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The Astrophysical Journal, 738:128 (20pp), 2011 September 10
C

doi:10.1088/0004-637X/738/2/128

2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

ON THE CONSTANCY OF THE PHOTON INDEX OF X-RAY SPECTRA OF 4U 1728-34 THROUGH ALL SPECTRAL STATES
Elena Seifina1 and Lev Titarchuk2
1 2

,3 ,4

Sternberg Astronomical Institute, Moscow State University, Universitetsky Prospect 13, Moscow 119992, Russia; seif@sai.msu.ru ` Dipartimento di Fisica, Universita di Ferrara, Via Saragat 1, I-44100 Ferrara, Italy; titarchuk@fe.infn.it, lev@milkyway.gsfc.nasa.gov 3 School of Physics, Astronomy, and Computational Sciences, George Mason University, Fairfax, VA 22030, USA 4 Goddard Space Flight Center, NASA, code 663, Greenbelt, MD 20770, USA Received 2011 January 5; accepted 2011 June 24; published 2011 August 18

ABSTRACT We present an analysis of the spectral properties observed in X-rays from neutron star X-ray binary 4U 1728-34 during transitions between the low- and high-luminosity states when the electron temperature kTe of the Compton cloud monotonically decreases from 15 to 2.5 keV. We analyze the transition episodes from this source observed with BeppoSAX and RXTE satellites. We find that the X-ray broadband energy spectra of 4U 1728-34 during all spectral states can be modeled by a combination of a thermal (blackbody-like) component, a Comptonized component (which we herein denote as COMPTB), and a Gaussian component. Spectral analysis using this model provides evidence that the photon power-law index is almost constant ( = 1.99 ± 0.02) when kTe changes from 15 to 2.5 keV during these spectral transitions. We explain this quasi-stability of the index by the model in which the spectrum is dominated by the strong thermal Comptonized component formed in the transition layer (TL) between the accretion disk and neutron star surface. The index quasi-stability takes place when the energy release in the TL is much higher than the flux coming to the TL from the accretion disk. Moreover, this index stability effect now established for 4U 1728-34 during spectral evolution of the source was previously suggested for a number of other neutron binaries. This intrinsic property of the neutron star is fundamentally different from that in black hole binary sources for which the index monotonically increases during spectral transition from the low state to the high state and saturates at high values of the mass accretion rate. Key words: accretion, accretion disks ­ black hole physics ­ radiation mechanisms: non-thermal ­ stars: individual (4U 1728-34) ­ stars: neutron Online-only material: color figures

1. INTRODUCTION The evolution of spectral parameters of compact objects in X-ray binaries is of great interest for understanding the nature of compact objects. It is well known that a number of black hole (BH) candidate sources demonstrate correlations between their 1­10 Hz quasi-periodic oscillation (QPO) frequencies L and photon power-law index during spectral transition when sources evolve from the low state (LS) to the high state (HS), see Shaposhnikov & Titarchuk (2009), hereafter ST09. Then the definition of the spectral state is related to the level of soft blackbody emission presumably related to the mass accretion rate. In the HSs of BHs, these index-QPO frequency correlations sometimes show a saturation of at high values of L . On the other hand, ST09 (see also Titarchuk & Seifina 2009, hereafter TS09) found that saturates with the mass accretion rate in almost any case of a BH binary. This saturation effect can be considered as a BH signature or as a signature of the converging flow (CF) into a BH (ST09 and TS09). The question that naturally arises is how the spectral index behaves as a function of the mass accretion rate or as a function of cutoff energy of the spectrum in neutron star (NS) sources. Recently, Farinelli & Titarchuk (2011, hereafter FT11), collected X-ray spectra obtained by BeppoSAX from quite a few NS sources: Sco X-1, GX 17+2, Cyg X-2, GX 340+0, GX 3+1, and GS 1826-238. Their results probably indicate that the value of the photon index slightly varies around 2 independently of the spectral state (or electron temperature of a Compton cloud) at least for this particular sample of NS spectra (see Di Salvo et al. 1

