U 1820-30 showed a prominent X-ray low­hard state at energies less than 10 keV as it was observed by the X-ray monitors on RXTE and Swift. We display the characteristics of the low­hard state obtained using the RXTE data in Figure 9. Since 2009 April 24 (MJD 54945), the source was steadily brightening in the 15­50 keV band of the Swift/Burst Alert Telescope (BAT), with a daily average of 0.032 ± 0.002 counts cm-2 s-1 (145 mCrab; Krimm et al. 13

2009). In contrast, the highest count rate detected by the Swift/BAT was 0.14 counts cm-2 s-1 . The ASM/RXTE and PCA/RXTE light curves showed that 4U 1820-30 was in an extended low state from MJD 54944 to MJD 54982. The ASM count rate took a sharp drop at MJD 54944 while the flux began rising in the BAT monitor. The ASM count rate was very low, approximately 6.0 ± 0.5 counts s-1 during this low-state period, with respect to a usual average count rate of 20 counts s-1 . During the same time period, the RXTE/PCA count rate decreased from 4000 counts s-1 to 1000 counts s-1 . This kind of long-time low state was not observed from 4U 1820-30 over the 10­15 year period. The typical low-state duration varies from 1 to 2 weeks. We also establish that the X-ray spectra of this source over the low-luminosity state (MJD 54955­54982) are quite stable in terms of the CompTB normalization value. But the electron temperature kTe of the Comptonized plasma increases from 3 keV up to 20 keV during the MJD 54950­54960 period and after that kTe gradually decreases again to 3 keV when the luminosity rises at the end of the quasi-plateau (at MJD 55000). In Table 4, we report the best-fit parameter values. During the


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Figure 10. Same as Figure 7 but for all RXTE sets (R 1­R 7) to demonstrate a long-time variability. (A color version of this figure is available in the online journal.)

IS-B transition period, we find that the photon index (or the spectral index = - 1) is almost constant; i.e., it only slightly varies around 2 (see the combined Figure 10). 5.3. Spectral State Transitions in 4U 1820-30 In 4U 1820-30, the hard­soft state transition is observed as kTe decreases from 15­20 keV to 3 keV (see Figure 9). The outburst hard-to-soft state transitions are seen when the soft photon flux NCOMPTB dramatically increases (see Figures 5 and 6). In general, following FT11, we consider the spectral state transitions in terms of the kTe change. Thus, the hard state is seen when the electron temperature reaches a maximum, kTemax , whereas the soft state is observed when the electron temperature, kTemin reaches a minimum. Note that kTe is well determined by the high-energy cut-off of the spectrum, Ecut , and can be well established by the spectral fits to the data. This spectral state definition is based on kTe (or Ecut 2kTe ). Unlike what occurs in the case of NS binaries, for a black hole (BH) case one can relate a spectral state change to the spectral (photon) index change (see Shaposhnikov & Titarchuk 2009, hereafter ST09). 14

We test the hypothesis of appr 2 using a 2 -statistic 2 criterion. We calculate the distribution of red (appr ) = N (1/N ) i =1 (i - appr /i )2 versus appr and we find a sharp 2 minimum of the function red (appr ) at 1 when appr = 1.99 ± 0.01 and appr = 1.99 ± 0.02 with confidence levels of 67% and 99% for 234 dof, respectively, (see the related figure of 2 red (appr ) for 4U 1728-34 in ST11). The index is almost constant when kTe (see Figure 11) and the COMPTB normal2 ization, Lsoft /d10 , change (see below). Based on an analysis of 39 BeppoSAX data, FT11 proposed that is about 2 for quite a few NS sources. FT11 also define the spectral state using a value kTe and they demonstrate that = 2 ± 0.2 (or = 1 ± 0.2) when kTe varies in the 2.9­21 keV interval. It should be noted that not all NSs show spectral state transitions but quite a few NSs do exhibit them. For instance, socalled atoll-sources (such as 4U 1820-30) usually demonstrate IS­B transitions. Particularly during such transitions, it can be possible to distinguish an NS from a BH. Specifically, NSs and BHs show drastically different variations of spectral characteristics. NSs, examples of which include 4U 1820-30 and 4U 1728-34, indicate variabilities of kTe and mass-accretion


