Black Hole Candidate XTE J1118 Research Articles

A closer look at the X-ray transient XTE J1908+094: identification of two new near-infrared candidate counterparts

We had reported in Chaty, Mignani, Israel (2002) on the near-infrared (NIR) identification of a possible counterpart to the black hole candidate XTE J1908+094 obtained with the ESO/NTT. Here, we present new, follow-up, CFHT adaptive optics observations of the XTE J1908+094 field, which resolved the previously proposed counterpart in two objects separated by about 0.8. Assuming that both objects are potential candidate counterparts, we derive that the binary system is a low-mass system with a companion star which could be either an intermediate/late type (A-K) main sequence star at a distance of 3-10 kpc, or a late-type ($>$K) main sequence star at a distance of 1-3 kpc. However, we show that the brighter of the two objects (J ~ 20.1, H ~ 18.7, K' ~ 17.8) is more likely to be the real counterpart of the X-ray source. Its position is more compatible with our astrometric solution, and colours and magnitudes of the other object are not consistent with the lower limit of 3 kpc derived independently from the peak bolometric flux of XTE J1908+094. Further multi-wavelength observations of both candidate counterparts are crucial in order to solve the pending identification.

Iron-line and continuum flux variations in the RXTE spectra of the black hole candidate XTE J1650-500

We present the results of spectral fits made to 57 pointed observations of the Galactic black hole candidate and X-ray transient XTE J1650−500, made with the Rossi X-ray Timing Explorer (RXTE) in 2001 when the source was in a transition from the hard state to the soft state. A strong and variable Fe Kα emission line is detected in these spectra. The line flux varies in a non-linear way with the hard X-ray flux, in apparent contradiction to the predictions of simple disc reflection models. We observe a change in the overall trend that coincides with changes in the continuum X-ray spectrum and the fast X-ray variability. The Fe-line flux versus hard X-ray flux variations are consistent with the predictions of reflection models that consider high disc-ionization states and with a model that considers gravitational light-bending effects. Indications for an anticorrelation between the Fe-line flux and the hard X-ray flux in the spectrally hardest observations and weak variations in the Fe-line energy (as observed with XMM–Newton and BeppoSAX) slightly favour the light-bending interpretation.

MMT Observations of the Black Hole Candidate XTE J1118+480 near and in Quiescence

We report on the analysis of new and previously published MMT optical spectra of the black hole binary XTE J1118+480 during the decline from the 2000 outburst to true quiescence. From cross-correlation with template stars, we measure the radial velocity of the secondary to derive a new spectroscopic ephemeris. The observations acquired during approach to quiescence confirm the earlier reported modulation in the centroid of the double-peaked Halpha emission line. Additionally, our data combined with the results presented by Zurita et al. (2002) provide support for a modulation with a periodicity in agreement with the expected precession period of the accretion disk of ~52 day. Doppler images during the decline phase of the Halpha emission line show evidence for a hotspot and emission from the gas stream: the hotspot is observed to vary its position, which may be due to the precession of the disk. The data available during quiescence show that the centroid of the Halpha emission line is offset by about -100 km/s from the systemic velocity which suggests that the disk continues to precess. A Halpha tomogram reveals emission from near the donor star after subtraction of the ring-like contribution from the accretion disk which we attribute to chromospheric emission. No hotspot is present suggesting that accretion from the secondary has stopped (or decreased significantly) during quiescence. Finally, a comparison is made with the black hole XRN GRO J0422+32: we show that the Halpha profile of this system also exhibits a behaviour consistent with a precessing disk.

On the Origin of Radio Emission in the X‐Ray States of XTE J1650−500 during the 2001–2002 Outburst

