arXiv subjects

S. Chaty

Publications and source records attributed to S. Chaty.

At least 109 records · Page 6Linked to original sources

arXiv2003

Optical and near-infrared observations of the microquasar V4641 Sagittarii during the 1999 September outburst

We present photometric and spectroscopic optical and near-infrared (NIR) observations (Based on observations collected at the European Southern Observatory, Chile (ESO ID 63.H-0493 and 64.H-0382)) taken during the outburst of the microquasar V4641 Sgr = SAX J1819.3-2525 (in'tzand et al., 2000) in September 1999. We observed an increase in the J-Ks colour between 5 and 8 days after the outburst, which we interpret as likely evidence for the presence of dust around the source. We also observed an extraordinarily strong, broad and variable H_alpha line, with a velocity width of 4560 km/s suggesting the presence of a high-velocity outflow component. We constrain the distance of the system between 3 and 8 kpc, locating it further away than previously derived from radio observations (Hjellming et al., 2000), but consistent with Orosz et al. (2001). We then discuss the nature of this system, showing that the companion star is either a B3-A2 main sequence star, or a B3-A2 sub-giant crossing the Hertzsprung gap. The system is therefore an Intermediate or High Mass X-ray Binary System (IMXB or HMXB). The distance derived by these optical/NIR observations implies that the jets observed by Hjellming et al. (2000) would then exhibit apparent velocities of ~ 10 c. We finally discuss the possibility of an interaction between the jets and surroundings of the source, and also of this source being a ``microblazar''.

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arXiv2002

Identification of a 12-17 day timescale in X-ray observations of GRS 1915+105

Measurement of the energy emitted from accreting astrophysical systems provides an observational constraint on the plasma processes that may be operating within the disk. Here we examine the continual time variation over the past six years of the total X-ray flux from the microquasar GRS 1915+105. The application of differencing and rescaling techniques to RXTE/ASM data shows that the small amplitude fluctuations scale up to 12-17 days. A 17-day timescale in the X-ray fluctuations corresponds to half the measured binary orbital period of this system (33.5 +/- 1.5 days). While this may be coincidental, it is possible that these two timescales may be linked by, for example, a turbulent cascade in the accretion disk driven by a tidally-induced two-armed spiral shock corotating with the binary system. Temporal scaling is found only in the ever-present small fluctuations, and not in the intermittent larger-amplitude fluctuations. This is consistent with the basic model for this source which consists of a steady, cold outer disk and an unstable inner disk.

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arXiv2002

The Infrared Counterpart of the X-ray Burster KS 1731--260

We present JK' infrared images of the X-ray transient KS 1731--260, obtained in 1997 and in 1998 with IRAC2b at the ESO/MPI 2.2-m telescope at La Silla. Using as a reference the recent Chandra position, we confirm the identification of the X-ray source with the previously proposed counterpart (Wijnands et al. 2001b), for which we measure J=17.32 +/- 0.2 and K' = 16.36 +/- 0.18. The source was entering a low X-ray state at the epoch of our observations, and the accretion disk was still dominating the infrared flux. Indeed, when compared with the only published magnitudes in the J-band (Orosz et al. 2001), obtained with the source in quiescence, our photometry confirms the fading of the counterpart during the decay of the RXTE/ASM lightcurve.

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arXiv2002

Study of the Largest Multiwavelength Campaign of the Microquasar GRS 1915+105

We present the results from a multiwavelength campaign of GRS 1915+105 performed from 2000 April 16 to 25. This is one of the largest coordinated set of observations ever performed for this source, covering the wide energy band in radio (13.3-0.3 cm), near-infrared (J-H-K), X-rays and Gamma-rays (from 1 keV to 10 MeV). During the campaign GRS 1915+105 was predominantly in the "plateau" (or low/hard) state but sometimes showed soft X-ray oscillations: before April 20.3, rapid, quasi-periodic (~= 45 min) flare-dip cycles were observed. The radio flares observed on April 17 shows frequency- dependent peak delay, consistent with an expansion of synchrotron-emitting region starting at the transition from the hard-dip to the soft-flare states in X-rays. On the other hand, infrared flares on April 20 appear to follow (or precede) the beginning of X-ray oscillations with an inconstant time delay of ~= 5-30 min. This implies that the infrared emitting region is located far from the black hole by >~ 10E13 cm, while its size is <~ 10E12 cm constrained from the time variability. We find a good correlation between the quasi-steady flux level in the near-infrared band and in the X-ray band. From this we estimate that the reprocessing of X-rays, probably occurring in the outer parts of the accretion disk, accounts for about 20-30% of the observed K magnitude in the plateau state. The OSSE spectrum in the 0.05-10 MeV band is represented by a single power law with a photon index of 3.1 extending to ~1 MeV with no cutoff. The power-law slope above ~30 keV is found to be very similar between different states in spite of large flux variations in soft X-rays, implying that the electron energy distribution is not affected by the change of the state in the accretion disk.