The Astrophysical Journal, 767:160 (23pp), 2013 April 20 Table 5 Comparison of the Parameters of atoll Sources 4U 1820-30, GX 3+11 , and 4U 1728-342 Source D (kpc) 4.53 5.8­84 4.2­6.46 Type of Donor Star A WD ? Mass of Donor Star (M ) 10 0.07 ··· Porb (minute) ··· 11.46 ± 0.04 ··· i (deg) ··· 43 ± 95 ··· kTe (keV) 2.3­4.5 2.9­21.0 2.5­15 NCOMPTB 2 (Lsoft /D10 ) 39 0.04­0.15 0.02­0.14 0.02­0.09 kTBB (keV) 0.6 0.6 0.6­0.7

Titarchuk, Seifina, & Frontera

kTs (keV) 1.16­1.7 1.1­1.7 1.3

f

tLS­HS­LS (days) 1000 170 15

GX 3+1 4U 1820-30 4U 1728-34

0.2­0.9 0.2­1 0.5­1

References. (1) ST12; (2) ST11; (3) Kuulkers & van der Klis 2000; (4) Shaposhnikov & Titarchuk 2004; (5) Arons & King 1993; (6) van Paradijs 1978.

diagram (right column) for A-C time events (see upper panel). All points [(events) A red (ID 94090-01-01-00), A blue (ID 94090-01-02-03), B red (ID 94090-01-02-02), C red (ID 9409001-04-03)], except B (blue) and C (red), are related to the IS (broadband noise, no VLFN). PDSs denoted by B (blue, ID 94090-02-02-00) and C (red, ID 94090-0103-00) exhibit the island-lower-banana state transition. For the blue PDS the VLFN (very low frequency noise) appears as a BLN component that transforms into a QPO (a broad Lorentzian with a h centroid frequency at 7­10 Hz). We present PDS (panels A1, B1, C1) along with corresponding E F (E )-spectral diagrams (panels A2, B2, C2). The related PDSs and energy spectral data are shown by blue and red points, respectively. In the left panel, we also show the electron temperature kTe associated with a given PDS. 6.2. Spectral and Timing Properties during the High-luminosity State Transition To compare with Figure 12, we show the evolution of spectral­timing characteristics during the high-luminosity state in Figure 13. In the top panel, we display the ASM light curve during the high-luminosity interval at R 3 (1999) transition events. Red/blue points A, B, and C are related to the moments at MJD = 51283.6/51300, 51313.7/51330.5, and 51389.4/ 51396.26 covering different transition phases. In the bottom left and bottom right panels, we present PDSs for the 15­30 keV energy range and the E F (E )-spectral diagram respectively, for the A (red, top), B (blue, middle), and C (blue, bottom) points of the X-ray light curve (see the upper panel). All points are related to the banana state with relatively strong broadband noise and VLFN with QPOs at l 6­7 Hz for the C moment (red). The PDSs in panel C (blue and red) illustrate the island-lower-banana state transition. Here the VLFN appears and the BLN component transforms into a QPO (a broad Lorentzian with h centroid frequency at 7­10 Hz, C red, 40017-01-12-00). The power spectra (panels A1, B1, C1) correspond to E F (E ) diagrams (panels A2, B2, C2). The corresponding energy spectra of 4U 1820-30 are related to the electron temperature of 3 keV. In Figure 14, we illustrate a typical power spectrum of 4U 1820-30 for different X-ray spectral states (shown in the right panel). The electron temperature values of corresponding energy spectra are indicated on the right vertical axis. The power spectra in the extreme island state (EIS), IS (multiplied by factor 10-2 for clarity), lower left banana state (LLB, â10-4 ), lower banana state (LB, â10-6 ), and upper banana state (UB, â10-8 ) are presented from the top to the bottom. The histograms show the best fits to the power spectra, which consist of three components: VLFN the peaked noise component, low-frequency QPOs (l and h ) and the high-frequency feature hHz (see van Straaten et al. 2003 for details of the terminology). 15

Figure 11. Photon index vs. electron temperature kTe (in keV) in the frame of our spectral model wabs (Blackbody + CompTB + Gaussian) during transition events (see Tables 3 and 4). Blue and red points correspond to BeppoSAX and RXTE observations, respectively. (A color version of this figure is available in the online journal.)