We report on simultaneous radio and X-ray observations of the black hole candidate XTE J1650-500 during the course of its 2001-2002 outburst. The scheduling of the observations allowed us to sample the properties of XTE J1650-500 in different X-ray spectral states, namely, the hard state, the steep power-law state, and the thermal dominant state, according to the recent spectral classification of McClintock & Remillard. The hard state is consistent with a compact jet dominating the spectral energy distribution at radio frequencies; however, the current data suggest that its contribution as direct synchrotron emission at higher energies may not be significant. In that case, XTE J1650-500 may be dominated by Compton processes (either inverse Comptonization of thermal disk photons and/or synchrotron self-Compton radiation from the base of the compact jet) in the X-ray regime. We surprisingly detect a faint level of radio emission in the thermal dominant state that may be consistent with the emission of previously ejected material interacting with the interstellar medium, similar (but on a smaller angular scale) to what was observed in XTE J1550-564 by Corbel and coworkers. Based on the properties of radio emission in the steep power-law state of XTE J1650-500 and taking into account the behavior of other black hole candidates (namely, GX 339-4, XTE J1550-564, and XTE J1859+226) while in the intermediate and steep power-law states, we are able to present a general pattern of behavior for the origin of radio emission in these two states that could be important for understanding the accretion-ejection coupling very close to the black hole event horizon.

The relativistic Fe emission line in XTE J1650-500 with BeppoSAX: evidence for black hole spin and light-bending effects?

We report spectral results from three BeppoSAX observations of the black hole candidate XTE J1650-500 during its 2001/2002 outburst. We find strong evidence for the presence of a broad and strongly relativistic Fe emission line. The line profile indicates an accretion disc extending down to two gravitational radii (or less) suggesting the presence of a rapidly rotating central Kerr black hole. Thanks to the broadband spectral coverage of BeppoSAX, we could analyze the 1.5-200 keV spectra of the three observations and report the presence of a strong reflection component from the accretion disc, which is totally consistent with the observed broad Fe emission line. The shape of the reflection component appears to be affected by the same special and general relativistic effects that produce the broad Fe line. We study the variation of the different spectral components from the first to the third observation and we find that they are well reproduced by a recently proposed light bending model.

Broadband spectra of XTE J1650-500 with BeppoSAX

Abstract We report on preliminary results of three Target of Opportunity (TOO) observations of the X-ray transient and black hole candidate XTE J1650-500 performed on 2001 September 11-12, September 21-23, and October 3-4 respectively, with the Narrow Field Instruments (0.1-200 keV) of the BeppoSAX satellite. Inspection of the light curves in different energy bands clearly shows an extreme variability both temporal and spectral. This variability has the consequence that spectra provide acceptable fits only if integrated on a temporal scale of order 104s or even less. However, all spectra are fit by the same model with some parameters that remain unchanged during the 3 TOOs, and others that dramatically change on the above time scale. The model used is a two-phase accretion disk corona model, in which a hot corona with a hybrid electron distribution comptonizes seed photons from a disk. Reflection is important and ionization seems to be very small or even zero. During the first TOO a broad Fe Kαline is clearly seen.

State transitions in the 2001/2002 outburst of XTE J1650–500

We present a study of the X-ray transient and black hole candidate XTE J1650-500 during its 2001/2002 outburst. The source made two state transitions between the hard and soft states, at luminosity levels that differed by a factor of ~5-10. The first transition, between hard and soft, lasted for ~30 days and showed two parts; one part in which the spectral properties evolve smoothly away from the hard state and another that we identify as the 'steep power law state'. The two parts showed different behavior of the Fe K emission line and QPO frequencies. The second transition, from soft to hard, lasted only \~15 days and showed no evidence of the presence of the 'steep power law state'. Comparing observations from the early rise and the decay of the outburst, we conclude that the source can be in the hard state in a range of more 10^4 in luminosity. We briefly discuss the state transitions in the framework of a two-flow model.

BeppoSAX study of the spectral and temporal behavior of the black hole candidate XTE J1118+480

Abstract The high galactic latitude black–hole candidate XTE J1118+480 was followed–up by BeppoSAX during its outburst and transition to quiescence with four TOOs from April to December 2000. In this paper, we summarize and briefly discuss the results of the spectral and temporal data analysis of these observations.

Radio and X-ray observations during the outburst decay of the black hole candidate XTE J1908+094