2000; Farinelli et al. 2008). However, the available data for those sources were taken when these sources were in the HS or in the low state (LS), but nobody has analyzed up to now the spectral evolution from the LS to the HS for any particular NS source. A suitable candidate for the study of the spectral evolution in an NS is the so-called atoll 4U 1728-34, which exhibits a remarkable spectral transition from the LS to the HS and vice versa. 4U 1728-34 (GX 354-0) was first resolved by UHURU scans of the Galactic center region in 1976 (see Forman et al. 1976 and Bradt et al. 1993). Then type I X-ray bursts from 4U 1728-34 were discovered during SAS-3 observations by Lewin et al. (1976) and Hoffman et al. (1976). Further, the bursting behavior was subsequently studied in detail using extensive observations by SAS 3, which accumulated 96 bursts in total. Using these data Basinska et al. (1984) presented evidence for a narrow distribution of peak burst fluxes, as well as a correlation between the peak flux and the burst phase. The distance to the source in the range of 4.2­6.4 kpc has been estimated by van Paradijs (1978) and confirmed by Basinska et al. (1984) and Kaminker et al. (1989) using measurements of the peak burst fluxes. A radio counterpart of 4U 1728-34 was detected during Very Large Array (at 4.86 GHz) observations with a variable flux density in the range of 0.3­0.6 mJy (Marti et al. 1998). The estimated extinction of the source is AV 14, and a precise position following from the detection of the radio counterpart allowed us to identify this source as a K = 15 infrared source (Marti et al. 1998). Long-term Ariel-5 measurements, as well as extensive monitoring by the All-Sky Monitor (ASM) on board RXTE, suggest the presence of a long-term quasi-periodicity about 63­72 days (Kong et al. 1998). RXTE/Proportional


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Counter Array (PCA) observations of the source in 1996 led to the discovery of nearly coherent millisecond oscillations during the X-ray bursts (Strohmayer et al. 1996). Titarchuk & Osherovich (1999) presented a model for the radial oscillations and diffusion of the perturbation in the transition layer (TL) surrounding the NS. Using dimensional analysis, they identified the corresponding radial oscillation and diffusion frequencies in the TL with the low-Lorentzian L and break frequencies b for 4U 1728-34. They predicted values for b , related to the diffusion in the boundary layer, that are consistent with the observed b . Subsets of the bursts observed during the PCA observations have also been studied by van Straaten et al. (2001) and Franco (2001) with particular attention to the relationship between the appearance of burst oscillations and a value of the mass accretion rate. Titarchuk & Shaposhnikov (2005, hereafter TS05) analyzed RXTE/PCA observations of 4U 1728-34 in the energy range from 3 keV to 40 keV and found that, using the model comprising two Comptonization components (BMC5 ), the photon index is consistent with being quasi-constant (around 2.2), while the low QPO frequency does not exceed 10 Hz. then monotonically increases to values of 6. Moreover, using broadband observations of 4U 1728-34 by BeppoSAX, Di Salvo et al. (2000) and Piraino et al. (2000) fitted the X-ray spectra of 4U 1728-34 in a wide energy range from 0.1 keV to 200 keV by a sum of Blackbody components plus a thermal Comptonization spectrum, usually described by the XSPEC COMPTT model (Titarchuk 1994; Hua & Titarchuk 1995). The TS05 model cannot fit the BeppoSAX data; moreover, this model has more parameters than does the model of Di Salvo et al. (2000), Piraino et al. (2000), and FT11. Naturally, we pose the question of whether the FT11 model can fit the BeppoSAX along with RXTE data and what dependence of index versus spectral state can be found. In this paper, we try to answer a fundamental question of the possibility of distinguishing a BH from NS systems, which others have extensively attempted to solve without considering the mass of the compact object as a main argument, in particular without using optical counterpart data to measure mass function. In this way, some methods have been proposed to identify systems that contain BHs using X-ray observational properties only. The strong rapid variability was first considered as a signature of the presence of BH (Oda et al. 1971), until the same rapid variability was detected in accreting NSs (Tennant et al. 1986). Now it is well established that Galactic BH candidates (BHCs) demonstrate two spectral states, the HS and LS, and transition between the two (Remillard & MacClintock 2006). However, sometimes the so-called atoll-NS sources6 also show the "high" and "low" spectral states (D' Ai et al. 2006; TS05). Therefore, this property requires more detailed investigations of BHs versus NSs. Specifically, the HS spectra of BHCs are characterized by thermal emission at 1 keV, presumably originated in the accretion disk, along with a steep power-law tail whose photon index ( = 2­3) monotonically increases with the mass accretion rate (see ST09). The LS spectra show much weaker disk emission than that in the HS spectra and
5

