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Register a new userOn the properties of thermal disk winds in X-ray transient sources: a case study of GRO J1655-40
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S. Luketic
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We present the results of hydrodynamical simulations of the disk photosphere irradiated by strong X-rays produced in the inner most part of the disk. As expected, the irradiation heats the photosphere and drives a thermal wind. To apply our results to the well-studied X-ray transient source GRO J1655-40, we adopted the observed mass of its black hole, and the observed properties of its X-ray radiation. To compare the results with the observations, we also computed transmitted X-ray spectra based on the wind solution. Our main finding is: the density of the fast moving part of the wind is more than one order of magnitude lower than that inferred from the observations. Consequently, the model fails to predict spectra with line absorption as strong and as blueshifted as those observed. However, despite the thermal wind being weak and Compton thin, the ratio between the mass-loss rate and the mass accretion rate is about seven. This high ratio is insensitive to the accretion luminosity, in the limit of lower luminosities. Most of the mass is lost from the disk between 0.07 and 0.2 of the Compton radius. We discovered that beyond this range the wind solution is self-similar. In particular, soon after it leaves the disk, the wind flows at a constant angle with respect to the disk. Overall, the thermal winds generated in our comprehensive simulations do not match the wind spectra observed in GRO J1655-40. This supports the conclusion of Miller et al. and Kallman et al. that the wind in GRO J1655-40, and possibly other X-ray transients, may be driven by magnetic processes. This in turn implies that the disk wind carries even more material than our simulations predict and as such has a very significant impact on the accretion disk structure and dynamics.
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Essentially all low-mass X-ray binaries (LMXBs) in the soft state appear to drive powerful equatorial disc winds. A simple mechanism for driving such outflows involves X-ray heating of the top of the disc atmosphere to the Compton temperature. Beyond the Compton radius, the thermal speed exceeds the escape velocity, and mass loss is inevitable. Here, we present the first coupled radiation-hydrodynamic simulation of such thermally-driven disc winds. The main advance over previous modelling efforts is that the frequency-dependent attenuation of the irradiating SED is taken into account. We can therefore relax the approximation that the wind is optically thin throughout which is unlikely to hold in the crucial acceleration zone of the flow. The main remaining limitations of our simulations are connected to our treatment of optically thick regions. Adopting parameters representative of the wind-driving LMXB GRO~J1655-40, our radiation-hydrodynamic model yields a mass-loss rate that is $simeq5times$ lower than that suggested by pure hydrodynamic, optically thin models. This outflow rate still represents more than twice the accretion rate and agrees well with the mass-loss rate inferred from Chandra/HETG observations of GRO~J1655-40 at a time when the system had a similar luminosity to that adopted in our simulations. The Fe XXV and Fe XXVI Lyman $rm{alpha}~$ absorption line profiles observed in this state are slightly stronger than those predicted by our simulations but the qualitative agreement between observed and simulated outflow properties means that thermal driving is a viable mechanism for powering the disc winds seen in soft-state LMXBs.
We report the identification and study of an unusual soft state of the black hole low-mass X-ray binary GRO J1655-40, observed during its 2005 outburst by the Rossi X-ray Timing Explorer. Chandra X-ray grating observations have revealed a high mass-outflow accretion disc wind in this state, and we show that the broadband X-ray spectrum is remarkably similar to that observed in the so-called `hypersoft state of the high mass X-ray binary Cyg X-3, which possesses a strong stellar wind from a Wolf-Rayet secondary. The power-spectral density (PSD) of GRO J1655-40 shows a bending power-law shape, similar to that of canonical soft states albeit with larger fractional rms. However, the characteristic bend-frequency of the PSD is strongly correlated with the X-ray flux, such that the bend-frequency increases by two decades for less than a factor 2 increase in flux. The strong evolution of PSD bend-frequency for very little change in flux or X-ray spectral shape seems to rule out the suppression of high-frequency variability by scattering in the wind as the origin of the PSD bend. Instead, we suggest that the PSD shape is intrinsic to the variability process and may be linked to the evolution of the scale-height in a slim disc. An alternative possibility is that variability is introduced by variable absorption and scattering in the wind. We further argue that the hypersoft state in GRO J1655-40 and Cyg X-3 is associated with accretion close to or above the Eddington limit.