Obtaining simultaneous radio and X-ray data during the outburst decay of soft X-ray transients is a potentially important tool to study the disc - jet connection. Here we report results of the analysis of (nearly) simultaneous radio (VLA or WSRT) and Chandra X-ray observations of XTE J1908+094 during the last part of the decay of the source after an outburst. The limit on the index of a radio - X-ray correlation we find is consistent with the value of 0.7 which was found for other black hole candidates in the low/hard state. Interestingly, the limit we find seems more consistent with a value of 1.4 which was recently shown to be typical for radiatively efficient accretion flow models. We further show that when the correlation-index is the same for two sources one can use the differences in normalisation in the radio - X-ray flux correlation to estimate the distance towards the sources if the distance of one of them is accurately known (assuming black hole spin and mass and jet Lorentz factor differences are unimportant or minimal). Finally, we observed a strong increase in the rate of decay of the X-ray flux. Between March 23, 2003 and April 19, 2003 the X-ray flux decayed with a factor ~5 whereas between April 19, 2003 and May 13, 2003, the X-ray flux decreased by a factor ~750. The source (0.5-10 keV) luminosity at the last Chandra observation was L~3x10^32 (d/8.5 kpc)^2 erg s^-1.

Chandra/High Energy Transmission Grating Spectrometer Spectroscopy of the Galactic Black Hole GX 339−4: A Relativistic Iron Emission Line and Evidence for a Seyfert‐like Warm Absorber

We observed the Galactic black hole GX 339-4 with the Chandra High Energy Transmission Grating Spectrometer (HETGS) for 75 ks during the decline of its 2002-2003 outburst. The sensitivity of this observation provides an unprecedented glimpse of a Galactic black hole at about a tenth of the luminosity of the outburst peak. The continuum spectrum is well described by a model consisting of multicolor disk blackbody (kT~=0.6 keV) and power-law (Gamma~=2.5) components. X-ray reflection models yield improved fits. A strong, relativistic Fe Kalpha emission line is revealed, indicating that the inner disk extends to the innermost stable circular orbit. The breadth of the line is sufficient to suggest that GX 339-4 may harbor a black hole with significant angular momentum. Absorption lines from H- and He-like O and He-like Ne and Mg are detected, as well as lines that are likely due to Ne II and Ne III. The measured line properties make it difficult to associate the absorption with the coronal phase of the interstellar medium. A scenario wherein the absorption lines are due to an intrinsic AGN-like warm-absorber geometry-perhaps produced by a disk wind in an extended disk-dominated state-may be more viable. We compare our results to Chandra observations of the Galactic black hole candidate XTE J1650-500 and discuss our findings in terms of prominent models for Galactic black hole accretion flows and connections to supermassive black holes.

Spectral and Temporal Behavior of the Black Hole Candidate XTE J1118+480 as Observed withBeppoSAX

XTE J1118+480 is a well-established black hole candidate with a mass estimate in the range of 7-10 M☉. With BeppoSAX we observed the source four times, from 2000 April to December. Results of the first observation have already been reported. Here we report spectral results of the later observations, performed in 2000 May, June, and December and compare them with the results obtained from the 2000 April observation. We observe a decrease of the column density from a value consistent with the Galactic N obtained from radio measurements to a value a factor of 2 lower. The spectra are well fitted with a thermal Comptonization plus blackbody model. The blackbody luminosity decreases with time, while the electron temperature of the Comptonizing electrons does not show significant change. A Compton reflection component is apparent and stable, although weak (mean value of Ω/2π = 0.21). The reflector shows a low metallicity (mean value of Z/Z☉ = 0.13). On the basis of the spectral results, a hot central disk appears to be the best scenario for the high-energy photons, while the temporal properties point to a nonthermal origin of a fraction of the soft X-ray photons, likely synchrotron emission internal to the hot disk.

Discovery of the near-infrared counterpart of the X-ray transient XTE J1908+094: The counterpart of XTE J1908+094

We report on the identification in the near-infrared (NIR) of the counterpart to the newly discovered black hole candidate XTE J1908+094, achieved partly through ‘Target of Opportunity’ (ToO) observations carried out at the ESO/NTT. Accurate astrometry and photometry performed on three sets of observations of the source covering 1.7 months have shown a decrease of its NIR flux, consistent with the X-ray decay. After dereddening for the measured column density, we computed the absolute magnitudes of the source for different distances. We derive that the source is a low-mass system and that the companion star is a main sequence star with a spectral type later than K.

