X-ray Dips Research Articles

2 Years ofINTEGRALMonitoring of GRS 1915+105. I. Multiwavelength Coverage withINTEGRAL,RXTE, and the Ryle Radio Telescope

We report the results of simultaneous monitoring observations of the Galactic microquasar GRS 1915+105 with INTEGRAL and RXTE from 3 up to ~300 keV, and the Ryle Telescope at 15 GHz. We first identify the classes of variability in which GRS 1915+105 is found, and report some direct transitions between them. The accretion ejection connections are studied in a model-independent manner through the source light curves, hardness ratio, and color-color diagrams. During a period of steady X-ray state (class χ) we observe a steady radio flux interpreted as the signature of a compact jet. We then turn to three particular observations during which we observe several types of soft X-ray dip and spike cycles, followed by radio flares, corresponding to classes ν, λ, and β types of variability. This is the first time ejections are reported during a class λ observation. We generalize the fact that a (nonmajor) discrete ejection always occurs, in GRS 1915+105, as a response to an X-ray sequence composed of a spectrally hard X-ray dip terminated by an X-ray spike marking the disappearance of the emission above 18 keV. We identify the trigger of the ejection as the X-ray spike. A possible correlation between the amplitude of the radio flare and the duration of the X-ray dip is found. The X-ray dips prior to ejections could thus represent the time during which the source accumulates energy and material that is ejected later. The fact that these results do not rely on any spectral modelling enhances their robustness.

2 Years ofINTEGRALMonitoring of GRS 1915+105. II. X‐Ray Spectro‐Temporal Analysis

This is the second paper presenting the results of 2 yr of monitoring of GRS 1915+105 with INTEGRAL, RXTE, and the Ryle Telescope. We present the X-ray spectral and temporal analysis of four observations showing strong radio to X-ray correlations. During one observation GRS 1915+105 was in a steady state, while during the three others it showed cycles of X-ray dips and spikes (followed by radio flares). Through time-resolved spectroscopy of these cycles, we suggest that the soft X-ray spike is the trigger of the ejection. The ejected medium is then the coronal material responsible for the hard X-ray emission. In the steady state observation, the X-ray spectrum is indicative of the hard-intermediate state, with the presence of a relatively strong emission at 15 GHz. The X-ray spectrum is the sum of a Comptonized component and an extra power law extending to energies >200 keV without any evidence for a cutoff. We observe a possible correlation of the radio flux with that of the power law component, which may indicate that we see direct emission from the jet at hard X-ray energies. We study the energy dependence of a ~4 Hz QPO during the hard-intermediate state observation. The QPO ``spectrum'' is well modeled by a power law with a cutoff at an energy of about 11 keV, and clearly differs from the relative contribution of the Comptonized component to the overall flux. This may rule out models of global oscillations of the Compton corona.

INTEGRAL observations of Hercules X-1

Aims. We investigate the X-ray spectral and timing properties of the accreting X-ray pulsar Her X-1 observed with the INTEGRAL satellite in July–August 2005. Methods. The data analyzed in this work cover a substantial part of one main-on state of the source. The short time scale pulse period development is measured. X-ray pulse profiles for different energy ranges and time intervals are constructed. Pulse-averaged and pulse-phase resolved broad band X-ray spectra are studied. Spectral changes during X-ray dips are explored. Results. The X-ray pulse profiles are found to change significantly during the period of observations. For the first time a strong spinup is measured within one 35 d cycle. Spectral characteristics observed during the X-ray dips are consistent with their interpretation as being due to partial covering as has been reported by several authors. The fundamental cyclotron absorption line is firmly observed in both pulse-averaged and pulse-phase resolved X-ray spectra. The energy, width, and the depth of the line are found to vary significantly with pulse phase.

A Parameterization Study of the Properties of the X‐Ray Dips in the Low‐Mass X‐Ray Binary X1916−053

