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The Astrophysical Journal, 747:99 (15pp), 2012 March 10
C

doi:10.1088/0004-637X/747/2/99

2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

GX 3+1: THE STABILITY OF SPECTRAL INDEX AS A FUNCTION OF MASS ACCRETION RATE
1

Elena Seifina1 and Lev Titarchuk2

,3 ,4

Sternberg Astronomical Institute, Moscow State University, Universitetsky Prospect 13, Moscow 119992, Russia; seif@sai.msu.ru 2 Dipartimento di Fisica, Universita di Ferrara, Via Saragat 1, I-44100 Ferrara, Italy; titarchuk@fe.infn.it ` 3 School of Physics, Astronomy, & Computational Sciences (SPACS), George Mason University, Fairfax, VA 22030, USA 4 Goddard Space Flight Center, NASA, Code 663, Greenbelt, MD 20770, USA; lev@milkyway.gsfc.nasa.gov Received 2011 August 10; accepted 2011 December 27; published 2012 February 21

ABSTRACT We present an analysis of the spectral and timing properties observed in X-rays from neutron star (NS) binary GX 3+1 (4U 1744-26) during long-term transitions between the faint and bright phases superimposed on short-term transitions between lower banana (LB) and upper banana (UB) branches in terms of its colorcolor diagram. We analyze all observations of this source obtained with the Rossi X-ray Timing Explorer and BeppoSAX satellites. We find that the X-ray broadband energy spectra during these spectral transitions can be adequately reproduced by a composition of a low-temperature blackbody component, a Comptonized component (COMPTB), and a Gaussian component. We argue that the electron temperature kTe of the Compton cloud monotonically increases from 2.3 keV to 4.5 keV, when GX 3+1 makes a transition from UB to LB. We also detect an evolution of noise components (a very low frequency noise and a high-frequency noise) during these LBUB transitions. Using a disk seed photon normalization of COMPTB, which is proportional to the mass accretion rate, we find that the photon power-law index is almost constant ( = 2.00 ± 0.02) when mass accretion rate changes by a factor of four. In addition, we find that the emergent spectrum is dominated by the strong Comptonized component. We interpret this quasi-stability of the index and a particular form of the spectrum in the framework of a model in which the energy release in the transition layer located between the accretion disk and NS surface dominates that in the disk. Moreover, this index stability effect now established for GX 3+1 was previously found in the atoll source 4U 1728-34 and suggested for a number of other low-mass X-ray NS binaries (see Farinelli & Titarchuk). This intrinsic behavior of NSs, in particular for atoll sources, is fundamentally different from that seen in black hole binary sources where the index monotonically increases during spectral transition from the low state to the high state and then finally saturates at high values of mass accretion rate. Key words: accretion, accretion disks black hole physics radiation mechanisms: non-thermal stars: individual (GX 3+1) stars: neutron Online-only material: color figures

1. INTRODUCTION Low-mass X-ray binaries (LMXBs) hosting a neutron star (NS) show a variety of spectral states and transitions between them. In this regard, the so-called atoll sources (see, e.g., Hasinger & van der Klis 1989; van der Klis 2005) are particularly interesting because they demonstrate a wide range of luminosities (most of them show from 0.01 to 0.5 of the Eddington limit LEdd ). It is worth noting that the name of atoll sources is associated with the shape traced in the colorcolor diagram (CD). This shape can be divided into two main regions, corresponding to two X-ray states of the source: the harder one is related to the island state (IS), and the softer one is related to the banana (B) state. These spectra of NS sources can be described by blackbody (BB) components, with color temperatures kTBB < 1 keV and kTs > 1 keV that are presumably related to the accretion disk and NS surface, respectively. In addition, there is a thermal Comptonization component with electron temperature kTe = 2.315 keV that is probably related to the transition layer (TL) located between the disk and NS (see Paizis et al. 2006; Farinelli & Titarchuk 2011; Seifina & Titarchuk 2011, hereafter FT11 and ST11, respectively). An analysis of X-ray power spectra of atoll sources indicates a tight relation between timing properties and the position on the CD, suggesting that the source timing and spectral properties are well determined by basic parameters such as mass accretion rate (see, e.g., Di Salvo et al. 2001). 1

GX 3+1 is one of the brightest atoll sources associated with a bulge component of our Galaxy. GX 3+1 along with GX 9+9, GX 9+1, and GX 13+1 form the subclass of persistently bright atoll sources, which are always in the banana state (see Hasinger & van der Klis 1989). In particular, two-branch structures have been observed in the CD and hardness-intensity diagram (HID) of GX 3+1 (Stella et al. 1985; Lewin et al. 1987; Schultz et al. 1989; Homan et al. 1998; Muno et al. 2002; Schnerr et al. 2003). Specifically, their tracks in the X-ray CD are long, diagonal, and slightly curved, while their fast timing properties are dominated only by a relatively weak (1%4% rms) power-lawshaped noise component. These aforementioned atoll sources are intermediate in terms of luminosity that changes in the range 0.10.5 of LEdd (see Christian & Swank 1997; Ford & van der Klis 2000). In contrast to other atoll sources and Z sources, these bright atoll sources have so far not shown kHz quasi-periodic oscillations (QPOs) (see Wijnands et al. 1998; Strohmayer 1998; Homan et al. 1998; Oosterbroek et al. 2001; Schnerr et al. 2003), which can be attributed to the fact that these objects do not reach the left lower banana (LLB), where other atoll sources exhibit kHz QPOs (van der Klis 2000). For example, the weaker atoll sources, such as, e.g., 4U 1608-52, 4U 0614+09, or 4U 1728-34, show kHz QPOs in LLB. X-ray data of GX 3+1 (4U 1744-26) obtained in broad energy ranges using BeppoSAX (0.1200 keV) and Rossi X-Ray Timing Explorer (RXTE) (3200 keV) offer a unique

The Astrophysical Journal, 747:99 (15pp), 2012 March 10

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possibility to further investigate, in detail, the evolution of X-ray spectral properties during transition events. This bright atoll source shows long-term transitions from the fainter phase to its brighter phase in X-rays and vice versa when the corresponding luminosity changes, at least, by a factor of four, while on timescales of hours GX 3+1 demonstrates low flux variabilities as transitions between LB and upper banana (UB) states. Naturally one can pose a fair question: what is the physical mechanism responsible for the source emission during these luminosity changes and particularly how the spectral index changes during these transitions? GX 3+1 was discovered during an Aer obee-rocket flight on 1964 June 16 (Bowyer et al. 1965). Subsequently, this source was observed many times during various observational campaigns. Detailed long-term monitoring observations of GX 3+1 were performed by the all-sky monitor (ASM) on GINGA (see Asai et al. 1993), EXOSAT (see Schultz et al. 1989), RXTE (see Bradt et al. 1993; Kuulkers & van der Klis 2000), Wide-Field Camera (WFC) of BeppoSAX (see den Hartog et al. 2003), and INTEGRAL (see Paizis et al. 2006). In particular, den Hartog et al. (2003) found three types of variability: short-term variation (of order of seconds), mild variability on a daily (hourly) timescale, and slow sinusoidal-like variation on a timescale of years. However, it is surprising that the hardness ratio, which can be a measure of the spectral shape, stays almost constant during these observations. Although an optical counterpart has not yet been identified (e.g., Naylor et al. 1991), GX 3+1 is presumably a low-mass X-ray binary in which a NS is accompanied by a low-mass star of spectral type A or a later. During an active stage, the companion overflows its Roche lobe and transfers matter onto the NS via an accretion disk. This process is possibly accompanied by nuclear burning of a helium or hydrogen layer of the NS surface as a result of the matter accumulation on the NS surface (Hanson & van Horn 1975). Unstable fusion occurs, leading to thermonuclear flashes that can be observable in the form of so-called type-I X-ray bursts (Woosley & Taam 1976). GX 3+1 does exhibit fast variability in the form of type-I X-ray bursts, which were extensively studied by a number of X-ray missions: GINGA (Asai et al. 1993), Gr anat (Pavlinsky et al. 1994; Molkov et al. 1999), RXTE (Kuulkers & van der Klis 2000; Kuulkers 2002), and INTEGRAL (Chenevez et al. 2006). A unique superburst with a decay time of 1.6 hr was detected with the ASM on RXTE (Kuulkers 2002). One of the shortest bursts ever seen exhibits a quick (i.e., less than 2 s) radius expansion phase, indicating that the burst luminosity was at the Eddington luminosity, causing the NS atmosphere to expand owing to radiation pressure. This implies that a distance to the source is about 4.5 kpc, assuming that the NS atmosphere is hydrogen-rich (see more details in Kuulkers & van der Klis 2000). In this paper, we concentrate our efforts on the spectral and timing properties of GX 3+1 related to changes in mass accretion rate, which are seen as the mild and slow variabilities. Previously, an analysis of the burst properties of GX 3+1 as a function of mass accretion rate on timescales larger than 1 minute was presented by Asai et al. (1993) and den Hartog et al. (2003). In particular, the slow variability has been revealed during transitions from the faint phase to the bright phase of luminosity and is generally caused by significant increase of mass accretion rate. The mild variability is presumably related to moderate local variability of mass accretion rate and revealed as local transitions between LB and UB states. The slow variability 2