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arXiv2001

Near Infrared Observations of Galactic Black Hole Candidates

We report on several European Southern Observatory (ESO) near-infrared (NIR) observational campaigns aimed at understanding the nature of galactic black hole candidates. Our results, including NIR photometry of the sources GRO J1655-40, GRS 1739-278, GRS 1716-249, GRS 1121-68 and GX 339-4, show that all the sources but GRO J1655-40 are consistent with low-mass stars as the companion star of the binary system. By locating the counterparts on a colour-magnitude diagram (CMD), we better constrain the spectral type of the companion star of three of the systems considered here, and confirm a fourth one. The spectral types are respectively: M0-5 V for GRS 1716-249, F8-G2 III for GX 339-4 and later than F5 V for GRS 1739-278. We confirm the already known spectral type of the companion in GRS 1121-68 (K0-5 V). The location of GRO J1655-40 on the CMD is consistent with the sub-giant luminosity class and with this source crossing the Hertzsprung gap. However, a non-stellar emission seems to contribute to the NIR flux of this source.

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arXiv2001

The Evolving Accretion Disc in the Black Hole X-ray Transient XTE J1859+226

We present HST, RXTE, and UKIRT observations of the broad band spectra of the black hole X-ray transient XTE J1859+226 during the decline from its 1999-2000 outburst. Our UV spectra define the 2175A interstellar absorption feature very well and based on its strength we estimate E(B-V)=0.58+/-0.12. Hence we deredden our spectra and follow the evolution of the spectral energy distribution on the decline from outburst. We find that the UV and optical data, and the X-ray thermal component when detectable, can be fit with a simple blackbody model of an accretion disc heated by internal viscosity and X-ray irradiation, and extending to close to the last stable orbit around the black hole, although the actual inner radius cannot be well constrained. During the decline we see the disc apparently evolving from a model with the edge dominated by irradiative heating towards one where viscous heating is dominant everywhere. The outer disc radius also appears to decrease during the decline; we interpret this as evidence of a cooling wave moving inwards and discuss its implications for the disc instability model. Based on the normalisation of our spectral fits we estimate a likely distance range of 4.6-8.0kpc, although a value outside of this range cannot securely be ruled out.

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arXiv2001

A search for radio emission from Galactic supersoft X-ray sources

We have made a deep search for radio emission from all the northern hemisphere supersoft X-ray sources using the VLA and MERLIN telescopes, at 5 and 8.4 GHz. Three previously undetected sources: T Pyx, V1974 Cygni and RX J0019.8+2156 were imaged in quiescence using the VLA in order to search for any persistent emission. No radio emission was detected in any of the VLA fields down to a typical 1 sigma RMS noise of 20 uJy/beam, however, 17 new point sources were detected in the fields with 5 GHz fluxes between 100 and 1500 uJy giving an average 100 uJy-source density of around 200 per square degree, comparable to what was found in the MERLIN HDF survey. The persistent source AG Draconis was observed by MERLIN to provide a confirmation of previous VLA observations and to investigate the source at a higher resolution. The core is resolved at the milliarcsec scale into two components which have a combined flux of around 1 mJy. It is possible that we are detecting nebulosity which is becoming resolved out by the higher MERLIN resolution. We have investigated possible causes of radio emission from a wind environment, both directly from the secondary star, and also as a consequence of the high X-ray luminosity from the white dwarf. There is an order of magnitude discrepancy between observed and modelled values which can be explained by the uncertainty in fundamental quantities within these systems.

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arXiv2001

Characterising anomalous transport in accretion disks from X-ray observations

Whilst direct observations of internal transport in accretion disks are not yet possible, measurement of the energy emitted from accreting astrophysical systems can provide useful information on the physical mechanisms at work. Here we examine the unbroken multi-year time variation of the total X-ray flux from three sources: Cygnus X-1, the microquasar GRS1915+105, and for comparison the nonaccreting Crab nebula. To complement previous analyses, we demonstrate that the application of advanced statistical methods to these observational time-series reveals important contrasts in the nature and scaling properties of the transport processes operating within these sources. We find the Crab signal resembles Gaussian noise; the Cygnus X-1 signal is a leptokurtic random walk whose self-similar properties persist on timescales up to three years; and the GRS1915+105 signal is similar to that from Cygnus X-1, but with self-similarity extending possibly to only a few days. This evidence of self-similarity provides a robust quantitative characterisation of anomalous transport occuring within the systems.