The ultracompact low-mass X-ray binary X1916–053, which is composed of a neutron star and a semidegenerate white dwarf, exhibits periodic X-ray dips of variable width and depth. We have developed a new method to parameterize this dipping behavior to systematically study its variations. This can help in further understanding binary and accretion disk behaviors. Observations by the Rossi X-Ray Timing Explorer from 1998 clearly show a 4.87 day periodic variation in the dip width. This is probably due to nodal precession of the accretion disk, although an epoch-folding search finds no significant sidebands in the spectrum. From the negative-superhump model of Larwood et al., the mass ratio can be estimated as q = 0.045. Combining more than 24 years of historical data, we find an orbital period derivative orb/Porb = (1.62 ± 0.34) × 10−7 yr−1 and establish a quadratic ephemeris for the X-ray dips. This period derivative seems inconsistent with the prediction from the standard model of binary orbital evolution proposed by Rappaport et al. On the other hand, Tavani's radiation-driven model can properly account for the period derivative, although the large mass outflow predicted by this model has never been observed in this system. With the best ephemeris, we find that the standard deviation of the primary dips is smaller than that of the secondary dips. This means that the primary dips are more stable than the secondary dips. Thus, we conclude that the primary dips of X1916–053 occur from the bulge where the accretion stream encounters the rim of the disk, rather than the inner disk ring as previously proposed by Frank et al.

X-ray Spectroscopy of Dips of Cir X-1

We present X-ray spectral analyses of the low-mass X-ray binary Cir X-1 during X-ray dips, using the Rossi X-ray Timing Explorer (RXTE) data. Each dip was divided into several segments, and the spectrum of each segment was fitted with a three-component blackbody model, in which the first two components are affected by partial covering and the third one is unaffected. A Gaussian emission line is also included in the spectral model to represent the Fe K alpha line at similar to 6.4keV. The fitted temperatures of the two partially covered components are about 2 keV and I keV, while the uncovered component has a temperature of similar to 0.5-0.6keV. The equivalent blackbody emission radius of the hottest component is the smallest and that of the coolest component is the largest. During the dips the fluxes of the two hot components are linearly correlated, while that of the third component does not show any significant variation. The Fe line flux remains constant, within the errors, during the short dips. However, during the long dips the line flux varies significantly and is positively correlated with the fluxes of the two hot components. These results suggest: (1) that the temperature of the X-ray emitting region decreases with radius, (2) that the Fe Ka line emitting region is close to the hot continuum emitting region, and (3) that the size of the Fe line emitting region is larger than that of the obscuring matter causing the short dips but smaller than the region of that causing the long dips.

Detection of the radial velocity curve of the B5-A0 supergiant companion star of Cir X-1?

In this paper, we report on phase-resolved I-band optical spectroscopic and photometric observations of Cir X-1 obtained with the Very Large Telescope. The spectra are dominated by Paschen absorption lines at nearly all orbital phases except near phase zero (coinciding with the X-ray dip) when the absorption lines are filled in by broad Paschen emission lines. The radial velocity curve of the absorption lines corresponds to an eccentric orbit (e = 0.45) whose period and time of periastron passage are consistent with the period and phase predicted by the most recent X-ray dip ephemeris. We found that the I-band magnitude decreases from 17.6 to ~16.8 near phase 0.9-1.0 this brightening coincides in phase with the X-ray dip. Even though it is likely that the absorption-line spectrum is associated with the companion star of Cir X-1, we cannot exclude the possibility that the spectrum originates in the accretion disc. However, if the spectrum belongs to the companion star, it must be a supergiant of spectral type B5-A0. If we assume that the compact object does not move through the companion star at periastron, the companion star mass is constrained to solar for a 1.4-M solar neutron star, whereas the inclination has to be . Alternatively, the measured absorption lines and their radial velocity curve can be associated with the accretion disc surrounding a 1.4-M solar neutron star and its motion around the centre of mass. An absorption-line spectrum from an accretion disc is typically found when our line of sight passes through the accretion disc rim implying a high inclination. In this scenario, the companion star mass is found to be ~0.4 M solar . However, from radio observations it was found that the angle between the line of sight and the jet axis is smaller than 5°. This would mean that the jet ploughs through the accretion disc in this scenario, making this solution less probable.

Disc precession in the M31 dipping X-ray binary Bo 158?

We present results from three XMM-Newton observations of the M31 low mass X-ray binary (LMXB) XMMU J004314.4+410726.3 (Bo 158), spaced over 3 d in 2004 July. Bo 158 was the first dipping LMXB to be discovered in M31. Periodic intensity dips were previously seen to occur on a 2.78-h period, due to absorption in material that is raised out of the plane of the accretion disc. The report of these observations stated that the dip depth was anticorrelated with source intensity. In light of the 2004 XMM-Newton observations of Bo 158, we suggest that the dip variation is due to precession of the accretion disc. This is to be expected in LMXBs with a mass ratio ≲0.3 (period ≲4 h), as the disc reaches the 3:1 resonance with the binary companion, causing elongation and precession of the disc. A smoothed particle hydrodynamics simulation of the disc in this system shows retrograde rotation of a disc warp on a period of ∼11Porb, and prograde disc precession on a period of 29 ± 1Porb. This is consistent with the observed variation in the depth of the dips. We find that the dipping behaviour is most likely to be modified by the disc precession, hence we predict that the dipping behaviour repeats on an 81 ± 3 h cycle.