has been investigated with the RXTE/ASM light curve, WFC of BeppoSAX (den Hartog et al. 2003), and the ASM on GINGA (Asai et al. 1993), and these observations indicate that the flux oscillates semi-sinusoidally with a period of 67 years (see Figure 1). Moreover, the GINGA measurements with its Large Area Detector (Asai et al. 1993) show a constant 120 keV spectral shape over the fainter phases and also during brighter phases on luminosity, suggesting a constancy of the spectral index. This stability of the X-ray spectral shape over the bright and faint phases has also been confirmed by the WFC BeppoSAX measurements in the 228 keV band, which showed minimal spectral changes (den Hartog et al. 2003). The stability of the index was noted previously by FT11, for a number of NS LMXBs at different luminosities. They collected X-ray spectra obtained by BeppoSAX and demonstrated the relative stability of spectral index of approximately 1 ( = + 1) for quite a few NS sources: X1658-298, GX 354-0, GS 1826-238, 1E 1724-3045, Cyg X-1, Sco X-1, GX 17+2, and GX 349+2 at different spectral states. Recently ST11 presented results of analysis of X-ray spectra for "atoll" source 4U 172834, detected with BeppoSAX and RTXE at different luminosities and values of the best-fit electron temperatures. These results indicate that the value of the index varies slightly about a value of 1 (or the photon index varies around 2) independently of the electron temperature of the Compton cloud (CC) kTe and luminosity. This unique stability of the photon index may be an intrinsic property of NSs at least for the atoll sources. It is probably determined by common physical conditions for this class of sources. FT11 and ST11 interpreted this quasistability of the index in the framework of a model in which the spectrum is dominated by a strong thermal Comptonized component formed in the TL located between the accretion disk and NS surface. Indeed, the index quasi-stability takes place when the energy release in the TL is much higher than the photon energy flux coming from the accretion disk and illuminating the TL. The evolution of spectral parameters of compact objects in X-ray binaries is of great interest for understanding their nature. It is well known that many black hole (BH) candidate binaries exhibit correlations between mass accretion rate M and photon power-law index (see Shaposhnikov & Titarchuk 2009 and Titarchuk & Seifina 2009, hereafter ST09 and TS09, respec tively). In the soft states of BHs these index-M correlations almost always show a saturation of at high values of the M . This saturation effect can be considered as a BH signature or equivalently as a signature of a converging flow into a BH (ST09 and TS09). In this paper, we present the analysis of the BeppoSAX available observations and RXTE/PCA observations during 19962010 for GX 3+1. In Section 2, we present the list of observations used in our data analysis, while in Section 3, we provide the details of X-ray spectral analysis. We analyze the evolution of X-ray spectral and timing properties during the state transition in Section 4. We make our conclusions in Section 5. 2. DATA SELECTION Broadband energy spectra of the source were obtained combining data from three BeppoSAX Narrow Field Instruments (NFIs): the Low Energy Concentrator Spectrometer (LECS) for 0.34 keV (Parmar et al. 1997), the Medium Energy Concentrator Spectrometer (MECS) for 1.810 keV (Boella

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Seifina & Titarchuk

- RXTE - BeppoSAX
R1 R2 R3 R4 R5 R6

GX 3+1 (4U 1744-26)
R7

Figure 1. Evolution of ASM/RXTE count rate during 19962010 observations of GX 3+1. Blue vertical strips (on the top of the panel) indicate time for the RXTE pointed observations. Whereas bright blue rectangles are related to the RXTE data sets listed in Table 1, green triangles show BeppoSAX NFI data listed in Table 2. (A color version of this figure is available in the online journal.) Table 1 The List of Bepp oSAX Observations of GX 3+1 Used in Our Analysis Obs. ID 20603001 20835001 Start Time (UT) 1999 Feb 28 11:02:15 1999 Aug 30 18:33:08 End Time (UT) 1999 Feb. 30 09:14:15 1999 Aug. 31 04:54:32 MJD Interval 51237.451238.9 51420.851421.91 CD State Upper banana Lower banana

Reference. (1) Oosterbroek et al. 2001.

et al. 1997), and the Phoswich Detection System (PhDS) for 1560 keV (Frontera et al. 1997). The SAXDAS data analysis package is used for processing data. For each of the instruments we performed the spectral analysis in the energy range for which the response matrix is well determined. The LECS data have been re-normalized based on MECS. Relative normalization of the NFIs was treated as free parameters in model fitting, except for the MECS normalization that was fixed at a value of 1. We then checked this fitting procedure if these normalizations were in a standard range for each instrument.5 In addition, spectra are rebinned accordingly to energy resolution of the instruments in order to obtain significant data points. We rebinned the LECS spectra with a binning factor that is not constant over energy (Section 3.1.6 of Cookbook for the BeppoSAX NFI spectral analysis) using template files in GRPPHA of XSPEC.6 Also we rebinned the PhDS spectra with a linear binning factor of two, grouping two bins together (resulting bin width is 1 keV). Systematic error of 1% has been applied to these analyzed spectra. In Table 1, we listed the BeppoSAX observations used in our analysis. We have analyzed the available data obtained with RXTE (Bradt et al. 1993), which have been found in the time period from 1996 October to 2010 September (see also a review by Galloway et al. 2008). In our investigation we selected 101 observations made at different count rates (luminosity states) with a good coverage of risedecay transition tracks. We have made an analysis of RXTE observations of GX 3+1 during 14 years for seven intervals indicated by blue rectangles in Figure 1 (top). RXTE/PCA spectra have been extracted and analyzed, wherein PCA Standard 2 mode data, collected in the 350 keV
5 6

energy range, use the most recent release of PCA response calibration (ftool pcarmf v11.1). The relevant dead-time corrections to energy spectra have been applied. We used the data that are available through the GSFC public archive (http://heasarc.gsfc.nasa.gov). In Table 2, we presented the groups of RXTE observations that cover the source evolution from faint to bright (phase) events. Note that available RXTE data contain one "br ig ht phase" set (R 4) and six "faint phase" sets (R 1R 3, R 5R 7). The PCA energy spectra were modeled using XSPEC astrophysical fitting software. A systematic error of 0.5% has been applied to the analyzed spectra. We have also used public GX 3+1 data from the ASM on board RXTE, which show long-term quasi-periodic variability of the mean soft flux during a 6 year cycle (Figure 1). We use definitions of the fainter and brighter luminosity phases to relate these phases to the source luminosity, and we demonstrate that during the bright/faint phase transition of GX 3+1 COMPTB normalization changes from 0.04 to 0.14 Lsoft /D 2 10 , where Lsoft 39 39 is the soft photon luminosity in units of 1039 erg s-1 and D10 is distance to the source in units of 10 kpc. 3. SPECTRAL ANALYSIS In our spectral data analysis, we use a model that consists of a sum of a Comptonization component (COMPTB) (COMPTB is an XSPEC contributed model,7 see Farinelli et al. 2008, hereafter F08), soft BB component of temperature TBB , and Gaussian line component. The COMPTB spectral component has the following parameters: temperature of the seed photons Ts , energy index of the Comptonization spectrum (= - 1), electron temperature Te , illumination (Comptonization) fraction
7

http://heasarc.nasa.gov/docs/sax/abc/saxabc/saxabc.html http://heasarc.gsfc.nasa.gov/FTP/sax/cal/responses/grouping

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

3

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Blackbody-like emission of NS (kTs>1 keV) Disk blackbody emission (kTBB<1 keV) Comptonized corona emission (kTe ~ 2-4 keV)

Transition layer

Neutron star

Accretion disk

Figure 2. Suggested geometry of GX 3+1. Disk and neutron star soft photons are upscattered off hotter plasma of the transition layer located between the accretion disk and NS surface. Some fraction of these photons are seen directly by the Earth observer. Red and blue photon trajectories correspond to soft and hard (upscattered) photons, respectively. (A color version of this figure is available in the online journal.) Table 2 The List of RXTE Observation Groups of GX 3+1 Number of Set R1 R2 R3 R4 R5 R6 R7 Dates, MJD 50365 50962-51118 51324-51445 51998-52554 52881 53108-53280 55440.6-55440.8 RXTE Proposal ID 10069 30042, 30048 40023, 404251 60022 80105 90022 94307 1996 1998 1999 2001 2003 2004 2010 Dates UT Oct 9 03:36:0004:08:00 May 29Nov 1 May 26Sept 24 March 302002 Oct 7 Aug 30 03:53:3606:29:13 Apr 13Oct 2 Sept 1 15:00:3221:07:58 Rem. Phase on Luminosity faint faint faint bright faint faint faint

Bepp oSAX

Reference. (1) Oosterbroek et al. 2001.

f of the CC by the soft (NS) photons, f = A/(1 + A), and the normalization of the seed (NS) photon spectrum NCOM . We include a simple Gaussian component in the model, whose parameters are a centroid line energy Eline , the width of the line line , and the normalization Nline to fit the data in the 68 keV energy range. We also use the interstellar absorption with a column density NH . It should be noted that we fixed certain parameters of the COMPTB component: = 3 (lowenergy index of the seed photon spectrum) and = 0 because we neglect an efficiency of the bulk inflow effect versus the thermal Comptonization for NS GX 3+1. We apply a value of hydrogen column NH = 1.6 в 1022 cm-2 , which was found by Oosterbroek et al. (2001). Initially, we have tried a model consisting of an absorbed thermal component (bbod y ) and a thermal Comptonization component (COMPTB), but this model gave a poor description of data. Significant positive residuals around 6.5 keV suggest the presence of fluorescent iron emission line. The addition of a 4

Gaussian line component at 6.4 keV considerably improves fit 2 quality and provides a statistically acceptable red . The fluorescent iron emission line in GX 3+1 was detected for the first time by Oosterbroek et al. (2001)using BeppoSAX on 1999 August 30 (id=20835001). Oosterbroek et al. (2001) successfully described this emission feature with the Gaussian line model and used a model consisting of a thermal component (dominating energy release around 1 keV) and a thermal Comptonization tail to describe the 0.150 keV continuum. However, they needed to add a 2% systematical un2 certainty to LECS and MECS data to obtain acceptable red . We investigate a possibility to further improve the quality of the fit. In Figure 2, we illustrate our spectral model as a basic model for fitting the BeppoSAX and RXTE spectral data for GX 3+1. We assume that accretion onto a NS takes place when the material passes through the two main regions: a geometrically thin accretion disk (the standard ShakuraSunyaev disk, see