BMC is the so-called Bulk Motion Comptonization XSPEC Model (see details in Section 6.2.10 of "User's Guide of an X-Ray Spectral Fitting Package XSPEC v.12.6.0" and http://heasarc.gsfc.nasa.gov/ xanadu/xspec/manual/Additive.html). 6 Here, we use a term of the atoll-NS sources to specify NS X-ray binaries characterized by a specific " "-shaped track in a color­color diagram.

a harder power-law tail (the photon index of which is around 1.7). This hard component is generally believed to be a result of thermal Comptonization of soft (disk) photons in a hot gas (Compton cloud) in the vicinity of the compact object. BHs, in contrast with NSs, sometimes demonstrate a more complicate X-ray spectrum. For example, the RXTE spectra of BH GRS 1915+105 require two Comptonization components, soft and hard ones (see Titarchuk & Seifina 2009). In this case one can clearly see the evolution of two photon indices 1 = 1.7­3.0 and 2 = 2.7­4.2 for the hard and soft components, respectively. Titarchuk & Seifina (2009) argued that the index saturation effect of the hard component is due to Comptonization of the soft (disk) photons in the CF into BH and that of the soft component is due to the thermal Comptonization in the TL when the mass accretion rate increases. These conclusions were later supported by Monte Carlo simulations by Laurent & Titarchuk (2011). Moreover, in the description of BH and NS X-ray LS spectra with the thermal Comptonization model, there is an essential difference between these types of the compact sources. The electron temperature of the Compton (scattering) cloud kTe is usually lower for NSs, kTe < 25 keV, than that for BHs, kTe > 50 keV (see Churazov et al. 1997). The lower electron temperature in NSs is a consequence of the additional cooling provided by the NS surface, which reflects X-ray photons and ultimately determines the value of the Compton cloud electron temperature (Titarchuk et al. 1998 and see also Sunyaev & Titarchuk 1989; Kluzniak 1993). This fact of the observed difference between the BH and NSs was recently also discussed by Reynolds & Miller (2011). They conclude the observable difference of kTe in these types of the sources is evidence of the absence and presence of a solid surface in BHs and NSs, respectively, and this fact can be considered as indirect evidence for the existence of the event horizon in BHs. It is worth pointing out that Titarchuk et al. (1998) and Titarchuk & Fiorito (2004) previously came to similar conclusions based on the analysis of the Compton cooling of the X-ray emission region (TL) in the presence (NS) and absence (BH) of the reflection surface. Thus, the basic property that distinguishes BHs from NSs is the presence of the event horizon as well as a CF in the vicinity of a BH (Ebisawa et al. 1996). In fact, close to the event horizon, the strong gravitational force dominates the pressure forces and leads to an almost free fall CF of accreting material into a BH. The dynamic Comptonization of low energy photons off fastmoving electrons dominates the thermal Comptonization at a high-mass accretion rate and the plasma temperature of the CF is less than 10­15 keV; as a result, an extended steep power law is formed (see Titarchuk et al. 1997; Titarchuk & Zannias 1998; Laurent & Titarchuk 1999). These kinds of spectra are observed in the soft state of BH binaries (see, e.g., ST09 and TS09). On the other hand, in NS sources the radiation pressure forces become dominant close to their surface; thus, a free fall should be suppressed at high-mass accretion rates. Does the presence of the firm surface in the NS make any difference in the dependence of the photon index versus mass accretion rate with respect to that established in BHs (see ST09; TS09; ST10)? Furthermore, can the index saturation detected in many BHs with the mass accretion rate exist only in BH sources, since it has not yet been observed in NS sources? For example, Di Salvo et al. (2006), studying low-mass X-ray binaries hosting NSs, concluded that it is unlikely to distinguish BHs from NSs based on their X-ray spectra. However, FT11 argue that in NS sources the index 2