rate M along with a quasi-constant index equal to about 2. Meanwhile, BHs demonstrate a monotonic growth of when M increases and reaches its final flattening (saturation; see ST09). 6. SPECTRAL-TIMING CORRELATIONS DURING SPECTRAL STATE TRANSITIONS We analyze the RXTE light curves applying the powspec task taken from FTOOLS 5.1. We implement the timing analysis RXTE/PCA data which we perform in the 13­30 keV range applying the event mode with a time resolution of 1.2 â 10-4 s. We make power density spectra (PDS) in the 0.1­500 Hz frequency range with a 0.001 s time resolution. The Poissonian statistic's contribution was subtracted. We apply the QDP/PLT package9 for PDS modeling. 6.1. Spectral and Timing Properties during the Low-luminosity State Transition In Figure 12, we present the generic behavior of X-ray timing-spectral characteristics for the low-luminosity state at R 7 (2009) transition events. We plot PDSs (left column) along with the EF (E )­spectral diagram (right column) for six moments at MJD = 54947.6/54956.5, 54958.6/55002.6, and 54997.7/54960.36, covering different transition phases. At the bottom, we demonstrate PDSs for the 15­30 keV energy range (left column) and plot these along with the E F (E )-spectral
9

http://heasarc.gsfc.nasa.gov/ftools/others/qdp/qdp.html


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Figure 12. Top: evolution of the RXTE/ASM count rate during low-luminosity state R 7 (2009) transition events. Red/blue points A, B, and C MJD = 54947.6/54956.5, 54958.6/55002.6, and 54997.7/54960.36 covering different transition phases. Bottom: PDSs for the 15­30 keV band plotted along with the E F (E )-diagrams (right column) for A, B, and C points of the X-ray light curve. The E F (E )-diagrams (panels A2, B2, the corresponding power spectra (panels A1, B1, C1). The data shown by blue and red points are related to the corresponding power spectra. In the we indicate kTe related to a given PDS. (A color version of this figure is available in the online journal.)

mark moments at (left column) are C2) are related to left bottom panel,

6.3. Comparison of Spectral and Timing Characteristics of atoll Sources 4U 1820-30, GX 3+1, and 4U 1728-34 In this paper, we also study the correlations of X-ray spectral­timing characteristics and M in a number of atolls during their spectral transitions. We search for similarities and differences between atoll sources. In this way, we can present a comparative analysis of three atoll sources: 4U 1820-30, GX 3+1, and 4U 1728-34. We apply the same spectral model that consists of a low-temperature BB, a Comptonized continuum, and Gaussian line components.
6.3.1. Constancy of the Photon Index

6.3.2. The Differences and Similarities of kTe Ranges in 4U 1820-30, GX 3+1, and 4U 1728-34

We demonstrate that atolls 4U 1820-30, GX 3+1, 4U 1728-34 show a similar pattern of photon index ver sus M (or NCOMPTB ). Namely, the photon index only slightly varies around 2 (see Figure 15). Following FT11, ST11, and ST12, we can suggest that the cooling flow of soft disk photons is much less than the energy release in the TL for each of these three sources. 16

One can see from Figure 16 that the ranges of kTe for an individual state evolution of these three sources are different. The electron temperature kTe changes in 4U 1728-34 from 3 to 15 keV, whereas kTe varies within a much narrow range of kTe around 3 keV in GX 3+1. In turn, source 4U 1820-30 demonstrates a wider interval of kTe in which kTe varies from 2.9 keV to 21 keV, similar to some extent to the temperature change in 4U 1728-34. Note that in a low-temperature regime, 4U 1820-30 and GX 3+1 are similar in terms of normalization 2 NCOMPTB = (4­15) â L39 /D10 , or mass accretion rate, (see Figure 5) and Comptonized fraction f = 0.2­0.8 (see Figure 16), whereas 4U 1820-30 and 4U 1728-34 are similar for a 2 range of the normalization NCOMPTB = (2­4) â L39 /D10 (see Figure 5) and f = 0.5­0.8 (see Figure 16) for the hightemperature regime. However, in contrast to 4U 1728-34, the source 4U 1820-30 has an additional branch of intermediate temperatures (8­12 keV) when the Comptonized fraction is