The Astrophysical Journal, 747:99 (15pp), 2012 March 10 Table 3 Best-fit Parameters of Spectral Analysis of Bepp oSAX Observations of GX 3+1 in 0.350 keV Energy Rangea Observational ID 20603001 20835001 MJD (day) 51237.5 51420.8 TBB (keV) 0.47(3) 0.62(5) NBB
b

Seifina & Titarchuk

Ts (keV) 1.30(3) 1.21(5)

= -1 0.99(7) 1.07(4)

Te (keV) 3.68(5) 2.4(2)

log(A) 0.09(4) -0.32(8)

NCOM

b

Eline (keV) 7.4(1) 6.0(1)

Nline

b

2 red

(dof)

2.65(2) 1.61(1)

4.18(3) 3.56(2)

0.55(4) 0.43(4)

1.08(457) 1.16(445)

Notes. Parameter errors correspond to 1 confidence level. a The spectral model is wabs (blackbody + COMPTB + Gaussian). b Normalization parameters of bl a ck bod y and COMPTB components are in units of Lsoft /d 2 , where Lsoft is the soft photon luminosity in units of 1037 erg s-1 , d 10 37 37 is the distance to the source in units of 10 kpc, and the Gaussian component is in units of 10-2 в total photons cm-2 s-1 in line.

10

GX 3+1 GX 3+1

B eppoS A X

GX 3+1 GX 3+1

R X TE

Figure 3. Top: the best-fit spectrum of GX 3+1 in E F (E ) units using BeppoSAX observation 20603001 carried out on 1999 February 28March 1. The data are presented by crosses and the best-fit spectral model wabs*(blackbody+COMPTB+Gaussian) by a green line. The model components are shown by blue, red, and crimson lines for blackbdody, COMPTB, and Gaussian components, respectively. Bottom: vs. photon energy in keV. The best-fit model parameters are = 1.99 ± 0.07, Te = 3.68 ± 0.05 keV, and Eline = 7.4 ± 0.1 keV (reduced 2 = 1.08 for 457 dof; see more details in Table 3). (A color version of this figure is available in the online journal.)

Figure 4. Top: a typical E F (E ) spectral diagram of GX 3+1 for PCA/RXTE observation 94307-05-01-000 on 2010 September 1 (MJD 55431.62) during the faint phase (green line). The spectral model, presented by its components, is shown by dashed red, blue, and purple lines for COMPTB, bl a ck bod y , and Gaussian components, respectively. Bottom: vs. photon energy in keV. The best-fit model parameters are = 2.04 ± 0.04, kTe = 3.16 ± 0.05 keV, and Eline = 6.59 ± 0.05 keV (reduced 2 = 0.87 for 67 dof; see more details in Table 4). (A color version of this figure is available in the online journal.)

Shakura & Sunyaev 1973) and the TL, where NS and disk soft photons are upscattered off hot electrons. In other words, in our picture, the emergent thermal Comptonization spectrum is formed in the TL, where thermal disk seed photons and soft photons from the NS are upscattered off the relatively hot plasma (electrons). Some fraction of these seed soft photons can also be seen directly. Red and blue photon trajectories shown in Figure 2 correspond to soft (seed) and hard (upscattered) photons, respectively. We show examples of X-ray spectra in Figures 3 and 4 for BeppoSAX and RXTE data, respectively. Spectral analysis of BeppoSAX and RXTE observations indicates that X-ray spectra of GX 3+1 can be described by a model with a Comptonization component represented by the COMPTB model. Moreover, for broadband BeppoSAX observations this spectral model component is modified by photoelectric absorption at low energies. On the top of Figure 3 we demonstrate the best-fit BeppoSAX spectrum of GX 3+1 in units of E F (E ) (top) (where F (E ) 5

is energy flux in erg, per keV and per second) using our model for the BeppoSAX observation (id=20603001) carried out on 1999 February 28March 1. The data are presented by crosses and the best-fit spectral model wabs*(blackbody + COMPTB + Gaussian) by a green line. The model components are shown by blue, red, and crimson lines for blackbdody, COMPTB, and Gaussian components, respectively. On the bottom we show versus photon energy in keV. The best-fit model parameters are = 1.99 ± 0.07, kTe = 3.68 ± 0.05 keV, and Eline = 7.4 ± 0.1 keV (reduced 2 = 1.08 for 457 dof; see more details in Table 3). In particular, we find that an addition of the soft thermal component with temperature kTBB = 0.50.7 keV to the model significantly improves the fit quality of the BeppoSAX spectra. For the BeppoSAX data (see Tables 1 and 3) we find that the spectral index is 1.03 ± 0.04 (or the corresponding photon index = + 1 is 2.03 ± 0.04). Unfortunately, RXTE detectors do not provide well-calibrated spectra below 3 keV, while the broad energy band of BeppoSAX

The Astrophysical Journal, 747:99 (15pp), 2012 March 10 Table 4 The Best-fit Parameters of Spectral Analysis of PCA/RXTE Observations of GX 3+1 in 350 keV Energy Rangea Observational ID 10069-03-01-00 30042-04-01-00 30042-04-02-00 30048-01-01-00 30042-04-03-00 30042-04-03-01 30042-04-04-00 40023-01-01-00 40023-01-01-01 40023-01-01-02 40023-01-03-00 40023-01-02-00 40023-01-02-01 40023-01-02-02 40023-01-02-03 40023-01-02-04 40023-01-02-05 40023-01-02-06 40023-01-02-07 40023-01-04-00 40023-01-03-01 40425-01-01-00 40425-01-01-01 40425-01-01-02 40425-01-01-03 40425-01-01-04 40425-01-01-05 40425-01-01-06 40023-01-05-00 40023-01-03-01 60022-01-01-00 60022-01-01-01 60022-01-01-02 60022-01-01-03 60022-01-02-00 60022-01-02-01 60022-01-01-04 60022-01-01-05 60022-01-01-06 60022-01-03-00 60022-01-03-01 60022-01-03-01 60022-01-04-03 60022-01-04-00 60022-01-04-02 60022-01-01-07 60022-01-03-03 60022-01-04-04 60022-01-03-04 60022-01-04-01 60022-01-05-00 60022-01-05-01 60022-01-06-01 60022-01-06-03 60022-01-06-05 60022-01-06-06 60022-01-06-07 60022-01-06-00 60022-01-06-09 60022-01-06-08 60022-01-06-10 60022-01-06-11 60022-01-06-12 60022-01-06-13 60022-01-07-00 MJD (day) 50365.172 50962.598 50973.668 51011.138 51113.941 51114.207 51118.739 51324.737 51325.310 51325.542 51390.248 51398.243 51398.188 51399.644 51400.131 51400.192 51400.259 51400.325 51400.723 51404.304 51405.307 51420.779 51420.846 51421.018 51421.084 51421.151 51421.218 51421.285 51445.320 51445.599 51998.64 51998.71 51998.78 51999.78 52028.61 52028.94 52029.47 52029.61 52029.69 52067.39 52067.58 52068.30 52102.363 52103.361 52103.685 52127.54 52128.667 52128.734 52129.386 52131.114 52131.507 52131.375 52166.493 52168.754 52169.610 52169.678 52170.534 52170.983 52171.194 52172.176 52172.241 52172.725 52172.792 52172.852 52201.445 = -1 1.00(8) 1.0(1) 1.02(7) 0.99(2) 1.01(2) 1.03(3) 1.1(1) 1.0(3) 1.0(2) 1.(1) 1.0(1) 0.9(2) 0.9(1) 0.9(1) 1.01(6) 1.01(9) 1.0(1) 1.0(1) 1.0(1) 0.99(1) 1.(1) 1.0(1) 1.09(5) 1.05(1) 1.01(1) 1.0(1) 1.08(9) 1.02(8) 0.99(3) 1.0(1) 1.06(3) 1.03(9) 1.01(3) 1.02(7) 1.00(9) 1.01(3) 1.07(3) 1.03(6) 1.05(3) 1.01(4) 1.03(5) 1.07(9) 1.03(9) 1.01(5) 1.0(1) 0.99(3) 0.99(5) 1.0(1) 1.02(6) 1.07(6) 1.0(1) 0.99(7) 1.0(1) 1.00(7) 1.01(4) 1.02(5) 1.03(5) 1.00(5) 1.06(7) 1.04(4) 1.0(1) 0.99(6) 1.00(3) 1.03(5) 1.04(4) Te (keV) 2.40(2) 2.52(1) 2.42(1) 2.44(2) 3.15(2) 3.17(2) 3.52(8) 2.48(2) 2.40(1) 2.46(1) 2.54(1) 2.53(1) 2.50(2) 2.49(8) 2.51(5) 2.4(1) 2.6(1) 2.45(7) 2.45(1) 4.50(4) 2.48(1) 3.05(7) 2.83(9) 2.8(1) 3.29(9) 2.85(8) 2.99(5) 2.69(6) 2.59(9) 2.35(4) 2.49(1) 2.52(2) 2.57(1) 2.47(3) 2.51(6) 2.54(2) 2.52(1) 2.51(2) 2.53(4) 2.53(1) 2.52(3) 3.16(2) 3.25(9) 2.72(5) 2.87(6) 2.53(7) 2.78(5) 2.93(5) 2.72(6) 2.87(6) 2.62(5) 2.89(8) 2.75(3) 2.96(6) 2.65(4) 3.04(6) 2.6(1) 2.65(7) 2.86(5) 2.99(8) 2.66(4) 2.62(9) 3.41(9) 2.83(3) 2.75(5) log(A) 0.03(1) -0.04(3) -0.1(1) 0.18(8) 0.39(5) 0.41(5) 0.45(6) 0.02(1) 0.23(3) -0.01(1) 0.01(1) 0.04(2) 0.02(2) 0.01(1) 0.06(2) 0.06(2) -0.09(5) 0.18(9) 0.11(2) -0.9(1) 0.09(2) -0.5(5) -0.31(7) -0.29(3) -0.29(3) -0.33(6) -0.43(3) -0.19(6) -0.06(4) 0.73(9) 0.04(1) 0.07(4) -0.04(1) 0.04(1) 0.03(1) 0.01(1) 0.06(2) 0.2(1) -0.01(1) 0.21(7) 0.04(1) -0.46(9) -0.57(8) -0.20(8) -0.1(1) 0.24(1) -0.26(5) -0.29(8) -0.17(7) -0.3(1) -0.09(1) -0.36(1) -0.17(2) -0.37(5) 0.08(7) -0.40(3) -0.04(3) -0.15(5) -0.23(4) -0.43(6) -0.25(5) -0.49(7) -0.54(4) -0.20(3) -0.13(3) NCOM
b