The Astrophysical Journal, 738:128 (20pp), 2011 September 10 Table 1 List of BeppoSAX Observations of 4U 1728-34 Used in the Analysis ObsID 20674001 20889003 Start Time (UT) 1998 Aug 23 19:15:27 1999 Aug 19 02:01:32 End Time (UT) 1998 Aug 24 09:14:15 1999 Aug 20 04:54:32

Seifina & Titarchuk

MJD Interval 51048.8­51049.41 51409.1­51410.22

References. 1 Di Salvo et al. 2000; 2 Piraino et al. 2000.

varies weakly until the soft photon illumination of the transition layer, Qd , is much smaller than the energy release in the TL, Qcor . We try to test further this kind of index behavior in the NS source using X-ray observations of the atoll source 4U 1728-34 and compare it, if possible, with the index dependence on the mass accretion rate established in BHs. In this paper, we present the analysis of the available BeppoSAX observations during 1998­1999 and RXTE/PCA/ HEXTE observations during 1996­2000 for 4U 1728-34. 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 an evolution of X-ray spectral and timing properties during the state transition in Sections 4 and 5. We discuss our results and make our conclusions in Sections 6 and 7. 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; Parmar et al. 1997) for 0.3­4 keV, the Medium Energy Concentrator Spectrometer (MECS; Boella et al. 1997) for 1.8­10 keV, and the Phoswich Detection System (PDS; Frontera et al. 1997) for 15­60 keV. The SAXDAS data analysis package was 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 were renormalized 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. After the fitting procedure we checked whether these normalizations were in a standard range for each instrument.7 Specifically, the LECS/MECS renormalization ratio is 0.92, and the PDS/MECS renormalization ratio is 0.97. In addition, spectra were rebinned according to energy resolution of the instruments in order to obtain significant data points. We rebinned the LECS spectra, applying a rebinning template for grouping (lecs_2.grouping) with an energy-dependent binning factor used in GRPPHA of XSPEC.8 We also rebinned the PDS spectra with a linear binning factor of two, grouping two bins together (resulting bin width is 1 keV). In Table 1 we list the BeppoSAX observations used in our analysis. We also analyzed the available data obtained with RXTE (Bradt et al. 1993), which were found in the time period from 1996 February to 2000 July (see also the review by Galloway et al. 2008). In our investigation we selected 127 observations made at different count rates (luminosity states) with a good coverage of rise­decay flare track. We performed an analysis of RXTE observations of 4U 1728-34 during four years for eight intervals, indicated by blue rectangles in Figure 1 (top). We also
7 8