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arXiv2001

Multiwavelength observations revealing the outbursts of the soft X-ray transients XTE J1859+226 and XTE J1118+480

We report multiwavelength observations of the two soft X-ray transients (SXTs) XTE J1859+226 and XTE J1118+480, which we observed with HST, RXTE and UKIRT. The two sources exhibited very different behaviour. XTE J1859+226 showed a thermal-viscous disc instability outburst modified by irradiation. XTE J1118+480, which we also observed with EUVE since it is located at a very high galactic latitude and suffers from very low extinction, is much more unusual. It exhibits i) a low X-ray to optical flux ratio and ii) a strong non-thermal contribution throughout the spectrum, which is likely to be due to synchrotron emission. We concentrate here on their evolution in the course of their outbursts.

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arXiv2001

Complete and Simultaneous Spectral Observations of the Black-Hole X-ray Nova XTE J1118+480

The X-ray nova XTE J1118+480 suffers minimal extinction (b = 62 degrees) and therefore represents an outstanding opportunity for multiwavelength studies. Hynes et al. (2000) conducted the first such study, which was centered on 2000 April 8 using UKIRT, EUVE, HST and RXTE. On 2000 April 18, the Chandra X-ray Observatory obtained data coincident with a second set of observations using all of these same observatories. A 30 ks grating observation using Chandra yielded a spectrum with high resolution and sensitivity covering the range 0.24-7 keV. Our near-simultaneous observations cover approximately 80% of the electromagnetic spectrum from the infrared to hard X-rays. The UV/X-ray spectrum of XTE J1118+480 consists of two principal components. The first of these is an approximately 24 eV thermal component which is due to an accretion disk with a large inner disk radius: > 35 Schwarzschild radii. The second is a quasi power-law component that was recorded with complete spectral coverage from 0.4-160 keV. A model for this two-component spectrum is presented in a companion paper by Esin et al. (2001).

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arXiv2001

Multiwavelength observations revealing the outbursts of the two soft X-ray transients XTE J1859+226 and XTE J1118+480

We report here on multiwavelength observations of the two new soft X-ray transients (SXTs) XTE J1859+226 and XTE J1118+480, which we observed with HST/RXTE/UKIRT. For XTE J1118+480 we also used EUVE since it is located at a very high galactic latitude and suffers from very low extinction. The two sources exhibited very different behaviour. XTE J1859+226 seems quite normal and therefore a good object for testing the accretion mechanisms in place during the outbursts, XTE J1118+480 is much more unusual because it exhibits i) a low X-ray to optical ratio and ii) a strong non-thermal contribution in the radio to optical domain, which is likely to be due to synchrotron emission. We concentrate here on the near-infrared (NIR) and optical observations of these two systems.

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arXiv2001

On the nature of the microquasar V4641 Sagittari

We present photometric and spectroscopic optical and near-infrared (NIR) observations (Based on observations collected at the European Southern Observatory, Chile (ESO ID 63.H-0493 and 64.H-0382)) taken during the outburst of the recently flaring source V4641 Sgr = SAX J1819.3-2525 (in't Zand et al., 2000), on September 1999. This source was independently detected as the RXTE source XTE J1819-254 (Markwardt et al. 1999), and afterwards identified with the variable object V4641 Sgr (Kato et al. 1999). It underwent a bright optical outburst on 1999 Sept. 15.7 UT, going from magnitude 14 to 8.8 in the V-band (Stubbings 1999), and reaching 12.2 Crab in the X-rays (Smith et al. 1999) and Ks nearly 13. This outburst was therefore bright, but very brief, with an e-fold decay time of 0.6 days. A radio source was resolved, making of V4641 Sgr a new microquasar (Hjellming et al. 2000). We discuss the nature of this system, showing that our observations suggest a distance farther than previously derived from the radio observations (Hjellming et al. 2000). The distance of the system would be between 3 and 8 kpc, the companion star being a B3-A2 main sequence star. Another possibility is that the companion star is crossing the Hertzsprung gap (type B3-A2 IV), and in this case the distance cited above would be the minimum distance of the system. The system is therefore an Intermediate or High Mass X-ray Binary System (IMXB or HMXB). The inconsistency regarding the distance between the radio and optical/NIR observations could be explained by the detection of an interaction between matter ejected before the X-ray outburst and the surrounding medium of the source. If this is confirmed, this source could be added to the short list of microquasars where such an interaction has been detected.