Seifina & Titarchuk

Ts (keV) 1.10(8) 1.65(5) 1.17(4) 1.45(8) 1.5(1) 1.5(1) 1.7(5) 1.6(2) 1.6(2) 1.6(2) 1.56(8) 1.4(1) 1.5(1) 1.2(1) 1.2(1) 1.18(3) 1.25(2) 1.25(8) 1.20(8) 1.82(3) 1.20(8) 1.59(8) 1.59(9) 1.59(5) 1.59(8) 1.55(7) 1.49(8) 1.35(7) 1.24(8) 1.16(7) 1.20(5) 1.20(7) 1.2(1) 1.20(3) 1.20(5) 1.20(6) 1.17(9) 1.20(5) 1.20(4) 1.20(7) 1.20(4) 1.20(5) 1.31(7) 1.30(6) 1.20(7) 1.18(4) 1.20(8) 1.20(9) 1.20(4) 1.20(8) 1.30(6) 1.20(4) 1.20(9) 1.2(1) 1.20(5) 1.20(9) 1.20(5) 1.30(6) 1.20(7) 1.20(9) 1.20(5) 1.30(6) 1.20(8) 1.20(5) 1.20(6)

NBbody

b

Eline (keV) 6.53(3) 6.51(2) 6.5(1) 6.42(8) 6.53(1) 6.58(1) 6.51(4) 6.43(9) 6.43(5) 6.45(5) 6.49(7) 6.45(5) 6.45(5) 6.46(9) 6.38(5) 6.45(9) 6.50(7) 6.46(8) 6.45(8) 4.55(5) 6.55(8) 6.5(1) 6.5(1) 6.5(1) 6.5(1) 6.38(2) 6.38(4) 6.38(3) 6.47(8) 6.54(9) 6.24(2) 6.5(1) 6.59(5) 6.55(4) 6.56(2) 6.50(6) 6.50(7) 6.51(6) 6.54(2) 6.51(2) 6.55(8) 6.36(3) 7.07(3) 7.09(4) 7.03(3) 6.58(3) 6.34(5) 7.09(4) 6.42(3) 7.09(5) 5.99(7) 5.98(7) 5.87(7) 5.57(8) 6.17(3) 6.28(2) 6.47(7) 5.87(6) 6.47(8) 6.57(6) 6.43(7) 6.40(3) 6.81(5) 6.17(7) 6.24(8)

line (keV) 0.58(5) 0.5(1) 0.5(1) 0.6(1) 0.50(8) 0.52(7) 0.6(2) 0.67(9) 0.67(8) 0.62(6) 0.60(7) 0.64(6) 0.48(8) 0.69(9) 0.8(1) 0.52(5) 0.46(7) 0.41(6) 0.47(5) 0.50(7) 0.56(9) 0.6(1) 0.62(9) 0.61(8) 0.65(5) 0.58(7) 0.59(4) 0.62(8) 0.46(4) 0.4(1) 0.5(1 0.56(8) 0.58(7) 0.55(7) 0.53(6) 0.59(5) 0.62(7) 0.50(8) 0.53(9) 0.5(1) 0.35(4) 0.41(8) 0.50(9) 0.52(9) 0.50(5) 0.36(8) 0.4(1) 0.38(7) 0.37(8) 0.46(6) 0.47(4) 0.41(8) 0.43(9) 0.39(4) 0.418) 0.38(6) 0.50(8) 0.39(7) 0.37(9) 0.4(1) 0.40(8) 0.38(7) 0.35(8) 0.32(5) 0.35(8)

Nline

b

2 red

(dof)

F1 /F

2

c

10.88(2) 6.3(1) 9.9(1) 6.3(2) 4.00(6) 4.07(6) 3.7(1) 5.98(8) 5.78(6) 5.85(4) 6.20(6) 5.61(5) 5.50(5) 6.93(8) 6.19(7) 5.77(6) 6.72(6) 6.19(5) 6.85(4) 3.99(9) 6.38(6) 6.05(8) 6.85(7) 6.23(2) 5.83(5) 4.77(4) 5.22(5) 4.24(2) 7.38(7) 6.44(7) 12.03(1) 12.97(3) 11.92(3) 12.31(4) 11.50(3) 11.73(2) 13.88(6) 13.36(3) 11.54(4) 13.35(3) 12.65(3) 10.82(4) 9.44(7) 9.7(1) 10.68(9) 13.08(3) 10.23(1) 10.40(7) 9.9(1) 9.36(3) 8.5(1) 8.2(1) 11.19(5) 11.36(6) 11.04(7) 12.17(8) 7.20(4) 9.4(1) 11.1(1) 10.40(5) 10.03(4) 9.34(5) 12.08(7) 11.63(6) 10.13(3)

2.74(5) 3.17(3) 2.6(1) 3.12(8) 3.05(2) 3.07(2) 2.86(3) 2.62(8) 2.64(9) 2.38(8) 2.6(1) 2.68(9) 2.78(7) 2.47(5) 3.16(4) 2.83(5) 2.9(1) 3.04(8) 2.38(7) 3.63(9) 2.32(9) 4.01(6) 4.09(9) 3.74(9) 4.19(6) 3.91(3) 4.11(2) 3.79(3) 2.7(1) 2.32(1) 2.86(2) 2.31(3) 3.26(2) 2.39(4) 2.74(4) 2.51(6) 3.15(2) 2.39(6) 2.84(9) 2.62(4) 2.36(5) 5.41(3) 5.47(4) 6.65(9) 4.01(9) 2.64(9) 6.8(1) 5.8(1) 5.97(9) 4.56(6) 5.8(1) 6.5(1) 5.53(5) 5.52(9) 5.26(8) 5.22(7) 5.33(3) 6.53(7) 5.70(5) 5.39(6) 6.26(8) 6.34(5) 5.24(9) 6.09(9) 5.50(8)

0.95(4) 0.46(2) 0.37(8) 0.7(1) 0.42(1) 0.44(1) 0.47(2) 0.9(1) 0.73(7) 0.73(8) 0.7(1) 0.76(9) 0.61(9) 0.96(8) 1.09(6) 0.61(6) 0.63(9) 0.45(9) 0.45(8) 0.70(6) 0.60(8) 1.05(4) 0.87(9) 0.85(2) 0.86(3) 0.99(2) 1.11(8) 0.87(6) 0.62(4) 0.52(8) 1.50(6) 1.33(5) 1.33(7) 1.57(6) 1.22(5) 1.23(3) 1.61(8) 1.03(5) 1.26(6) 1.22(3) 1.28(3) 1.58(4) 1.64(5) 1.86(5) 1.63(3) 1.14(3) 1.57(3) 1.65(4) 1.83(3) 1.64(4) 1.69(3) 1.72(6) 1.61(3) 1.63(3) 1.65(6) 1.63(8) 1.68(7) 1.64(4) 1.74(5) 1.71(4) 1.68(3) 1.74(2) 1.81(5) 1.77(7) 1.69(3)

1.1(67) 1.50(67) 1.12(67) 0.82(67) 0.87(67) 0.91(67) 1.18(67) 1.35(67) 0.75(67) 1.26(67) 0.94(67) 1.01(67) 1.01(67) 0.53(67) 0.71(67) 0.85(67) 0.77(67) 0.92(67) 0.92(67) 1.70(67) 1.28(67) 0.82(67) 1.05(67) 1.34(67) 1.30(67) 1.15(67) 1.39(67) 1.36(67) 0.84(67) 1.70(67) 1.07(67) 1.09(67) 0.87(67) 1.15(67) 1.13(67) 1.05(67) 0.98(67) 0.88(67) 1.40(67) 1.09(67) 1.30(67) 1.20(67) 1.07(67) 1.24(67) 1.04(67) 1.10(67) 1.04(67) 1.05(67) 1.01(67) 1.15(67) 1.16(67) 1.05(67) 0.96(67) 1.16(67) 1.06(67) 1.11(67) 1.12(67) 1.1(67) 1.06(67) 1.1(67) 0.95(67) 1.08(67) 1.01(67) 1.05(67) 1.07(67)