analyzed two BeppoSAX observations, whose dates are marked by green triangles in Figure 1. Standard tasks of the HEASOFT/FTOOLS 5.3 software package were utilized for data processing. For spectral analysis we used PCA Standard 2 mode data, collected in the 3­20 keV energy range. The standard dead time correction procedure was applied to the data. The average dead time correction is in the range 3%­10% depending on the count rate value. HEXTE data were used in order to construct broadband spectra. We subtracted the background corrected in off-source observations. To exclude the channels with the largest uncertainties, we used only data in the 20­60 keV energy range for the spectral analysis. The HEXTE data were renormalized based on the PCA. Typical PCA/HEXTE renormalization factor is 0.98. We used the data that are available through the GSFC public archive (http://heasarc.gsfc.nasa.gov). In Table 2 we list the groups of RXTE observations that cover the source evolution from quiescent to flare events. Note that we did not use any normalization factor to normalize between BeppoSAX and RXTE data. We also used public 4U 1728-34 data from the ASM on board RXTE. We retrieved the ASM light curves (in the 2­12 keV energy range) from the public RXTE/ASM archive at MIT.9 In the bottom panel of Figure 1 we show a mean count rate (blue dashed line) during the 1996­2010 interval of ASM/RXTE monitoring observations of 4U 172834. In this panel one can also see a long-term quasi-periodic variability of mean soft flux during a cycle of six years. We investigate available periods of slow variability (indicated by green) during which we also have the BeppoSAX observations of 4U 1728-34 (see also the upper panel of Figure 1). We use definitions of the low- and high-luminosity states to relate these states to the source luminosity, and we demonstrate that during the high­low state transition the electron temperature of the Compton cloud changes from 2.5 keV to 15 keV, and vice versa, respectively. Thus, the "high spectral state" corresponds to the "low electron temperature state" and vice versa the "low spectral state" corresponds to the "high electron temperature state." During the flare seen in the ASM light curve the electron temperature kTe usually decreases from 15 keV to 2.5 keV. We introduce a definition of a "burst" to point out a significant increase in X-ray flux (about a factor of five) with respect to the persistent emission level. Specifically, we identify a "burst" when the ASM count rate is greater than 10 counts s-1 . We associate the count rate increase with the increase in the mass accretion rate. 3. SPECTRAL ANALYSIS In our spectral data analysis we use a model that consists of a sum of the Comptonization (COMPTB) component (COMPTB is the XSPEC Contributed model;10 see Farinelli et al. 2008,
9 10

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

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

3


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- RXTE - BeppoSAX

4U 1728-34

R1- R8

Figure 1. Top: evolution of the ASM/RXTE count rate during 1996­2006 observations of 4U 1728-34. Blue rectangles indicate the RXTE/PCA & HEXTE data of pointed observations, and green triangles show BeppoSAX NFI data used for analysis. Bottom: ASM/RXTE 1 day-type light curve of 4U 1728-34 during 1996­2010. The blue dashed line shows a mean count rate and indicates the long-term quasi-periodic variability of mean soft flux during a cycle of six years. The green double arrow points out the 1996­2000 time interval of RXTE observations used in our analysis (see R1­R8 intervals). (A color version of this figure is available in the online journal.) Table 2 List of Groups of the RXTE Observation of 4U 1728-34 Used in the Analysis Number of Set R1 R2 R3 R4 R5 R6 R7 R8 Dates (MJD) 50128­50143 50710­50728 51086­51196 51409­51443 51237­51359 51198­51213 51667­51733 51652­51657 RXTE Proposal ID 10073 20083 30042 40019 40027 40033 50023 50029 Dates (UT) 1996 Feb 15­Mar 1 1997 Sep 19­Oct 1 1998 Sep 30­1999 Jan 18 1999 Aug 19­Sep 22 1999 Feb 27­Jul 10 1999 Jan 20­Feb 4 2000 Mar 7­Jul 8 2000 Apr 18­23 Rem. Ref. 1, 2, 3, 4, 5, 7, 8 3, 4, 5, 7, 8 4 8 6, 8 6, 8 6, 8 6, 8

BeppoSAX

References. (1) Strohmayer et al. 1996; (2) Ford & van der Klis 1998; (3) van Straaten et al. 2001; (4) Di Salvo et al. 2001; (5) Mendez et al. 2001; (6) Migliari et al. 2003; (7) Jonker et al. 2000; (8) TS05.

hereafter F08), a soft blackbody component of temperature TBB , and the 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 , Comptonization fraction f (f = A/(1 + A)), and the normalization of the seed photon spectrum NCOMPTB (see the Appendix for the definition of NCOMPTB ). 4