Evidence of Spin and Energy Extraction in a Galactic Black Hole Candidate: The [ITAL]XMM-Newton[/ITAL]/EPIC-[CLC]pn[/CLC] Spectrum of XTE J1650−500

We observed the Galactic black hole candidate XTE J1650-500 early in its fall of 2001 outburst with the XMM-Newton European Photon Imaging pn Camera (EPIC-pn). The observed spectrum is consistent with the source having been in the state. We find a broad, skewed Fe Kα emission line that suggests the primary in this system may be a Kerr black hole and that indicates a steep disk emissivity profile that is hard to explain in terms of a standard accretion disk model. These results are quantitatively and qualitatively similar to those from an XMM-Newton observation of the Seyfert galaxy MCG -6-30-15. The steep emissivity in MCG -6-30-15 may be explained by the extraction and dissipation of rotational energy from a black hole with nearly maximal angular momentum or from material in the plunging region via magnetic connections to the inner accretion disk. If this process is at work in both sources, an exotic but fundamental general relativistic prediction may be confirmed across a factor of 106 in black hole mass. We discuss these results in terms of the accretion flow geometry in stellar-mass black holes and the variety of enigmatic phenomena often observed in the very high state.

Observational Signatures of Black Holes: Spectral and Temporal Features of XTE J1550−564

The theoretical predictions of the converging inflow, or Bulk-Motion Comptonization model are discussed and some predictions are compared to X- and gamma-ray observations of the high-soft state of Galactic black hole candidate XTE J1550+564. The approx. 10(exp 2)-Hz QPO phenomenon tends to be detected in the high-state at times when the bolometric luminosity surges and the hard-powerlaw spectral component is dominant. Furthermore, the power in these features increases with energy. We offer interpretation of this phenomenon, as oscillations of the innermost part of the accretion disk, which in turn supplies the seed photons for the converging inflow where the hard power-law is formed through Bulk Motion Comptonization (BMC). We further argue that the noted lack of coherence between intensity variations of the high-soft-state low and high energy bands is a natural consequence of our model, and that a natural explanation for the observed hard and soft lag phenomenon is offered. In addition, we address some criticisms of the BMC model supporting our claims with observational results.

Correlated fast X-ray and optical variability in the black-hole candidate XTE J1118+480.

Black holes become visible when they accrete gas, a common source of which is a close stellar companion. The standard theory for this process (invoking a 'thin accretion disk') does not explain some spectacular phenomena associated with these systems, such as their X-ray variability and relativistic outflows, indicating some lack of understanding of the actual physical conditions. Simultaneous observations at multiple wavelengths can provide strong constraints on these conditions. Here we report simultaneous high-time-resolution X-ray and optical observations of the transient source XTE J1118+480, which show a strong but puzzling correlation between the emissions. The optical emission rises suddenly following an increase in the X-ray output, but with a dip 2-5 seconds in advance of the X-rays. This result is not easy to understand within the simplest model of the optical emission, where the light comes from reprocessed X-rays. It is probably more consistent with an earlier suggestion that the optical light is cyclosynchrotron emission that originates in a region about 20,000 km from the black hole. We propose that the time dependence is evidence for a relatively slow (<0.1c), magnetically controlled outflow.

Multiwavelength Observations of the Black Hole Candidate XTE J1550−564 during the 2000 Outburst

We report optical, infrared, and X-ray light curves for the outburst, in 2000, of the black hole candidate XTE J1550-564. We find that the start of the outburst in the H and V bands precedes that seen in the Rossi X-Ray Timing Explorer All-Sky Monitor by 11.5 ± 0.9 and 8.8 ± 0.6 days, respectively; a similar delay has been observed in two other systems. About 50 days after the primary maxima in the VIH light curves, we find secondary maxima, most prominently in H. This secondary peak is absent in the X-ray light curve but coincides with a transition to the low/hard state. We suggest that this secondary peak may be due to nonthermal emission associated with the formation of a jet.

X‐Ray States and Radio Emission in the Black Hole Candidate XTE J1550−564

We report on radio and X-ray observations of the black hole candidate (BHC) XTE J1550-564 performed during its 2000 X-ray outburst. Observations were conducted with the Australia Telescope Compact Array and allowed us to sample the radio behavior of XTE J1550-564 in the X-ray low hard and intermediate/very high states. We observed optically thin radio emission from XTE J1550-564 5 days after a transition to an intermediate/very high state, but we observed no radio emission 6 days later, while XTE J1550-564 was still in the intermediate/very high state. In the low hard state, XTE J1550-564 is detected with an inverted radio spectrum. The radio emission in the low hard state most likely originates from a compact jet; optical observations suggest that the synchrotron emission from this jet may extend up to the optical range. The total power of the compact jet might therefore be a significant fraction of the total luminosity of the system. We suggest that the optically thin radio emission detected 5 days after the transition to the intermediate/very high state is due to a discrete ejection of relativistic plasma during the state transition. Subsequent to the decay of the optically thin radio emission associated with the state transition, it seems that in the intermediate/very high state the radio emission is quenched by a factor greater than 50, implying a suppression of the outflow. We discuss the properties of radio emission in the X-ray states of BHCs.