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Figure 13. Top: evolution of the RXTE/ASM count rate during the high-luminosity state R 3 (1999) transition events. Red/blue points A, B, and C mark moments at MJD = 51283.6/51300, 51313.7/51330.5, and 51389.4/51396.26 covering different transition phases. Bottom: PDSs for the 15­30 keV band (left column) are plotted along with the E F (E )-diagram (right column) for A (red, top), B (blue, middle), and C (blue, bottom) points of the X-ray light curve. All points are related to the banana state [strong broadband noise, VLFN and QPOs at l 6­7 Hz (C red)]. The E F (E )-diagrams (panels A2, B2, C2) are related to the corresponding power spectra (panels A1, B1, C1). The data are shown by black points. The electron temperature kTe of the corresponding energy spectra of 4U 1820-30 is about 3keV. (A color version of this figure is available in the online journal.)

relatively low, f < 0.5 (see Figure 16). Note that all objects have a common temperature interval 3­4 keV when NCOMPTB (or mass accretion rate) is relatively high. Thus, according to FT11, ST11, ST12, and the present study, the electron temperature kTe for atolls and Z -sources varies in the 2.5­25 keV range. Specifically, the change of kTe around 3 keV is similar for all three source 4U 1820-30, GX 3+1, and 4U 1728-34. The minimum value of kTe at 2.5 keV occurs at the peak luminosity for 4U 1728-34 (see ST11), during a local rise of luminosity for GX 3+1 (ST12), and at high-luminosity phases for 4U 1820-30. For all three of these objects, the values of color seed photon temperatures kTs = 1.1­1.7 keV and BB temperatures kTBB 0.6 keV are comparable (see Table 5). In contrast, the variability extent of kTe is not similar. The reason for the difference in electron temperature ranges is evident. Sources 4U 1820-30 and 4U 1728-34 show a complete cycle of state evolution: IS­LB­UB stages for 4U 1820-30 and an EIS-UB state for 4U 1728-34 EIS­UB state (see Di Salvo et al. 2001; ST11). But GX 3+1 demonstrates a short evolution behavior on 17

the CCD from the LB to the UB. This evolutionary picture is also clear from Figure 16 which shows that the track of GX 3+1 is only a part of the full track (see the definition of a state sequence and the standard atoll­Z scheme in Hasinger & van der Klis 1989). Note that 4U 1820-30 shows almost the same kTe range as that of 4U 1728-34 and an almost identical timing evolution. But clear differences between these atolls can be seen in Figure 17, where we show spectral hardness (10­50 keV/3­ 50 keV) versus flux in the 3­60 keV range. In fact, 4U 1728-34 (blue points) is fainter and harder and demonstrates a much wider range of spectral hardness than that of 4U 1820-30 (red points).
6.3.3. Comparison of Spectral Evolution as a Function of the Luminosity for 4U 1820-30, GX 3+1, and 4U 1728-34

Now we present a comparison of X-ray spectral evolution for sources 4U 1820-30, GX 3+1, and 4U 1728-34 based on the luminosity value that is presumably proportional to


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Figure 14. PDSs of 4U 1820-30 related to its X-ray spectral states. kTe values (in keV) corresponding to the energy spectra are indicated on the right vertical axis. PDSs in the extreme island state (EIS), island state (IS, multiplied by a factor 10-2 for clarity), lower left banana state (LLB, â10-4 ), lower banana state (LB, â10-6 ), and upper banana state (UB, â10-8 ) are presented from top to bottom. The histograms consist of four components: VLFN (very low frequency noise in banana states), the peaked noise component, low-frequency QPOs fit by Lorentzians (l , h ), and high-frequency QPOs (hHz ). (A color version of this figure is available in the online journal.)

CompTB normalization and, consequently, to the mass accretion rate (taking into account the fact that these objects' distances to the Earth are similar; see Table 5). For 4U 1820-30, the distance range is within 5.8­8 kpc (Shaposhnikov & Titarchuk 2004), whereas for 4U 1728-34 and GX 3+1 the distances are estimated as 4.5 kpc and 4.2­6.4 kpc, respectively (see van Paradijs 1978; Kuulkers & van der Klis 2000). We show the CompTB normalization (related to the soft photon luminosity value) for these sources as a function of 18

kTe in Figure 5. 4U 1820-30 subtends a wider interval in CompTB normalization than that of 4U 1728-34. Note that in the high-luminosity state (or NCOMPTB ) 4U 1820-30 is simi2 lar to GX 3+1: NCOMPTB = (4­15) â L39 /D10 , the Comptonized fraction f = 0.2­0.8 (see Figure 16), and the electron temperatures kTe vary little around 3 keV. While in the low-luminosity state, 4U 1820-30 is closer to source 4U 1728-34: NCOMPTB = 2 (2­5) â L39 /D10 (see Figure 5), f = 0.5­0.8, and kTe changes from 5 to 20 keV (see Figure 16).