In Figure 2 we illustrate our spectral model as a basic model for fitting the BeppoSAX and RXTE spectral data for 4U 1728-34. We assume that accretion onto an NS takes place when the material passes through the main two regions, a geometrically thin accretion disk (standard Shakura­Sunyaev disk; see 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


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Transition layer

Neutron Star

Disk photons

Figure 2. Suggested geometry of the system. Disk and NS soft photons are upscattered (Comptonized) in the relatively hot plasma of the TL (between the accretion disk and the NS surface). Some fraction of these photons is 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.)

spectrum is formed in the TL region, where disk BB-like seed photons and NS soft photons are upscattered in the relatively hot plasma. Some fraction of these seed soft photons can also be seen directly by the Earth observer. 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 Figure 3 (for BeppoSAX data) and in Figures 4 and 5 (for RXTE data). Spectral analysis of BeppoSAX and RXTE observations indicates that X-ray spectra of 4U 1728-34 can be described by the model, while its Comptonization component can be presented by the COMPTB model. Moreover, for broadband BeppoSAX observations, this spectral model component is modified by photoelectric absorption at low energies. Also following the suggestions of Di Salvo et al. (2000) and Piraino et al. (2000), we add a Gaussian line at 6.7 keV and a thermal blackbody component at low energies (1­4 keV) to improve the fit statistics. Along with these components, Di Salvo et al. (2000) included a narrow Gaussian line to fit an excess in the residuals around 1.7 keV. We also test the presence of this line feature, but the addition of this component to the model does not improve the quality of the model fit. It is worth noting that D' Ai et al. (2006) analyzed simultaneous Chandra and RXTE observations of 4U 1728-34 (2002 March 2­5). They fitted the 1.2­35 keV continuum spectrum with a blackbody plus a Comptonized component, and they fitted large residuals at 6­10 keV by a broad (FWHM 2keV) Gaussian emission line or, alternatively, by two absorption edges associated with low ionized iron and Fe xxv/xxvi. However, in the framework of this model, D' Ai et al. (2006) found no evidence of broad or narrow Fe K lines between 6 and 7 keV. However, using our model wabs*(blackbody+COMPTB+Gaussian) we found an iron line feature during all BeppoSAX and RXTE observations. 5

On the top of Figure 3 we demonstrate the best-fit BeppoSAX spectrum using our model, and in the bottom right panel we show the best-fit spectrum along with for the model (reduced 2 = 1.16 for 445 dof). In particular, we find that the addition of the soft blackbody-like component of temperature TBB = 0.5­0.7 keV to the model significantly improves the fit quality of the BeppoSAX spectra. The line emission is clearly detected in the range from 5 to 8 keV, as one can see from the left bottom panel of Figure 3. We show that this line is quite broad, and it is much wider than the instrumental response whose width is smaller than 0.02 keV.11 Thus, we include in the model asimple Gaussian component 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 range. We also include in the model the interstellar absorption with a column density NH . It should be mentioned that we fixed certain parameters of the COMPTB component: = 3 (low energy index of the seed photon spectrum) and = 0 because we neglect the efficiency of the bulk inflow effect versus the thermal Comptonization in the case of NS source 4U 1728-34. For the BeppoSAX data (see Tables 1 and 3) we find that the spectral index is of 1.03 ± 0.04 (or the corresponding photon index = + 1 is 2.03 ± 0.04). While the temperature of the seed photons Ts of the COMPTB component changes from 1.2 to 1.3 keV, the color temperature of the soft Blackbody component TBB is around 0.6 keV. Unfortunately, RXTE detectors cannot provide wellcalibrated spectra below 3 keV, while the broad energy band of BeppoSAX telescopes allows us to determine the parameters of blackbody components at soft energies. Thus, in order to fit the RXTE data we have to fix the temperature of the blackbody component at a value of TBB = 0.7 keV, obtained as an upper
11

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


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keV (keV/cm2 s keV)

10

-4

10

-3

0.01

0.1

1

1

10 channel energy (keV)