RXTE observations of the dipping low-mass X-ray binary 4U 1624–49

We analyse ~ 360 ks of archival data from the Rossi X-Ray Timing Explorer (RXTE) of the 21 hr orbital period dipping low-mass X-ray binary 4U 1624-49. We find that outside the dips the tracks in the colour-colour and hardness-intensity diagrams (CDs and HIDs) are reminiscent of those of atoll sources in the middle and upper parts of the banana branch. The tracks show secular shifts up to ~ 10%. We study the power spectrum of 4U 1624-49 as a function of the position in the CD. This is the first time power spectra of this source are presented. No quasi-periodic oscillations (QPOs) are found. The power spectra are dominated by very low frequency noise (VLFN), characteristic for atoll sources in the banana state, and band limited noise (BLN) which is not reliably detected but may, uncharacteristically, strengthen and increase in frequency with spectral hardness. The VLFN fits to a power law, which becomes steeper when the source moves to the harder part of the CD. We conclude that 4U 1624-49 is an atoll source which in our observations is in the upper banana branch. Combining this with the high (0.5-0.7 L_Edd) luminosity, the long-term flux stability of the source as seen with the RXTE All-Sky Monitor (ASM), and with the fact that it is an X-ray dip source, we conclude that 4U 1624-49 is most likely a GX atoll source such as GX 3+1 and GX 9+9, but seen edge on.

A possible new dipping X-ray source in the field of M 31

We report the discovery of the new dipping X-ray source, XMMU J004308.6+411247, in M31, during a systematic search for periodicities in XMM-Newton archival observations. During the 2002 January 6 observation, the dips recur with a 107 min period and the source count rate is consistent with zero at the dip minimum. Dips with the same modulation and period are also observed during the XMM-Newton observations carried out on 2000 June 25, 2001 June 29 and the Chandra observation of 2001 October 5. The dips of XMMU J004308.6+411247 show no evidence of energy dependence. The average X-ray flux of XMMU J004308.6+411247 is nearly constant across different observations (~1.e37 erg/s for an assumed M31 distance of 780 kpc in the 0.3-10 keV band); the spectrum is well fit by an absorbed power law with a photon index ~0.8 or an absorbed Comptonization model. The photo-electric absorption is consistent with the Galactic value in the source direction. If XMMU J004308.6+411247 is located in M31 its properties are consistent with those of dipping low mass X-ray binaries in the Galaxy. Present observations do not allow to distinguish between dips and eclipses. The possibility that XMMU J004308.6+411247 is a foreground X-ray source cannot be ruled out at present; in this case the source might be a magnetic cataclysmic variable.

A dipping black hole X-ray binary candidate in NGC 55

XMM-Newton EPIC observations have revealed a bright point-like X-ray source in the nearby Magellanic-type galaxy NGC 55. At the distance of NGC 55, the maximum observed X-ray luminosity of the source, designated as XMMU J001528.9-391319, is L(x) ~ 1.6 x 10^{39} erg/s, placing the object in the ultraluminous X-ray source (ULX) regime. The X-ray lightcurve exhibits a variety of features including a significant upward drift over the 60 ks observation. Most notably a series of X-ray dips are apparent with individual dips lasting for typically 100-300 s. Some of these dips reach almost 100 percent diminution of the source flux in the 2.0-4.5 keV band. The EPIC CCD spectra can be modelled with two spectral components, a very soft powerlaw continuum (Gamma ~ 4) dominant below 2 keV, plus a multi-colour disc (MCD) component with an inner-disc temperature kT ~ 0.8 keV. The observed upperward drift in the X-ray flux can be attributed to an increase in the level of the MCD component, whilst the normalisation of the powerlaw continuum remains unchanged. The dipping episodes correspond to a loss of signal from both spectral components, although the blocking factor is at least a factor two higher for the MCD component. XMMU J001528.9-391319 can be considered as a candidate black-hole binary (BHB) system. A plausible explanation of the observed temporal and spectral behaviour is that we view the accretion disc close to edge-on and that, during dips, orbiting clumps of obscuring material enter our line of sight and cause significant blocking or scattering of the hard thermal X-rays emitted from the inner disc. In contrast, the more extended source of the soft powerlaw flux is only partially covered by the obscuring matter during the dips.