Correlated X‐Ray Spectral and Timing Behavior of the Black Hole Candidate XTE J1550−564: A New Interpretation of Black Hole States

We present an analysis of RXTE data of the X-ray transient XTE J1550-564. The source went through several states, which were divided into spectrally soft and hard states. These states showed up as distinct branches in the color-color diagram, forming a structure with a comb-like topology; the soft state branch forming the spine and the hard state branches forming the teeth. Variability was strongly correlated with the position on the branches. The broad band noise became stronger, and changed from power law like to band limited, as the spectrum became harder. Three types of QPOs were found: 1-18 Hz and 102-284 Hz QPOs on the hard branches, and 16-18 Hz QPOs on and near the soft branch. The frequencies of the high and low frequency QPOs on the hard branches were correlated with each other, and anti-correlated with spectral hardness. The changes in QPO frequency suggest that the inner disc radius only increases by a factor of 3-4 as the source changes from a soft to a hard state. Our results on XTE J1550-564 strongly favor a 2-dimensional description of black hole behavior, where the regions near the spine of the comb in the color-color diagram can be identified with the high state, and the teeth with transitions from the high state, via the intermediate state (which includes the very high state) to the low state, and back. The two physical parameters underlying this behavior vary to a large extent independently and could for example be the mass accretion rate through the disk and the size of a Comptonizing region.

Optical Observations of the Black Hole Candidate XTE J1550−564 during Reflare and Quiescence

We report optical monitoring of the soft X-ray transient XTE J1550-564 during the 1999 season (1999 January 4-August 24). The first optical observations available in 1999 show that the peak "reflare" brightness had exceeded the peak brightness of the initial optical flare in 1998 September by over half a magnitude. We compare the optical reflare light curves with the total X-ray flux, the power-law flux, and disk flux light curves constructed from the spectral fits to RXTE/PCA data. During the first 60 days of the observed optical reflare, we find no correspondence between the thermal component of the X-rays often associated with a disk and the optical flux—the former remains essentially flat, whereas the latter declines exponentially and exhibits three substantial dips. However, the power-law flux is anticorrelated with the optical dips, suggesting that the optical flux may be up scattered into the X-ray by the hot corona. Periodic modulations were discovered during the final stage of the outburst (May-June), with P = 1.546 ± 0.038 days, and during quiescence (July and August), with P = 1.540 ± 0.008 days. The analysis of the combined data set reveals a strong signal for a unique period at P = 1.541 ± 0.009 days, which we believe to be the orbital period.

The Power Spectral Properties of the Z Source GX 340+0

We present an analysis of ~390 ksec of data of the Z source GX 340+0 taken during 24 observations with the Rossi X-ray Timing Explorer satellite. We report the discovery of a new broad component in the power spectra. The frequency of this component varied between 9 and 14 Hz, and remained close to half that of the horizontal branch quasi-periodic oscillations (HBO). Its rms amplitude was consistent with being constant around ~5%, while its FWHM increased with frequency from 7 to 18 Hz. If this sub-HBO component is the fundamental frequency, then the HBO and its second harmonic are the second and fourth harmonic component, while the third harmonic was not detected. This is similar to what was recently found for the black hole candidate XTE J1550-564. The profiles of both the horizontal- and the normal branch quasi-periodic oscillation peaks were asymmetric when they were strongest. We describe this in terms of a shoulder component at the high frequency side of the quasi-periodic oscillation peak, whose rms amplitudes were approximately constant at ~4% and \~3%, respectively. The peak separation between the twin kHz quasi-periodic oscillations was consistent with being constant at 339+-8 Hz but a trend similar to that seen in, e.g. Sco X-1 could not be excluded. We discuss our results within the framework of the various models which have been proposed for the kHz QPOs and low frequency peaks.