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arXiv2000

A search for possible interactions between ejections from GRS 1915+105 and the surrounding interstellar medium

We have observed an extended region surrounding the first discovered galactic superluminal source GRS 1915+105, seeking evidence of interaction between the relativistic ejecta of that object and the interstellar medium. We find two radio sources axisymmetrically aligned along the sub-arcsecond relativistic ejecta of GRS 1915+105 and roughly 17' distant from it, which coincide with the luminous IRAS sources 19124+1106 and 19132+1035. We have observed these sources at centimeter (VLA), millimeter (IRAM 30m), and infrared (ISO, UKIRT, ESO/MPI 2.2m) wavelengths in both line and continuum emission. At centimeter wavelengths a non-thermal jet-like feature aligned along the outflow axis is located adjacent to the inner edge of the southern source. Strong density enhancements are found in the millimeter tracers CO and H13CO+ at the positions of both sources and some of the morphology is reminiscent of shock-like interactions; however, linewidths are narrow. At infrared wavelengths strong hydrogen recombination lines and weak lines of molecular hydrogen are observed at the southern source. We discuss these results as possible evidence of the sought-after interaction, both in terms of the regions undergoing ongoing shock-heating and in terms of them being locations of shock-induced star formation. The evidence for each of these is inconclusive. Millimeter line mapping of a portion of W 50 where the relativistic jets of the X-ray binary SS 433 interact with the interstellar medium shows roughly similar morphology as GRS 1915+105, suggesting that the phenomena observed at the IRAS sources may not be unusual for such a long distance interaction.

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arXiv2000

On the nature of the microquasar V4641 Sagittarii

We present photometric and spectroscopic optical and near-infrared (NIR) observations taken during the outburst on September 1999 of the source V4641 Sgr = SAX J1819.3-2525 (in'tzand et al., 2000) = XTE J1819-254 (Markwardt et al., 1999). We show that our observations suggest a distance between 4 and 8 kpc, the spectral type of the companion star being constrained between B3 and A2 V, making the system a High Mass X-ray Binary System (HMXB). In view of the radio and optical/NIR observations, it is possible that the ejecta of the source interacted with the surrounding medium of the source.

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arXiv2000

The X-ray transient XTE J1118+480: Multiwavelength observations of a low-state mini-outburst

We present multiwavelength observations of the newly discovered X-ray transient XTE J1118+480 obtained in the rising phase of the 2000 April outburst. This source is located at unusually high Galactic latitude and in a very low absorption line of sight. This made the first EUVE spectroscopy of an X-ray transient outburst possible. Together with our HST, RXTE, and UKIRT data this gives unprecedented spectral coverage. We find the source in the low hard state. The flat IR-UV spectrum appears to be a combination of optically thick disk emission and flat spectrum emission, possibly synchrotron, while at higher energies, including EUV, a typical low hard state power-law is seen. EUVE observations reveal no periodic modulation, suggesting an inclination low enough that no obscuration by the disk rim occurs. We discuss the nature of the source and this outburst and conclude that it may be more akin to mini-outbursts seen in GRO J0422+32 than to a normal X-ray transient outburst.

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arXiv2000

Search for interactions between ejections of GRS 1915+105 and its environment

To unravel the effect of likely interactions between the energetic ejections of the galactic superluminal source GRS 1915+105 and its surrounding interstellar medium, we observed its environment. Two IRAS sources are symmetrically placed with respect to GRS 1915+105, and are aligned with the sub-arcsec ejections of this source. We analyzed these two sources IRAS 19124+1106 and IRAS 19132+1035 through near-infrared, millimeter and centimeter wavelengths. The evidence for these regions being interaction zones seems inconclusive.

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arXiv2000

A Search for the Optical/Infrared Counterpart of the Anomalous X-ray Pulsar 1E 1841-045

We have carried out a search for the optical and infrared counterpart of the Anomalous X-ray Pulsar 1E 1841-045, which is located at the center of the supernova remnant Kes73. We present the first deep optical and infrared images of the field of 1E 1841-045, as well as optical spectroscopy results that exclude the brightest objects in the error circle as possible counterparts. A few of the more reddened objects in this region can be considered as particularly interesting candidates, in consideration of the distance and absorption expected from the association with Kes73. The strong interstellar absorption in the direction of the source does not allow to completely exclude the presence of main sequence massive companions.

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arXiv2000

VLT observations of GRS 1915+105

We present near infrared spectroscopy of the superluminal microquasar \grs\ obtained with the first unit of the VLT (Based on observations collected at the European Southern Observatory, Chile (ESO No 63.H-0261).) and the ISAAC spectro-imager. The emission features detected in the VLT data have been identified as He I, Br $γ$, He II and Na I. The detection of Na I is reported here for the first time, while our confirmation of weak He II emission provides support to previous marginal detections of this feature. By comparing the observed spectra with those of massive stars, we find that our results are very consistent with \grs being a high mass X-ray system with an early type primary, as previously proposed by Chaty et al. (1996) and Mirabel et al. (1997). The VLT spectra also provide evidence of P Cygni profiles, that turn into blue emission wings when the system is in outburst. This observed line profile evolution implies that \grs must be surrounded by an expanding envelope, that is partially blown out during the X-ray outbursts. The presence of such circumstellar gaseous material around \grs is more naturally understood in the context of a massive luminous star than if the system was a low-mass X-ray binary.

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