6.79/1.28 4.13/0.80 2.69/1.49 4.68/1.03 3.26/1.31 3.62/1.56 3.03/1.34 4.68/1.03 4.15/0.88 4.06/0.77 4.33/0.88 3.99/0.81 3.93/0.76 4.74/0.94 4.34/0.86 3.97/0.75 4.64/0.93 4.26/0.92 4.70/0.98 4.16/0.94 4.39/0.92 4.84/0.97 5.36/1.21 4.88/1.10 4.87/0.98 4.87/0.98 4.47/0.88 3.77/0.74 4.96/1.03 4.80/1.13 7.94/1.69 8.43/1.91 8.43/1.91 8.02/1.69 7.56/1.62 7.62/1.65 9.15/2.02 8.80/2.21 7.57/1.58 8.87/2.22 8.22/1.82 8.24/1.94 7.44/1.60 8.38/1.78 7.78/1.72 8.71/2.21 8.24/1.94 8.04/1.96 8.38/1.86 7.15/1.53 7.49/1.60 7.61/1.64 8.73/2.13 8.76/2.05 8.34/2.31 8.94/2.20 6.58/1.38 7.98/1.72 8.75/2.13 8.22/1.82 8.44/2.02 8.30/1.83 8.84/2.15 8.88/2.24 8.06/1.98

6

The Astrophysical Journal, 747:99 (15pp), 2012 March 10 Table 4 (Continued) Observational ID 60022-01-07-02 60022-01-07-03 60022-01-07-04 60022-01-08-00 60022-01-09-00 60022-01-10-00 60022-01-10-01 60022-01-10-02 60022-01-10-03 60022-01-10-04 60022-01-08-00 60022-01-11-00 60022-01-11-01 60022-01-11-03 60022-01-11-02 60022-01-12-00 60022-01-13-00 60022-01-13-01 90022-05-01-00 90022-05-01-01 90022-05-02-00 90022-05-03-00 90022-05-04-00 90022-05-04-01 90022-05-05-00 90022-05-06-00 90022-05-06-01 90022-05-06-02 90022-05-06-03 90022-05-07-00 90022-05-07-00 94307-05-01-00 94307-05-01-000 94307-05-01-00 94307-05-01-01 94307-05-01-02 MJD (day) 52204.424 52207.202 52207.071 52230.164 52295.306 52321.559 52320.301 52320.876 52324.647 52325.760 52230.164 52356.747 52356.814 52357.083 52394.572 52394.891 52554.152 52554.484 53108.101 53108.256 53238.243 53245.990 53252.490 53253.859 53263.468 53265.412 53265.541 53265.675 53265.806 53279.124 53280.960 55129.659 55440.307 55440.62 55440.701 55440.766 = -1 1.06(9) 1.03(5) 1.01(4) 1.0(5) 1.0(4) 1.0(5) 1.0(6) 1.0(3) 0.99(2) 0.99(2) 1.00(5) 0.99(3) 0.99(1) 0.99(2) 1.0(1) 1.0(1) 1.0(1) 1.00(5) 1.04(8) 1.00(8) 1.01(8) 1.06(9) 1.00(4) 1.0(1) 1.01(4) 1.09(6) 1.0(1) 1.01(2) 1.02(5) 1.00(7) 1.03(9) 1.02(3) 1.04(4) 1.03(8) 1.03(9) 1.02(3) Te (keV) 2.75(5) 2.76(3) 2.69(3) 2.58(4) 2.51(4) 2.50(4) 2.51(2) 2.52(1) 2.44(2) 2.44(3) 2.58(4) 2.50(2) 2.47(1) 2.41(1) 2.51(2) 2.73(4) 2.46(2) 2.56(5) 2.42(9) 3.1(1) 2.56(4) 2.36(2) 2.5(1) 2.47(2) 2.52(1) 2.52(2) 2.50(1) 2.51(2) 2.56(2) 2.53(1) 2.42(1) 3.16(4) 3.16(5) 2.50(2) 3.16(4) 3.26(7) log(A) -0.23(6) -0.02(1) -0.01(1) -0.01(1) 0.23(1) 0.11(1) 0.09(1) 0.06(1) 0.14(5) 0.15(6) -0.02(1) 0.08(7) 0.12(4) 0.6(1) 0.41(4) 0.08(7) 0.6(1) 0.03(1) -0.18(6) -0.46(7) 0.10(6) 0.30(6) 0.10(7) 0.06(2) 0.04(1) 0.01(1) 0.01(1) 0.04(2) -0.02(1) 0.10(6) 0.31(3) 0.41(3) 0.45(4) 0.25(2) 0.41(3) -0.56(3) NCOM
b

Seifina & Titarchuk

Ts (keV) 1.20(5) 1.20(7) 1.20(3) 1.3(1) 1.2(1) 1.20(9) 1.20(6) 1.20(5) 1.10(8) 1.10(9) 1.2(1) 1.10(8) 1.10(5) 1.10(8) 1.10(4) 1.10(7) 1.1(1) 1.20(5) 1.5(1) 1.59(9) 1.19(7) 1.19(8) 1.19(6) 1.50(6) 1.50(8) 1.45(7) 1.39(5) 1.41(4) 1.6(1) 1.70(8) 1.50(9) 1.4(1) 1.50(8) 1.4(1) 1.40(6) 1.35(7)

NBbody

b

Eline (keV) 6.36(6) 5.85(7) 6.52(5) 5.87(7) 5.97(8) 5.85(9) 5.78(7) 5.87(6) 6.26(6) 6.47(7) 6.75(6) 6.55(4) 6.46(3) 6.57(7) 6.38(5) 6.49(9) 6.35(5) 6.48(7) 6.38(6) 6.34(5) 6.39(9) 6.5(1) 6.6(1) 6.50(9) 6.49(7) 6.6(1) 6.62(9) 6.67(5) 6.6(1) 6.64(7) 6.65(8) 6.58(2) 6.59(5) 6.64(9) 6.58(3) 6.21(2)

line (keV) 0.3(1) 0.35(8) 0.4(1) 0.50(9) 0.46(8) 0.41(6) 0.46(7) 0.4(1) 0.47(9) 0.51(6) 0.53(8) 0.49(6) 0.5(1) 0.50(9) 0.39(7) 0.37(8) 0.35(4) 0.36(9) 0.53(2) 0.60(8) 0.62(3) 0.6(1) 0.65(8) 0.66(6) 0.67(8) 0.63(6) 0.62(4) 0.61(8) 0.62(9) 0.64(5) 0.62(8) 0.4(1) 0.40(7) 0.62(9) 0.4(1) 0.5(1)

Nline

b

2 red

(dof)

F1 /F

2

c

9.11(4) 11.05(5) 10.37(3) 7.64(1) 12.25(4) 13.26(4) 13.77(1) 12.19(4) 11.36(4) 11.91(5) 7.64(2) 9.94(6) 11.79(3) 13.29(4) 12.56(4) 13.26(7) 12.5(9) 13.6(7) 4.22(3) 4.75(7) 4.20(7) 5.42(5) 6.05(9) 6.08(3) 5.33(2) 5.42(3) 5.56(5) 5.54(6) 4.98(3) 3.53(1) 5.42(3) 4.53(7) 4.53(7) 4.67(2) 4.53(7) 4.57(1)

5.42(9) 5.07(9) 5.56(8) 5.67(7) 3.53(8) 2.71(8) 2.61(8) 2.60(8) 1.57(6) 2.23(6) 5.67(7) 1.29(6) 1.58(6) 2.8(1) 6.2(1) 5.76(8) 9.48(4) 2.85(7) 4.07(3) 3.68(5) 2.58(9) 1.5(1) 1.51(8) 2.49(5) 2.24(2) 2.46(3) 2.08(4) 2.34(6) 2.22(5) 2.45(3) 2.18(3) 3.08(2) 3.08(3) 4.17(4) 3.08(2) 4.13(2)

1.65(4) 1.62(3) 1.76(5) 1.64(3) 1.67(6) 1.71(8) 1.61(4) 1.65(4) 1.87(3) 1.79(6) 1.86(4) 1.86(7) 1.85(7) 1.87(4) 1.94(3) 1.85(6) 1.91(4) 1.86(3) 0.87(5) 1.03(7) 0.77(9) 0.79(8) 0.86(9) 0.83(5) 0.97(7) 0.87(5) 0.79(7) 0.79(5) 0.81(9) 0.85(6) 0.79(7) 0.32(1) 0.34(2) 0.68(9) 0.32(6) 0.21(7)

0.89(67) 1.15(67) 1.12(67) 1.15(67) 1.14(67) 1.2(67) 1.1(67) 1.04(67) 1.11(67) 1.09(67) 0.88(67) 1.2(67) 1.15(67) 1.1(67) 1.19(67) 1.01(67) 1.07(67) 1.16(67) 1.36(67) 1.38(67) 1.34(67) 1.00(67) 1.00(67) 1.25(67) 1.15(67) 1.15(67) 1.05(67) 1.07(67) 1.08(67) 1.18(67) 0.97(67) 1.09(67) 0.87(67) 1.13(67) 0.89(67) 0.98(67)

7.57/1.67 8.31/2.32 8.26/2.06 6.84/1.49 8.55/2.04 8.72/1.97 9.08/2.18 8.01/1.76 6.39/1.37 6.39/1.37 6.84/1.49 6.39/1.37 7.54/1.64 8.56/1.94 9.92/2.36 9.92/2.36 9.92/2.36 8.87/1.99 3.85/0.79 3.99/0.84 3.07/0.64 3.76/0.75 3.11/0.83 4.23/0.85 4.23/0.85 3.78/0.75 3.79/0.77 3.82/0.78 3.44/0.69 3.44/0.69 3.44/0.69 3.21/1.28 3.21/1.28 3.78/0.78 3.21/1.28 3.21/1.28