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Figure 15. From top to bottom: plots vs. CompTB normalization (left column) and vs. Comptonized fraction f (right column) for GX 3+1 (top), 4U 1820-30 (middle), and 4U 1728-34 (bottom) obtained using the wabs (Blackbody + CompTB + Gaussian) model. In the top panels, crimson and blue points are for GX 3+1 taken from ST12 and in the middle panels for 4U 1820-30 red and blue points correspond to RXTE and BeppoSAX data, respectively (current study). In the bottom panels, blue and green points correspond to RXTE and BeppoSAX data, respectively, for 4U 1728-34 (data taken from ST11). (A color version of this figure is available in the online journal.)

6.3.4. The Differences and Similarities of the Timescales of State Evolution for 4U 1820-30, GX 3+1, and 4U 1728-34

We should point out that all three of these atoll sources show transitions between low-luminosity and high-luminosity states over different timescales. Specifically, the timescales of X-ray flux variability for 4U 1728-34, 4U 1820-30, and GX 3+1 which are probably dictated by variability of mass accretion rate are 10 days, 100 days, and 1000 days, respectively. 19

However, these sources demonstrate an LB­UB transition and make this transition in the narrow interval of the low temperature kTe (around 3 keV) and during the same short time interval (hours­day). We remind the reader that the comparison between these three sources is facilitated by the fact that they show almost the same kTBB and kTs temperature values and they are located at approximately the same distance. The only difference in spectral evolution of these objects is related to their different ranges in electron temperature of the Comptonized component.


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Figure 16. kTe (in keV) plotted vs. illumination (comptonized) fraction f for 4U 1820-30, GX 3+1 (taken from ST12), and 4U 1728-34 (taken from ST11) during spectral state transitions obtained using the wabs (Blackbody + CompTB + Gaussian) model. Red, green, and blue points correspond to RXTE observations of 4U 1820-30, GX 3+1, and 4U 1728-34, respectively. The curved arrows are related to an increase in mass accretion rate. On the right-hand side of the figure, we show a sequence of CCD states (EIS: extreme island state; IS: island state; LLB: lower left banana state; LB: lower banana state; UB: upper banana state) which are listed according to the standard atoll-Z scheme (Hasinger & van der Klis 1989). One can see that kTe is directly related to the sequence of CCD stages. (A color version of this figure is available in the online journal.)

6.3.5. Correlation of Illumination Parameter f versus Electron Temperature kTe and Its Relation with Different Stages in the Color­Color Diagram

Using Table 5, one can see that the ranges of the bestfit illumination fraction f are 0.2­1.0, 0.2­0.9, and 0.5­1 for 4U 1820-30, GX 3+1, and 4U 1728-34, respectively. These values of f indicate a different geometry of the TL and thus different illumination for these X-ray sources. In Figure 16, we demonstrate that the electron temperature kTe directly correlates with a sequence of CCD states, EIS­IS­LLB­LB­UB (see Hasinger & van der Klis 1989 for this CCD classification). Note that ST12 reveal a relation between spectral states, kTe , and f for atoll sources GX 3+1 and 4U 1728-34. We show these kTe ­f relations for these two atolls in Figure 16. The direction in which the inferred M increases is indicated by arrows. Now we present three different tracks on the kTe ­f diagram for the three sources 4U 1820-30, GX 3+1, and 4U 1728-34 and show how these tracks are related to the standard CCD sequence (see Figure 16). The track of 4U 1820-30 consists of three segments (branches) related to kTe : high-(12­21 keV), intermediate-(7­12 keV), and low-temperature (2.9­6 keV) ones, wherein each segment has a negative correlation of kTe and f. GX 3+1 demonstrates only the so-called lowtemperature branch track. Namely, when the fraction f increases, kTe decreases from 4.5 keV to 2.3 keV. For 4U 1728-34, we see a more complicated pattern. In contrast to 4U 1820-30 and GX 3+1, 4U 1728­34 has a segment with a positive correlation of kTe versus f from 4 to 12 keV. Specifically, at the high-temperature state (EIS), f changes only slightly from 0.9 to 1 when kTe decreases. As kTe further drops from 12 keV to 4 keV, f also drops from 0.9 to 0.5. Finally, f goes up from 0.5 to 0.8 when the source enters the lowtemperature state (LB­UB). As a result, we demonstrate that 20