We report the results of multiwavelength observations of the superluminal X-ray transient GRO J1655-40 during and following the prominent hard X-ray outburst of March-April 1995. GRO J1655-40 was continuously monitored by BATSE on board CGRO, and repeatedly observed in the radio and optical bands from the ground. About a month after the onset of the hard X-ray outburst, GRO J1655-40 was observed twice by HST on April 25 and 27 1995, with the aim of detecting faint optical emission from ejected plasmoids. Despite the similarity of the hard X-ray emission in April 1995 with previous events in 1994, no radio or optical emission related to plasmoids was detected. We conclude that GRO J1655-40 is subject to a complex behavior showing: radio-loud hard X-ray outbursts with strong radio emission (of flux $f_r goe 100$~mJy) both from a `core source and from propagating plasmoids (as those in 1994), and radio-quiet hard X-ray outbursts with no detectable radio emission and plasmoid activity ($f_r loe 0.5$~mJy) (as those in 1995). Our results can constrain models of particle acceleration and radiation of relativistic plasmoids.
Chandra obtained two High Energy Transmission Grating (HETG) spectra of the stellar-mass black hole GRO J1655-40 during its 2005 outburst, revealing a rich and complex disk wind. Soon after its launch, the Neil Gehrels Swift Observatory began monitoring the same outburst. Some X-ray Telescope (XRT) observations were obtained in a mode that makes it impossible to remove strong Mn calibration lines, so the Fe K-alpha line region in the spectra was previously neglected. However, these lines enable a precise calibration of the energy scale, facilitating studies of the absorption-dominated disk wind and its velocity shifts. Here, we present fits to 15 Swift/XRT spectra, revealing variability and evolution in the outflow. The data strongly point to a magnetically driven disk wind: both the higher velocity (e.g., v ~ 10^4 km/s) and lower velocity (e.g., v ~ 10^3 km/s) wind components are typically much faster than is possible for thermally driven outflows (v < 200 km/s), and photoionization modeling yields absorption radii that are two orders of magnitude below the Compton radius that defines the typical inner extent of thermal winds. Moreover, correlations between key wind parameters yield an average absorption measure distribution (AMD) that is consistent with magnetohydrodynamic wind models. We discuss our results in terms of recent observational and theoretical studies of black hole accretion disks and outflows, and future prospects.
Context: The detection of overabundances of $alpha$-elements and lithium in the secondary star of a black-hole binary provides important insights about the formation of a stellar-mass black-hole. $alpha$-enhancement might theoretically also be the result of pollution by the nucleosynthesis occurring during an outburst, or through spallation by the jet. Aims: We study the abundances, and their possible variations with time, in the secondary star of the runaway black-hole binary GRO J1655--40, in order to understand their origin. Methods: We present a detailed comparison between a Keck spectrum obtained in 1998 found in the literature, archival VLT-UVES data taken in 2004 and new VLT-UVES spectra obtained early 2006. We carefully determine the equivalent widths of different $alpha$-elements (Mg, O, Ti, S and Si) with their associated uncertainty. We use the well-studied comparison star HD 156098 as well as synthetic spectra to match the spectrum of GRO J1655--40 in order to determine the abundances of these elements. Results: We see no significant variations of equivalent widths with time. Our fit using HD 156098 reveals that there is significant overabundance of oxygen in all our spectra, but no overabundances of any of the other $alpha$-elements. Finally, we do not detect the lithium line at 6707 AA. Conclusions: We show that there is no detected pollution in GRO J1655--40 after the burst in 2005. Moreover, we argue that uncertainties in the equivalent widths were previously underestimated by a factor of $sim$3. Consequently, our results challenge the existence of general overabundances of $alpha$-elements observed in this galactic black-hole binary, and thus the accepted interpretation that they are of supernova origin. The physical cause of the overabundance of oxygen remains unclear.
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