Notes. Parameter errors correspond to 1 confidence level. a The spectral model is wabs (blackbody + COMPTB + Gaussian), where N is fixed at a value of 1.6 в 1022 cm-2 (Oosterbroek et al. 2001); color temperature T H BB is fixed at 0.6 keV (see comments in the text). b Normalization parameters of bl a ck bod y and COMPTB components are in units of Lsoft /d 2 ,where Lsoft is the source luminosity in units of 1037 erg s-1 , d is the 10 10 37 37 distance to the source in units of 10 kpc, and the Gaussian component is in units of 10-2 в total photons cm-2 s-1 in line. c Spectral fluxes (F /F ) in units of в10-9 erg s-1 cm-2 for (310) and (1050) keV energy ranges, respectively. 1 2

telescopes allows us to determine the parameters of BB components at low energies. Thus, in order to fit the RXTE data, we have to fix the temperature of the BB component at a value of kTBB = 0.6 keV, obtained as an upper limit in our analysis of the BeppoSAX data. The best-fit spectral parameters using RXTE observations are presented in Table 4. In particular, we find that electron temperature kTe of the COMPTB component varies from 2.3 to 4.5 keV, while the photon index is almost constant ( = 1.99 ± 0.02) for all observations. It is worth noting that the width line of the Gaussian component does not vary significantly and is in the range of 0.50.8 keV. Color temperature kTs of the COMPTB component changes from 1.2 keV to 1.7 keV, which is consistent with that using the BeppoSAX data set of our analysis (see Table 3) and previous studies by Oosterbroek et al. (2001), den Hartog et al. (2003), and Chenevez et al. (2006). We should also emphasize that the temperature of the seed photons kTs of the COMPTB component usually increases up to 1.7 keV in the fainter phases and generally decreases to 1.2 keV in the bright phases. 7

In Figure 4, we show an example of the best-fit RXTE spectrum of GX 3+1 for the fainter luminosity phases and the residuals (bottom panel) with for the 94307-05-01-000 observation. Blue, red, and purple lines stand for BB, COMPTB, and Gaussian components, respectively. In Figure 5, we also show examples of typical photon spectra E F (E ) spectral diagrams of GX 3+1 during the fainter phase (94307-05-01-00, blue) and the brighter phases (60022-01-1301, red) detected with RXTE on MJD 55440.62 and 52544.48, respectively. The adopted spectral model shows a very good fidelity throughout all data sets used in our analysis. Namely, a 2 value of reduced red = 2 /Ndof , where Ndof is the number of degrees of freedom, is less than or about 1.0 for most observations. For a small fraction (less than 2%) of spectra 2 with high counting statistics red reaches 1.5. However, it never exceeds our rejection limit of 1.7. Note that the energy range for the cases in which we obtain the poor fit statistic (two among 101 spectra with 2 = 1.7 for 67 dof) is related to the iron line region. It is possible that the shape of the iron line is more complex than

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a simple Gaussian (i.e., a blend of different energies, presence of the edge, or broadening by Comptonization). The fits tend to favor a broad line (see Table 4), which might be caused by Comptonization. However, this possible complexity is not well constrained by our data. It is worth noting that we find some differences between our values of the best-fit model parameters and those in the literature. In particular, the photon index , estimated by Oosterbroek et al. (2001) for observation id=20835001, is 1.60 ± 0.25. This discrepancy in index values can be a result of using slightly different spectral models than Oosterbroek et al. (2001). Thus, using the broadband BeppoSAX observations, we can accurately determine all of the parameters of our spectral model while using the extensive observations of GX 3+1 by RXTE.We are able to investigate the overall pattern of the source behavior during the spectral transitions in the 350 keV energy range. 4. OVERALL PATTERN OF X-RAY PROPERTIES 4.1. Evolution of X-Ray Spectral Properties during Transitions As was mentioned above, at timescales larger that 1 minute, GX 3+1 exhibits two kinds of variability, slow and mild. The former one (slow) has a timescale of order years. This slow variability is seen in the faint phases and bright phases, which are related to low and high luminosities, respectively. On the other hand, the mild variability has a timescale of order of days, and modulation depth in the 310 keV band is typically 20%. The ASM (212 keV) mean flux correlates with COMPTB normalization (NCOM ) and tends to anticorrelate with the electron plasma temperature of CC Te (see Figure 6). Such mild variability is detected for both the fainter and brighter phases for GX 3+1. It should be noted that the COMPTB normalization changes are larger in the bright phase than those during the faint phase, while the electron temperature Te variations are almost the same for both phases. One can relate slow and mild variabilities of GX 3+1 to slow and mild changes of mass accretion rate, respectively. The slow variability by a factor of four has been seen in the 19962010 observations by ASM/RXTE. The same kind of changes of the flux were also observed in the earlier observations by Makishima et al. (1983). In turn, in the next section it will be shown that the slow variability can be related to transitions between the brighter and fainter phases along with small variations of the electron temperature kTe . We found that the X-ray spectra of GX 3+1 over the bright and faint phases are quite stable. Moreover, we detected a constant 350 keV spectral shape over the slow and mild variability stages. The best-fit parameters are listed in Table 4. The faint/ bright phase transitions are characterized by the spectra with an almost constant spectral index of 1 or photon index of 2 (see Figure 7). We have also established common characteristics of the risedecay spectral transition of GX 3+1 based on their spectral parameter evolution of X-ray emission in the energy range from 3 to 50 keV using PCA/RXTE data. In Figures 4 and 5 we present typical examples of the RXTE bright and faint phase spectra for GX 3+1. In fact, one can clearly see from these figures that the normalization of the thermal component is a factor of two higher in the bright phase than that in the faint phase, although the photon indices for each of these spectra are concentrated around 2 (see Figures 6 and 7). In fact, we test the hypothesis of appr 2 using the 2 2 statistic criterion. We calculate the distribution of red (appr ) = N 1 2 versus appr and find a sharp i =1 (i - appr /i ) N 8
Figure 5. Examples of X-ray spectra, presented as E F (E ) spectral diagrams, of GX 3+1 during faint phase (94307-05-01-00, blue) and bright phase (60022-0113-01, red) detected with RXTE on MJD 55440.62 and 52544.48, respectively. (A color version of this figure is available in the online journal.)
2 minimum of function red (appr ) around 1, which takes place in the range of appr = 1.99 ± 0.01 with a confidence level of 67% and appr = 1.99 ± 0.02 with a confidence level of 99% 2 for 101 dof (see the similar figure of red (appr ) for 4U 172834 in ST11). Using BeppoSAX data, FT11 suggested that the photon index is approximately 2 for many NS binaries that are observed in different spectral states. FT11 characterize the spectral state by a value of electron temperature Te and show that = 2 ± 0.2 (or = 1 ± 0.2) when kTe changes from 2.5 to 25 keV. A number of X-ray flaring episodes of GX 3+1 have been detected with RXTE during 20012002 (R 4 set) with a good risedecay coverage. We have searched for common spectral and timing features that can be revealed during these spectral transition episodes. We present the combined results of the spectral analysis of these observations using our spectral model wabs (blackbody + COMPTB + Gaussian) in Figures 6 and 7. ASM/RXTE count rate is shown in the top panel of these figures. Further, from the top to the bottom, we show the model flux in two energy bands 310 keV (blue points) and 1050 keV (crimson points). In the next panel we show a change of the TL electron temperature kTe . One can clearly see the lowamplitude spectral transition on timescales of 12 days from the brighter phase to the fainter phase during the time period from MJD 52000 to MJD 52200, while kTe only varies from 2.3 keV to 4.5 keV during this transition. Normalizations of the COMPTB and BB component (crimson and blue points, respectively) are shown in the next panel of Figures 6 and 7. In particular, one can see from Figures 6 and 7 how the COMPTB normalization NCOM correlates with the variations of ASM count rate and the model flux in the 310 keV energy band. On the other hand, the normalization of the BB component NBB is almost constant except at the mild variability episode peak, when NBB increases from 0.02 to 0.14 (see blue points in Figure 6 at MJD = 52130 and 52170). Moreover, these spectral variability transitions are related to a noticeable increase of flux in the 310 keV energy range and corresponding decrease of flux that takes place in the 1050 keV energy range (see the second panels from above in Figures 6 and 7). The illumination fraction f varies from 0.1 to 0.9 as seen from Figure 8, while the index only slightly varies with time around

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Figure 6. From top to bottom: evolutions of the ASM/RXTE count rate, model flux in 310 keV and 1060 keV energy ranges (blue and crimson points, respectively), electron temperature Te in keV, COMPTB and BB normalizations (crimson and blue points, respectively), and spectral index = - 1 during 20012002 outburst transition set (R4). The rising phases of the mild transition are marked with blue vertical strips. The peak burst times are indicated by the arrows on the top of the plot. (A color version of this figure is available in the online journal.)