Figure 17. Spectral hardness (10­50 keV/3­10 keV) vs. flux in the 3­60 keV range for 4U 1820-30 (red, current study), 4U 1728-34 (blue, taken from ST11), and GX 3+1 (green, taken from ST12). (A color version of this figure is available in the online journal.)

the CCD state evolution can also be seen using the kTe ­f correlation. 6.4. Comparison of Spectral Hardness Diagrams for atolls 4U 1820-30, GX 3+1, and 4U 1728-34 We use the plot HC (10­50 keV/3­50 keV) versus the 3­60 keV flux in the form of HIDs for the three sources: 4U 1820-30 (red), GX 3+1 (green), and 4U 1728-34 (blue, see Figure 17) to compare transition properties of these atolls in terms of their flux (or luminosity). In fact, 4U 1820-30 shows


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Figure 18. Examples of diagrams of the photon index vs. the COMPTB normalization (proportional to mass accretion rate) for BH sources (right column, GRS 1915+105 (TS09), GRO J1655-40 (ST08), and GX 339-4 (ST08)] along with atoll NS sources (left column, 4U 1820-30, 4U 1728-34 (ST11), and GX 3+1 (ST12)). For all plots, the RXTE data were used along with BeppoSAX data (indicated by blue points in the left column). One can see a noticeable change in followed by the saturation plateau for BHs while for NSs the index only slightly varies around 2. = 2 is indicated by the blue dashed line. (A color version of this figure is available in the online journal.)

a "J'-like inclined (or diagonal) shape in the HID with short upper and elongated lower branches (see also Figure 6). The short branch is close to the low-luminosity state, whereas the elongated branch covers a wide luminosity range. Our comparative analysis of the HID track branches for 4U 1820-30, GX 3+1, and 4U 1728-34 indicates that these objects have similar physical properties. The spectral and timing characteristics are very similar along corresponding segments. 21

Specifically, the short branch of a "J'-like track of 4U 1820-30 is adjacent to a low-luminosity area of 4U 1728-34 (blue points) and it is related to a high electron temperature regime of 4U 1820-30, as in 4U 1728-34. In turn, the elongated branch of 4U 1820-30 is closer to the GX 3+1 (green points) branch area and it is associated with low electron temperatures kTe (3­4 keV) and softer spectra, which are also seen in GX 3+1.


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Note that among the considered atolls, superbursts are observed only in GX 3+1 and 4U 1820-30 during the elongated branch. Furthermore, superbursts are detected at low­soft states, i.e., during the low-luminosity interval of a light curve when the electron temperature kTe is about 4 keV. Thus, 4U 1820-30 shows a property similar to GX 3+1 and it is situated at an intermediate position between 4U 1728-34 and GX 3+1 in terms of its luminosity. This observational fact can be related to the same intermediate rate of mass transfer in these two sources, 4U 1820-30 and GX 3+1. The comparison of HIDs allows us to diagnose physical properties of different objects with adjacent HID tracks. 7. DISCUSSION 7.1. Stability of the Photon Index is a Signature of an NS Source Thus, we demonstrate that the photon spectral index only slightly varies around 2 using numerous observations of NS sources 4U 1820-30, GX 3+1 (ST12), and 4U 1728-34 (ST11) by BeppoSAX and RXTE. In Figures 11, 15, and 18 (left column) we show as a function of the spectral model parameters: kTe (in keV), NCOMPTB -normalization, and illumination fraction f. These results for NS 4U 1820-30 were obtained when we applied our thermal Comptonization model to BeppoSAX and extensive RXTE observations. FT11 and ST11 performed similar investigations and found the photon (energy) index to be stable in other observations of NS binaries. We explain this index stability using the Comptonization model. Namely, the photon (energy) index is almost constant when the soft photon flux-illuminated TL is much less than the gravitational energy release in the TL (see, e.g., ST12). This model of the index stability can probably resolve the index stability effect now clearly established in these three NS sources using extensive BeppoSAX and RXTE observations. 7.2. On the Hard Tail Origin in atoll Source 4U 1820-30 The radio emission detected from 4U 1820­30 (Migliari et al. 2004) suggests the presence of a jet, which may also generate extended power-law X-ray emission. In this case, the power law can be a result of the inverse Compton effect on nonthermal electrons of the jet. Note, X-ray nonthermal power-law tails are also observed in soft states of BHs (see, for example, a review by ST09; see also McConnell et al. 2002 and Wardzi'nski et al. 2002 on the detection of the extended hard tails in the hard states of BHs, Cyg X-1, and GX 339-4 respectively, and NS Z-sources; see Di Salvo et al. 2000; Farinelli et al. 2005; D'Amico et al. 2001; Asai et al. 1994). However, these extended hard tails are also found in atolls (see, e.g., Piraino et al. 1999). Additive models that have been applied to fit the spectra of 4U 1820-30 need to use an additional power-law component (a pure one or a component of CompPS) to describe a hard spectral tail above 80 keV (see, e.g., Tarana et al. 2006). However, such an approach invokes an unknown non-thermal origin of hard tail emission. On the other hand, our suggestion allows us to explain X-ray spectra of 4U1820-30 in all spectral states using the same model without a specific composition of the model components at different states. In fact, in our model (see Sections 5.2 and 6.1), we describe the hard tail emission using the thermal Comptonization component in which the TL electron temperature kTe increases up to 20 keV and the illumination factor f decreases as the source goes to the hard state (see Figures 4 and 16). 22