X-ray behaviour of Circinus X-1 - I. X-ray dips as a diagnostic of periodic behaviour

We examine the periodic nature of detailed structure (particularly dips) in the Rossi X-Ray Timing Explorer/All Sky Monitor (RXTE/ASM) light-curve of Circinus X-1. The significant phase wandering of the X-ray maxima suggests their identification with the response on a viscous time-scale of the accretion disc to perturbation. We find that the X-ray dips provide a more accurate system clock than the maxima, and thus use these as indicators of the times of periastron passage. We fit a quadratic ephemeris to these dips, and find its predictive power for the X-ray light-curve to be superior to ephemerides based on the radio flares and the full archival X-ray light-curve. Under the hypothesis that the dips are tracers of the mass transfer rate from the donor, we use their occurrence rate as a function of orbital phase to explore the (as yet unconstrained) nature of the donor. The high P term in the ephemeris provides another piece of evidence which shows that Cir X-1 is in a state of dynamical evolution, and thus is a very young post-supernova system. We further suggest that the radio 'synchrotron nebula' immediately surrounding Cir X- 1 is, in fact, the remnant of the event that created the compact object, and discuss briefly the evidence for and against such an interpretation.

Discovery of narrow X-ray absorption features from the low-mass X-ray binary X 1254-690 with XMM-Newton

We report on two XMM-Newton observations of the low-mass X-ray binary X 1254-690. During an XMM-Newton observation of the low-mass X-ray binary in 2001 January a deep X-ray dip was seen while in a second observation one year later no dips were evident. The 0.5–10 keV EPIC spectra from both non-dipping intervals are very similar being modeled by a disk-blackbody and a power-law continuum with additional structure around 1 keV and narrow absorption features at 7.0 keV and 8.2 keV which are identified with the Kα and Kβ absorption lines of Fe xxvi. The low-energy structure may be modeled as a 175 eV (σ) wide emission line at ~0.95 keV. This feature is probably the same structure that was modeled as an absorption edge in an earlier BeppoSAX observation. The absorption line properties show no obvious dependence on orbital phase and are similar in both observations suggesting that the occurrence of such features is not directly related to the presence of dipping activity. Narrow Fe absorption features have been observed from the two superluminal jet sources GRO J1655-40 and GRS 1915+105, and the four low-mass X-ray binaries GX 13+1, MXB 1658-298, X 1624-490 and X 1254-690. Since the latter 3 sources are dipping sources, which are systems viewed close to the accretion disk plane, and the two microquasars are thought to be viewed at an inclination of ~70°, this suggests that these features are more prominent when viewed at high-inclination angles. This, together with the lack of any orbital dependence, implies a cylindrical geometry for the absorbing material.

Observational evidence for the accretion-disk origin for a radio jet in an active galaxy.

Accretion of gas onto black holes is thought to power the relativistic jets of material ejected from active galactic nuclei (AGN) and the 'microquasars' located in our Galaxy. In microquasars, superluminal radio-emitting features appear and propagate along the jet shortly after sudden decreases in the X-ray fluxes. This establishes a direct observational link between the black hole and the jet: the X-ray dip is probably caused by the disappearance of a section of the inner accretion disk as it falls past the event horizon, while the remainder of the disk section is ejected into the jet, creating the appearance of a superluminal bright spot. No such connection has hitherto been established for AGN, because of insufficient multi-frequency data. Here we report the results of three years of monitoring the X-ray and radio emission of the galaxy 3C120. As has been observed for microquasars, we find that dips in the X-ray emission are followed by ejections of bright superluminal knots in the radio jet. The mean time between X-ray dips appears to scale roughly with the mass of the black hole, although there are at present only a few data points.