1 (or 2) (see Figures 6 and 7). However, in most cases the soft disk radiation of GX 3+1 is subjected to reprocessing in a CC and only some fraction of the soft photon emission component (1 - f ) is directly seen by the Earth observer. Note that f changes with COMPTB normalization (see Figure 8, the inner panel in the left-hand upper corner). The energy spectrum of GX 3+1 during almost all states is dominated by a Comptonized component, while the direct soft photon emission is always weaker and detectable in the flaring episodes only (see also Figures 6 and 7). Note that for BHs a definition of spectral transition involves a change of photon index (see, e.g., ST09). However, there is no one-to-one correspondence between and cutoff (or e-fold) energy Efold . Titarchuk & Shaposhnikov (2010) demonstrate, using RXTE data for BH binary XTE J1550-564, that Efold decreases when increases from 1.4 to 2.12.2 until reaches 2.2 and then Efold increases. Thus, for a BH the main parameter 9

39

used for the spectral transition definition is a variable photon index , which monotonically increases when the source goes into the br ig ht phase. It is important to emphasize once again that in the NS binary GX 3+1 the transition from the faint phase to the br ig ht phase 2 takes place when COMPTB normalization NCOM = Lsoft /D10 39 changes from 0.04 to 0.15. Thus, we define the NS spectral transition in terms of the COMPTB normalization. In this case the faint phase is characterized by the low normalization value while the bright phase is related to the high normalization value. In Figure 9, we demonstrate the dependence of 2 COMPTB normalization Lsoft /D10 on kTe using these best-fit 39 parameters for GX 3+1 and 4U 1728-34 for the BeppoSAX and RXTE data. From this figure one can clearly see a monotonic behavior NCOM versus kTe , namely, kTe decreases when the soft flux increases. It is worth noting that the kTe values obtained using GX 3+1 data for BeppoSAX and RXTE reach the

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Figure 7. Same as Figure 6 but for all the RXTE sets (R1R7) to demonstrate slow (long-term) variability. (A color version of this figure is available in the online journal.)

asymptotic value of about 2.5 keV at high values of the soft flux (NCOM > 0.05). To demonstrate transition properties of GX 3+1 in terms of flux (or luminosity), we define the hard color (HC) as a ratio of the flux in the 1050 keV to that in the 350 keV energy band, and the soft color (SC) as a ratio of the flux in the 310 keV to that in the 1050 keV energy band. Plotting HC versus SC, we created our CD (see the left panel of Figure 10) for two atoll sources GX 3+1 (pi nk ) and 4U 1728-34 (blue). As it appears from this figure, the tracks of these two sources display a smooth and continuous (monotonic) function, pointing out the similar physical mechanism of hard/soft flux transition during a long-term source evolution for these two objects. In Figure 10 (right panel), we demonstrate a fragment of the ASM light curve of GX 3+1, which illustrates two types of flux variability. The long-term time trend (from bright to faint) is related to COMPTB normalization changes, while the second one shows short-term time variations (UBLB) related to the CC electron temperature variations. The blue line displays a mean count rate and indicates long-term variability of GX 3+1 flux. Note that the track of Figure 10 (left panel) reflects a long-term evolution of GX 3+1. 10

It is worth noting that among all NSs only a few atoll and Z-sources demonstrate a full track on the CD in a wide range of luminosity. For example, atoll sources, such as 4U 1728-34, usually show a wide range of spectral states during transitions that are related to changes of the total luminosity and mass accretion rate. One can establish a substantial difference between a NS and a BH owing to these flare episodes when a source evolves from the faint phase to the br ig ht phase and when the plasma temperature of a Comptonized region changes remarkably (like in 4U 1728-34 from 2.5 keV to 15 keV during ISB states). On the other hand, GX 3+1 shows significant changes in the total luminosity but with only a slight variation of electron temperature kTe in its ba na na state. However, the photon index stays around a value of two and is independent of the soft photon luminosity in both the faint phase and the bright phase. 4.2. Timing Properties during LBUB Transitions The RXTE light curves have been analyzed using the powspec task from FTOOLS 5.1. The timing analysis PCA/RXTE data were performed in the 1330 keV energy range using the event mode. The time resolution for this mode is 1.2в10-4 s.

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GX 3+1/

RXTE RXTE

GX 3+1/

BeppoSAX BeppoSAX

4U 1728-34/

4U 1728-34/

EIS

IS

kHz Q PO

LLB

LB UB

Figure 8. Electron temperature Te (in keV) plotted vs. illumination fraction f = A/(1 + A) for atoll sources GX 3+1 and 4U 1728-34 during mild variability. Pink/bright blue and blue/green points correspond to RXTE/BeppoSAX observations of GX 3+1 and 4U 1728-34, respectively. For GX 3+1 COMPTB normalization 2 measured in Lsoft /D10 units vs. illumination fraction f is plotted in the incorporated panel (top left) during long-term (slow) variability (see Table 4). The bended 39 arrows are related to an increase of mass accretion rate. On the right-hand side we show a sequence of CD states (EIS: the extreme island state, IS: island state, LLB: lower left banana state, LB: lower banana state, and UB: upper banana state) that are listed according to the standard atollZ scheme (Hasinger & van der Klis 1989). One can see that Te is directly related with the sequence of CD states. Along the track for 4U 1728-34 we indicate points of Te f correlation where kHz QPOs are detected. (A color version of this figure is available in the online journal.)

We generated power density spectra (PDS) in the 0.1500 Hz frequency range using light curves with 10-3 s time resolution. We subtracted the contribution due to Poissonian noise and Very Large Event Window for all PDS. We used the QDP/PLT plotting package to model PDS. Previously, timing analysis of PCA/RXTE data for GX 3+1 observed on 1999 August (our R3 set), when the source was in LB phase, was made by Oosterbroek et al. (2001). We investigated a timing behavior of GX 3+1 for our data set during all transitions between LB and UB phases (see Figure 11). In particular, the power spectrum of GX 3+1 consists of very low frequency noise (VLFN, described by a power law) and highfrequency noise (HFN, described by a cutoff power law; see van der Klis 2005). In the LB phases (A red, 60022-01-13-01, MJD = 52554; A blue, 60022-01-01-00, MJD = 51998) the best fit to the average power spectrum results in an rms VLFN component of 2% (in the 0.11 Hz range) described by power law -LF with the index of LF 1.7, whereas HFN rms (in the 1100 Hz range) has 4.7% with HF 1.0 and cutoff 30 Hz. Generally the index of VLFN LF gradually decreases from 1.7 to 1.4 toward UB. However, in the vicinity of a transition point between LB and UB (red histogram of panel (B) of Figure 11) LF jumps to 1.8 (B red, 94307-05-01-00, MJD = 55129) and decreases again to 1.4 (B blue, 60022-01-11-03, MJD = 52357). In general, the UB 11

power spectra of GX 3+1 are dominated by the VLFN with the brake at around 20 Hz at the lowest kTe = 2.4 keV (see blue B point in the right-hand panel of Figure 11). Specifically, during UB (blue histogram of panel (B) of Figure 11) one can see strong VLFN (rms = 5.1% ± 0.4%, LF = 1.4 ± 0.3, 2 = 139 for 102 dof; all parameter errors correspond to 1 confidence level) and HFN with rms = 1.7% ± 0.3% and break frequency shifted from 30 Hz to 6 Hz. After UB GX 3+1 again returns to the LB, showing properties of PDS similar to panel (A) (see panel (C) of Figure 11). Note that these components and their CD evolution are typical for atoll sources in the banana state (Hasinger & van der Klis 1989) and caused by mass accretion rate change (van der Klis 2005). This phase identification is supported by a combination of spectral (see Section 4.1) and timing properties in agreement with the atollZ scheme. While the aforementioned CD evolution of power spectra of GX 3+1 occurs on timescales from hours to days, we detected similar patterns for power spectrum evolution during LBUB transitions for both faint phase and bright phase on luminosity during long-term variability within 14 years with a quasi-periodic trend during 6 years. The similarity of LBUB transitions, which are independent of bright/faint phases on luminosity, indicates similar accretion configurations in all phases.

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brighter periods of GX 3+1

GX 3+1/

RXTE RXTE

4U 1728-34/ GX 3+1/
fainter periods of GX 3+1

BeppoSAX BeppoSAX

4U 1728-34/

brighter periods of 4U 1728-34 fainter periods of 4U 1728-34

2 Figure 9. COMPTB normalization measured in Lsoft /D10 units vs. electron temperature Te (in keV) obtained using the best-fit spectral model 39 wabs (blackbody + COMPTB + Gaussian) for atoll sources GX 3+1 (pink) and 4U 1728-34 (blue, taken from ST11) for RXTE data and bright blue and green points for Bepp oSAX data. Mass accretion rate continuously increases along this correlation from the right to the left. (A color version of this figure is available in the online journal.)

GX 3+1 4U 1728-34

GX 3+1 GX 3+1
40

ASM count/s

0

54000 MJD, days

lo

ng-term

20
short-term B UB-L

faint phase - bright phase

55000

Figure 10. Left: the CD (310/1050 keV flux ratio vs. 1050/350 keV flux ratio) of GX 3+1 (pink) and 4U 1728-34 (blue) during faintbright transitions (long-term variability). Right: fragment of ASM light curve of GX 3+1 that shows two types of flux variability. One is a long-term trend (from faint to bright) related to COMPTB normalization changes, and another one is short-term time variations (UBLB) related to the electron temperature changes. The blue line presents a mean count rate and indicates a long-term variability of GX 3+1. (A color version of this figure is available in the online journal.)