100

10

10

1

1

-1

normalized counts s-1 keV

0.01

normalized counts s-1 keV
S
2

-1

0.1

0.1

0.01

10

-3

10

-3

10

-4

10

-4

10

-5

10

-5

10

-6

4 2
â

2 0 -2

2

â

S

0 -2 -4 2 5 10 Energy (keV) 20 50

2

5

10
Energy (keV)

20

50

100

Figure 3. Top: best-fit spectrum of 4U 1728-34 in E F (E ) units using BeppoSAX observation 20889003 carried out on 1999 April 19. The data are presented by crosses and the best-fit spectral model wabs*(blackbody+COMPTB+Gaussian) by the green line. The model components are shown by red, crimson, and blue lines for blackbdody, COMPTB, and Gaussian components, respectively. Bottom panels: spectrum in units of counts along with . Left bottom panel: best-fit spectrum and for the model fit without the line component (reduced 2 = 2.15 for 445 dof). Right bottom panel: same as that on the left one but with the addition of the Gaussian (K -line) component (reduced 2 = 1.16 for 445 dof). The best-fit model parameters are = 2.07 ± 0.04, Te = 3.29 ± 0.04 keV, Eline = 6.0 ± 0.1 keV, and EWline = 51 ± 11 eV (see more details in Table 3). (A color version of this figure is available in the online journal.)

6


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Figure 4. Best-fit RXTE spectrum of 4U 1728-34 for the low-luminosity state in units E F (E ) (top) and the spectra in count units (bottom panels) with 2 for the 30042-03-01-00 observation. Left bottom panel: fit of the model wabs COMPTB,(red = 2.1 for 61 dof). Right bottom panel: same as the latter one but with 2 the addition of an iron Gaussian line and the blackbody component, namely, using the model wabs (blackbody + COMPTB + Gaussian) (red = 1.18 for 57 dof). The best-fit model parameters are = 1.99 ± 0.02, Te = 10.4 ± 0.3 keV, and Eline = 6.54 ± 0.03 keV (see more details in Table 4). Red, violet, and blue lines stand for blackbody, COMPTB, and Gaussian components, respectively. (A color version of this figure is available in the online journal.)

7


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Figure 5. Best-fit RXTE spectrum of 4U 1728-34 for the high-luminosity state in units E F (E ) 2 for the 50023-01-12-00 observation. Left bottom panel: fit of the model wabs COMPTB (red = 1.79 the addition of an iron Gaussian line and the Blackbody component, namely, using the model wabs The best-fit model parameters are = 1.99 ± 0.03, Te = 5.5 ± 0.1 keV, and Eline = 6.75 ± 0.04 keV for Blackbody, COMPTB, and Gaussian components, respectively. (A color version of this figure is available in the online journal.)

(top) and the spectra in count units (bottom panel) with for 61 dof). Right bottom panel: same as the latter one but with 2 (blackbody + COMPTB + Gaussian) (red = 1.2 for 57 dof). (see more details in Table 4). Red, violet, and blue lines stand

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 Te of the COMPTB component varies from 2.5 to 15 keV, while photon index is 8

almost constant ( = 1.99 ± 0.02) for all observations. It is worth noting that the width line of the Gaussian component does not vary much and is in the range of 0.3­0.6 keV. Color temperature Ts of the COMPTB component is around 1.3 keV,


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Table 3 Best-Fit Parameters of the Spectral Analysis of BeppoSAX Observations of 4U 1728-34 in the 0.3­60 keV Energy Rangea Observational ID 20674001 20889003 MJD (day) 51048.70 51409.50 TBB (keV) 0.47(3) 0.62(5) NBB
b

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

= -1 0.99(7) 1.07(4)

Te (keV) 3.76(8) 3.29(4)

log(A) 0.10(4) 1.06(6)

N

COMPTB

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

Nline

b

EWline (eV) 52(16) 51(11)



2 red

(dof)

2.65(2) 1.61(1)

4.18(3) 3.56(2)

0.55(4) 0.43(4)

1.25(457)