Optical Identification of the X-Ray Burster X1746−370 in the Globular Cluster NGC 6441

We find convincing observational evidence to confirm the optical identification of the X-ray burster X1746-370, located in the globular cluster NGC 6441. Chandra HRC-I imaging yields a much improved X-ray position for the source, which we show to be fully consistent with our rederived position of a UV-excess star, U1, in the same astrometric reference frame. In addition, the smaller Chandra X-ray error circle excludes the only other blue stars previously identified in the old Einstein circle. We have also obtained Hubble Space Telescope STIS time-resolved optical spectra of star U1. Although there are no strong line features, the flux distribution demonstrates U1 to be unusually bright in the blue and faint in the red, consistent with earlier WFPC2 photometry. More notably, the flux level of the continuum is seen to vary significantly compared with stars of similar brightness. Indeed, the light curve can plausibly be fitted by a sinusoid with period 5.73 hr, which is the period of the recurring X-ray dips seen in this source. The presence of modulations in both wavelengths strengthens the case for an orbital origin and, therefore, deepens the puzzle of the unusual energy-independent X-ray dips. Lastly, we note that X1746-370 remains the longest-period confirmed X-ray burster in a globular cluster, and the only one with a period typical of the Galactic population as a whole.

Ejection of the inner accretion disk in GRS 1915+105: The magnetic rubber-band effect

We examine theoretically the behaviour of the inner accretion disk in GRS 1915+105 when soft X-ray dips are present in the X-ray light curve. We assume the presence of a radial shock in the accretion disk, as in some of the Two Component Advective Flow (TCAF) solutions. We discuss the behaviour of the flux tubes inside a TCAF (which we name Magnetized TCAF or MTCAF model for brevity) and compare various competing forces on the flux tubes. In this MTCAF model, we find that the magnetic tension is the strongest force in a hot plasma of temperature $\gsim 10^{10}$K and as a result, magnetic flux tubes entering in this region collapse catastrophically, thereby occasionally evacuating the inner disk. We postulate that this magnetic `rubber-band' effect induced evacuated disk matter produces the blobby components of outflows and IR/radio jets. We derive the size of the post-shock region by equating the time scale of the Quasi-Periodic Oscillations to the infall time of accreting matter in the post-shock region and found the shock location to be $\sim 45-66 r_g$. We calculate the transition radius $r_{tr}$, where the Keplerian disk deviates into a sub-Keplerian flow, to be $\sim 320r_g$. Based on the derived X-ray spectral parameters, we calculate the mass of this region to be $\sim10^{18}$g. We conclude that during the X-ray dips the matter in the post-shock region, which manifests itself as the thermal-Compton component in the X-ray spectrum, is ejected, along with some sub-Keplerian matter in the pre-shock region.

BeppoSAX observation of the eclipsing dipping X-ray binary X 1658-298

Results of a 2000 August 12{13 BeppoSAX observation of the 7.1 hr eclipsing, dipping, bursting, transient, low-mass X-ray binary (LMXRB) X 1658-298 are presented. The spectrum outside of eclipses, dips and bursts can be modeled by the combination of a soft disk-blackbody and a harder Comptonized component with a small amount (1:3 10 21 atom cm 2 ) of low-energy absorption. In contrast, an RXTE observation 18 months earlier during the same outburst, measured an absorption of 5:0 10 22 atom cm 2 . Such a change is consistent with a thinning of the accretion disk as the outburst progresses. Structured residuals from the best-t spectral model are present which are tentatively identied with Ne-K/Fe-L and Fe-K shell emission. The spectral changes during dips are complex and may be modeled by a strong (3 10 23 atom cm 2 ) increase in absorption of the Comptonized component only, together with reductions in normalizations of both spectral components. This behavior is in contrast to the \complex continuum model for X-ray dip sources, where the softer blackbody component rapidly suers strong absorption. It is however, similar to that found during recent XMM-Newton observations of the eclipsing, dipping, LMXRB EXO 0748-676. An updated orbital ephemeris is provided which does not require a quadratic term, in contrast to that of Wachter et al. (2000).

Observational evidence for mass ejection during soft X-ray dips in GRS 1915+105

We investigate the connection between the X-ray and radio properties of the Galactic microquasar GRS1915+105, by analyzing the X-ray data observed with RXTE, during the presence of a huge radio flare (~450 mJy). The X-ray lightcurve shows two dips of ~100 second duration. Detailed time resolved spectral analysis shows the existence of three spectral components: a multicolor disk-blackbody, a Comptonized component due to hot plasma and a power-law. We find that the Comptonized component is very weak during the dip. This is further confirmed by the PHA ratio of the raw data and ratio of the lightcurves in different energy bands. These results, combined with the fact that the 0.5 -- 10 Hz QPO disappears during the dip and that the Comptonized component is responsible for the QPO lead to the conclusion that during the dips the matter emitting Comptonized spectrum is ejected away. This establishes a direct connection between the X-ray and radio properties of the source.