In a previous analysis of other RXTE data of GX 3+1 Oosterbroek et al. (2001) report VLFN and HFN values, in LB state, which are similar to our values in interval R3, with the exception of VLFN strength, for which they report 7.5% rms while we find 1.7% rms. All of the VLFN and HFN values of the analysis of EXOSAT data reported by Hasinger & van der Klis (1989) for GX 3+1 agree with our results. 12

4.3. Comparison of Spectral and Timing Characteristics of Atoll Sources GX 3+1 and 4U 1728-34
4.3.1. Quasi-constancy of the Photon Index

The atoll sources GX 3+1 and 4U 1728-34 demonstrate a similar behavior of the parameter versus mass accretion rate (or our COMPTB normalization), namely, the quasi-constancy

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A

UB

LB

B

A AC

B C

B C

Figure 11. Left column: PDS presented in terms of P ( ) for 1330 keV energy band correspond to LB and UB states of GX 3+1 and related to different electron temperatures indicated by points A, B, and C in the right panel. The strong noise component HFN seen in the 150 Hz range and relatively weak VLFN at the frequencies below 1 Hz are present before and after transition (see panels (A) and (C)) from LB to UB. In UB (panel (B), bl ue histogram) the power spectra of GX 3+1 are dominated by the VLFN with a break at about 20 Hz. Right panel: photon index plotted vs. electron temperature Te (in keV) using our spectral model wabs (blackbody + COMPTB + Gaussian) during spectral transitions (see Table 4). Violet and black points correspond to BeppoSAX and RXTE observations of GX 3+1, respectively. The violet arrow in the top part of the right panel points in the direction of the LBUB transition related to a decrease of Te . (A color version of this figure is available in the online journal.) Table 5 Comparisons of the Best-fit Parameters of Atoll Sources GX 3+1 and 4U 1728-341 Source Name 4U 1744-26 4U 1728-34 Alternative Name GX 3+1 GX 354-0 Class2 Atoll, Sp, B Atoll, Su, D Distance (kpc) 4.53 4.26.44 Presence of kHz QPO none5 +6 kTe (keV) 2.34.5 2.515 Ncomptb, 2 Lsoft /D10 39 0.040.15 0.020.09 kTBB (keV) 0.6 0.60.7 kTs (keV) 1.161.7 1.3 f 0.20.9 0.51

References. (1) ST11; (2) Classification of the system in the various schemes (see the text): Sp: supercritical, Su: subcritical, B: bulge, D: disk; (3) Kuulkers & van der Klis 2000; (4) van Paradijs 1978; (5) Strohmayer 1998; (6) Titarchuk & Osherovich 1999.

of the photon index 2 and almost identical long-term variations of the ASM mean count rate (see also ST11). According to FT11 and ST11, this observational fact can presumably indicate that the energy release in the TL for these two sources is much higher than the cooling flux of the soft (disk) photons.
4.3.2. A Difference of the Electron Temperature kTe Ranges in GX 3+1 and 4U 1728-34

4.3.3. Comparison of Spectral Evolution as a Function of the COMPTB Normalization for GX 3+1 and 4U 1728-34

A comparison of the best-fit spectral parameters for these two atoll sources shows that a slow variability is generally related to changes of COMPTB normalization, and a mild variability is mainly correlated with the electron temperature variations (see Figure 10). Note that the ranges of disk and NS temperatures are similar for both of these objects, namely, kTBB 0.6 keV and kTs = 1.21.7 keV, respectively. On the other hand, variations of the electron temperature kTe are quite different. The electron temperature kTe changes in a wide range kTe = 2.515 keV for 4U 1728-34, while for GX 3+1 kTe varies in a narrow range from 2.3 to 4.5 keV (see Figures 8 and 9). The reason for this difference of temperature ranges is quite obvious. While 4U 1728-34 shows an evolution from the extreme island state (EIS) to the UB state (see Di Salvo et al. 2001; ST11), GX 3+1 demonstrates only a short LBUB track on the CD (see Figure 8). These ranges of CD states are related to the ranges of kTe . 13

We can also compare spectral parameter evolution for GX 3+1 and 4U 1728-34 using COMPTB normalization because the distances to these sources are almost the same (see Table 5). Namely, for GX 3+1 the distance is in the range of 4.26.4 kpc (Kuulkers & van der Klis 2000), whereas for 4U 1728-34 it is 4.5 kpc (van Paradijs 1978). In Figure 9, we show a correlation of COMPTB normalization presumably proportional to mass accretion rate and the electron temperature kTe for these two atoll sources. GX 3+1 demonstrates a wider range of COMPTB normalization (by a factor of two higher than that for 4U 172834), while kTe varies only from 2.5 to 4.5 keV. The common interval of kTe for GX 3+1 and 4U 1728-34 is in the range 2.54.5 keV only, and the low limit of the electron temperature of 2.5 keV takes place at the peak luminosity for 4U 1728-34 (see ST11) and during increases in luminosity for GX 3+1, i.e., during the so-called UB state (see Figure 8).
4.3.4. Correlation of Illumination Parameter f versus Electron Temperature kTe and Its Relation with Different States on the ColorColor Diagram

One can see from Table 5 that the range of illumination fraction of CC (TL) f is wider for GX 3+1 (0.10.9) than that

The Astrophysical Journal, 747:99 (15pp), 2012 March 10

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for 4U 1728-34 (0.51). It can be related to different illumination of the TL for these two sources. For 4U 1728-34 the solid angle viewed from NS changes by a factor of two, whereas in GX 3+1 that changes by a factor of four. However, the photon index 2 for these two sources, which indicates that the energy release in the TL for these two sources is much higher than the cooling flux of the disk photons (see FT11 and ST11 for details of X-ray spectral formation in TL). In Figure 8, we present a plot kTe versus f = A/(1 + A) for atoll sources GX 3+1 and 4U 1728-34 during mild variability. Pink/bright blue and blue/green points correspond to RXTE/BeppoSAX observations of GX 3+1 and 4U 1728-34, 2 respectively. COMPTB normalization measured in Lsoft /D10 39 units versus f is plotted in the incorporated top left panel for long-term (slow) variability of GX 3+1 (see Table 4 for details). The bended arrows along the corresponding tracks correspond to an increase in mass accretion rate. On the right-hand side of the figure we also present the sequence of CD states, which are listed according to the standard atollZ scheme (Hasinger & van der Klis 1989). Here we also show that the CD sequence is definitely related to the electron temperature kTe . The diagram of Te versus f demonstrates a clear correlation of Te and f, while the diagram NCOM versus f, presented in the incorporated panel of Figure 8, shows only chaotic scattering of points in a wide range of f 0.20.9. Moreover, we find two different track shapes on the diagram of Te versus f for atoll sources GX 3+1 and 4U 1728-34 and their relations with the standard sequence of CD states (Figure 8). When the fraction f increases, the electron temperature Te decreases monotonically from approximately 4.5 keV to 2.3 keV for GX 3+1, while 4U 1728-34 demonstrates a more complicated behavior pattern. At EIS, with a decrease of Te , the fraction f slightly varies from 0.9 to 1. When Te further decreases from 12 keV to 4 keV, f decreases from 0.9 to 0.5. Finally, during the LBUB state transition f increases from 0.5 to 1. Thus, we show that the evolution of CD states can be traced by the correlation between Te and f too. Note that the tracks of f Te on this diagram resemble the atoll tracks on the standard CD (Schultz et al. 1989). 5. CONCLUSIONS We present our analysis of the spectral properties observed in X-rays from the NS X-ray binary GX 3+1 during long-term transitions between the faint phase and the bright phase superimposed by short-term transitions between LB and UB states. We analyze all transition episodes for this source observed with BeppoSAX and RXTE. For our analysis we apply the good spectral coverage and resolution of BeppoSAX detectors from 0.1 to 200 keV along with extensive RXTE coverage in the energy range from 3 to 50 keV. We show that the X-ray broadband energy spectra during all spectral states can be adequately fitted by the combination of a BB, a Comptonized, and a Gaussian component. We also show that photon index of the best-fit Comptonized component in GX 3+1 is almost constant, with a value of two, and consequently is almost independent of COMPTB normalization 2 Lsoft /D10 , which is presumably proportional to mass accretion 39 rate m (see Figures 6, 7, and 11). We should remind the reader that this index stability has recently been suggested using X-ray observations of quite a few other NS sources, namely, atoll sources X1658-298, GS 1826-238, and 1E 1724-3045 and also Z-sources Cyg X-2, Sco X-1, GX 17+2, GX 340+0, and GX 349+2 were observed by BeppoSAX at different spectral 14

states, as well as atoll source 4U 1728-34 observed by BeppoSAX and RXTE (see details in FT11 and ST11, respectively). A wide variation of parameter f = 0.10.9, obtained in the framework of our spectral model, points out a significant variation of the illumination of the Comptonization region (TL) by X-ray soft photons in GX 3+1. Using BeppoSAX observations, we find that there are two sources of blackbody emission: one is presumably related to the accretion disk, and another one is related to the NS surface, for which temperatures of soft photons are about 0.7 keV and 1.3 keV, respectively. We demonstrate that our analysis of X-ray spectral and timing properties in atoll source GX 3+1 allows us to distinguish between mild and long-term variabilities, and we link them with LBUB state transitions and transitions between bright and faint phases in luminosity, respectively. In this way we described mild flux variability between LB and UB states on a timescale of hours to days in terms of two basic spectral parameters, the electron temperature kTe and illumination fraction f. We argue that kTe monotonically increases from 2.3 keV to 4.5 keV when GX 3+1 makes a transition from UB state to LB state. We also find two noise components (VLFN and HFN) and their evolution during LBUB transitions: the X-ray power spectra (PDS) in UB are dominated by VLFN with the break around 20 Hz, whereas in LB the PDS are dominated by an HFN in the 1100 Hz range and accompanied by reduced VLFN below 1 Hz. We demonstrate that the photon index 2 is approximately constant when the source moves from the faint phase to the bright phase, as well as during local transitions from LB to UB. In ST11 we presented strong theoretical arguments that the dominance of the energy release in the TL with respect to the soft flux coming from the accretion disk, Qdisk /Qcor 1, leads to almost constant photon index 2. Thus, we argue that the stability of this index is an intrinsic signature of atoll sources, while in BHs the index monotonically changes with mass accretion rate and ultimately saturates (see ST09 and ST11). Photon indices of BH candidates (GRS 1915+105, GX 339-4, SS 433, H1743-322, 4U 1543-47, Cyg X-1, XTE J1550-564, and GRO J1655-40) show clear correlation with mass accretion rate m (ST09, TS09, and Seifina & Titarchuk 2010). This correlation is accompanied by an index saturation when m exceeds a certain level. The behaviors of the index for GX 3+1 and 4U 1728-34 are clearly different from that for the sample of black hole candidate sources. The photon index 2, while mass accretion rate changes by a factor of four. We acknowledge discussion and editing of the paper content with Chris Shrader. We are very grateful to the referee, whose constructive suggestions helped us improve the paper quality. REFERENCES
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