HST Observations of GRO J1655-40 in Outburst

We examine the results of a coordinated HST-RXTE-CGRO campaign to study the microquasar GRO J1655-40 during its 1996-7 outburst, focusing on interpretation of the overall anti-correlation seen between optical and X-ray fluxes during the early months of the outburst. Our tools include echo-mapping, optical/UV continuum spectral modelling and analysis of spectral variability. We conclude by suggesting one possible interpretation for the anti-correlation.Comment: 4 pages with 11 postscript figures included. To appear in New Astronomy Reviews, as part of proceedings of 2nd Workshop on Galactic Sources with Relativistic Jets. Uses newarcrc.sty (included

Hubble Space Telescope Observations of the Black Hole X-ray Transient GRO J0422+32 Near Quiescence

We present HST/FOS ultraviolet and optical spectroscopy of the black hole X-ray transient GRO J0422+32 shortly before the system reached quiescence. We find that the accretion spectrum from 2500-9000A can be very well fit by a self-absorbed synchrotron model, with superposed HI and MgII emission lines. The explanations we suggest for this spectrum are that it is either due to active coronal regions above a geometrically thin accretion disc, or that the disc is evaporated into an advective flow.Comment: 7 pages with 7 postscript figures included, uses mn.sty. Accepted for publication in MNRA

Echo-Mapping of Swift J1753.5-0127

We present two epochs of coordinated X-ray-optical timing observations of the black hole candidate Swift J1753.5-0127 during its 2005 outburst. The first epoch in July occurred at outburst peak. Two consecutive nights of observations using the McDonald Observatory Argos camera with the Rossi X-ray Timing Explorer show a consistent correlation with an immediate response and an extended tail lasting ~5s. The properties of the variability and the correlation are consistent with thermal reprocessing in an accretion disk. The shortness of the lag suggests a short orbital period consistent with that recently claimed. The second epoch in August used the VLT FORS2 HIT mode again in conjunction with RXTE. Again a repeatable correlation is seen between two independent subsets of the data. In this case, though, the cross-correlation function has an unusual structure comprising a dip followed by a double-peak. We suggest that this may be equivalent to the dip plus single peak structure seen by Kanbach et al. (2001) in XTE J1118+480 and attributed there to synchrotron emission; a similar structure was seen during later activity of Swift J1753.5-0127 by Durant et al. (2008).Comment: 7 pages, accepted for publication in Monthly Notices of the Royal Astronomical Societ

The 1996 outburst of GRO J1655-40: disc irradiation and enhanced mass transfer

We show that the 1996 outburst of the X-ray binary transient system GRO J1655-40 can be explained by the standard dwarf-nova type disc instability, followed by an episode of enhanced mass transfer from the secondary if the mass transfer rate in GRO J1655-40 is within a factor < 10 of the stability limit. We argue that irradiation of the secondary during the onset of the outburst driven by the thermal instability in the outer disc can increase the mass transfer rate above the minimum value required for stable accretion. This will then produce the period of near-constant X-ray emission seen in this system. This scenario can also explain the observed anti-correlation between the optical and X-ray fluxes. It is generally accepted that optical emission in low-mass X-ray binaries is produced by irradiation of the outer disc by X-rays. There is also strong circumstantial evidence that in order for the outer disc to see the irradiating flux, it must be warped. Depending on the warp propagation mechanism, either a burst of mass from the secondary or viscous decay are likely to decrease the degree of warping, thereby causing the decrease in the observed optical flux while the X-ray flux remains constant or even increases, exactly as observed in GRO J1655-40. Finally, the decrease of the disc warping and, therefore, irradiation will cause the disc to become unstable once again, terminating the outburst.Comment: Astronomy and Astrophysics - in pres