Timing Analysis of the Light Curve of the Dipping‐Bursting X‐Ray Binary X1916−053

We present the timing analysis results for our observations of the X-ray dip source X1916-053 conducted with RXTE between February and October of 1996. Our goal was to finally measure the binary period—as either the X-ray dip period or the ~1% longer optical modulation period—thereby establishing whether the binary has a precessing disk (SU UMa model) or a third star (triple model). Combined with historical data (1979-1996), the X-ray dip period is measured to be 3000.6508 ± 0.0009 s with a 2 σ upper limit || ≤ 2.06 × 10-11. From our quasi-simultaneous optical observations (1996 May 14-23) and historical data (1987-1996), we measure the optical modulation period to be 3027.5510 ± 0.0052 s with a 2 σ upper limit || ≤ 2.28 × 10-10. The two periods are therefore each stable (over all recorded data) and require a 3.9087 ± 0.0008 day beat period. This beat period, and several of its harmonics, is also observed as variations in the dip shape. Phase modulation of X-ray dips, observed in a 10 consecutive day observation, is highly correlated with the ~3.9 day dip shape modulation. The 1987-1996 optical observations show that the optical phase fluctuations are a factor of 3 larger than those in the X-ray. We discuss SU UMa versus triple models to describe the X1916-053 light-curve behavior and conclude that the X-ray dip period, with smaller phase jitter, is probably the binary period, so that the required is most likely similar to that observed in SU UMa and X-ray nova systems. However, the precession period stability, and especially the fact that the times of X-ray bursts may partially cluster to occur just after X-ray dips, continues to suggest that this system may be a hierarchical triple.

Discovery of Nearly Coherent Oscillations with a Frequency of ∼567 H[CLC]z[/CLC] during Type I X-Ray Bursts of the X-Ray Transient and Eclipsing Binary X1658−298

We report the discovery of nearly coherent oscillations with a frequency of approximately 567 Hz during type I X-ray bursts from the X-ray transient and eclipsing binary X1658-298. If these oscillations are directly related to the neutron star rotation, then the spin period of the neutron star in X1658-298 is approximately 1.8 ms. The oscillations can be present during the rise or decay phase of the bursts. Oscillations during the decay phase of the bursts show an increase in frequency of approximately 0.5-1 Hz. However, in one particular burst the oscillations reappear at the end of the decay phase at about 571.5 Hz. This represents an increase in oscillation frequency of about 5 Hz, which is the largest frequency change seen so far in a burst oscillation. It is unclear if such a large change can be accommodated by present models used to explain the frequency evolution of the oscillations. The oscillations at 571.5 Hz are unusually soft compared to the oscillations found at 567 Hz. We also observed several bursts during which the oscillations are detected at much lower significance or not at all. Most of these bursts happen during periods of X-ray dipping behavior, suggesting that the X-ray dipping might decrease the amplitude of the oscillations (although several complications exist with this simple picture). We discuss our discovery in the framework of the neutron star spin interpretation.

A Hard Tail in the Broadband Spectrum of the Dipper XB 1254−690

We report on the results of spectral analysis of the dipping source XB 1254-690 observed by the BeppoSAX satellite. We find that the X-ray dips are not present during the BeppoSAX observation, in line with recent RXTE results. The broadband (0.1-100 keV) energy spectrum is well fitted by a three-component model consisting of a multicolor disk blackbody with an inner disk temperature of ~0.85 keV, a Comptonized spectrum with an electron temperature of ~2 keV, and bremsstrahlung at a temperature of ~20 keV. Adopting a distance of 10 kpc and taking into account a spectral hardening factor of ~1.7 (because of electron scattering which modifies the blackbody spectrum emitted by the disk), we estimated that the inner disk radius is Rin 1/2 ~ 11 km, where i is the inclination angle of the system with respect to the line of sight. The Comptonized component could originate in a spherical corona or boundary layer, surrounding the neutron star, with optical depth ~19. The bremsstrahlung emission, contributing ~4% of the total luminosity, probably originates in an extended accretion disk corona with radius ~1010 cm. In this scenario, we calculated that the optical depth of this region is ~0.71 and its mean electron density is Ne ~ 2.7 × 1014 cm-3. This last component might also be present in other low-mass X-ray binaries, but, because of its low intrinsic luminosity, it is not easily observable. We also find an absorption edge at ~1.27 keV with an optical depth of ~0.15. Its energy could correspond to the L edge of Fe XVII or K edge of Ne X or neutral Mg.