8. CONCLUSIONS We analyze the X-ray spectral and timing characteristics of 4U 1820-30 observed during the hard­soft state transitions. We find a number of spectral transitions in 4U 1820-30 using BeppoSAX and RXTE data. For our investigation, we take advantage of the BeppoSAX broad spectral extension over the 0.3­200 keV range and abundant RXTE observations taken with 3­200 keV energy coverage. We demonstrate that the X-ray broadband spectra can be successfully fit by a combination of a Blackbody, Comptonization (CompTB), and Gaussian-line components for all spectral states. Also, we show an observable relation of the photon index and the normalization of the Comptonized component, CompTB, which is proportional to M . We demonstrate the stability of the photon index 2 when the source goes from the hard state to the soft state, in other words, when the electron temperature of the Comptonized region (TL), kTe decreases from21to3keV (seeFigure 11). We also show that only slightly varies with the CompTB normalization (M ). Note, this stability of the index in NS sources has been recently suggested for a number of other NSs, Sco X-1, Cyg X-2, GX 17+2, GX 3+1, GX 340+0, GX 349+2, X 1658-298, 1E 1724-3045, and GS 1826-238, which were analyzed using BeppoSAX data (see details in FT11; ST11; ST12). The use of the disk seed photon normalization, (CompTB), which is proportional to M , is fundamental in order to find the stability of during the hard­soft state transition. We do find stability (constancy) of the photon index of the Comptonized component versus both the CompTB normalization and the electron temperature kTe at about 2 for all spectral states. In our analysis of NS sources (see FT11; ST11; ST12; Seifina et al. 2013, and this paper) we do not find any particular case in which the photon index changes beyond the limits 2 ± 0.1. Thus, this index stability can be taken as an intrinsic property of NS binaries (as an NS signature), which is drastically different from that of BH binaries (e.g., GX 339-4, GRO J1655-40, XTE J1650-500, XTE J1550-564, 4U 154347, XTE J1859+226, H 1743-322, (ST09), GRS 1915+105 (TS09), SS 433 (ST10)), where monotonically rises during the hard­soft state transition and is followed by a saturation at high M -values (see Figure 18). In Figure 18, we show the ­M correlation for a number of BHs (right column) and that is almost independent of M in NSs (left column). Indices in BHs show a clear correlation with M or with the normalization 2 L39 /D10 (where L39 is the flux of soft (seed) photons). The ­M correlation is followed by -saturation when the mass accretion rate M exceeds the Eddington limit. The behavior of for the considered sample of NSs (4U 1820-30, versus M 4U 1728-34, and GX 3+1) is drastically different from that for given examples of BHs. The relatively wide interval of the illumination fraction f = 0.2­1 that we obtain in the framework of our model points to variable soft (disk) photon illumination of the TL in 4U 1820-30. Using BeppoSAX data, we also find two types of BB photons. One type is characterized by a color temperature of 0.7 keV, which is typical for the disk photons, and the other one has a color temperature of 1.3 keV, which can be associated with NS surface temperatures. We detect an evolution of 6­20 Hz QPOs and noise components during the island­banana state evolution (LLB­UB, see Figure 13).


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