Search

We present Swift observations of the black hole X-ray transient, GRO J1655−40, during the recent outburst. With its multiwavelength capabilities and flexible scheduling, Swift is extremely well suited for monitoring the spectral evolution of such an event. GRO J1655−40 was observed on 20 occasions and data were obtained by all instruments for the majority of epochs. X-ray spectroscopy revealed spectral shapes consistent with the ‘canonical’ low/hard, high/soft and very high states at various epochs. The soft X-ray source (0.3–10 keV) rose from quiescence and entered the low/hard state, when an iron emission line was detected. The soft X-ray source then softened and decayed, before beginning a slow rebrightening and then spending ∼3 weeks in the very high state. The hard X-rays (14–150 keV) behaved similarly but their peaks preceded those of the soft X-rays by up to a few days; in addition, the average hard X-ray flux remained approximately constant during the slow soft X-ray rebrightening, increasing suddenly as the source entered the very high state. These observations indicate (and confirm previous suggestions) that the low/hard state is key to improving our understanding of the outburst trigger and mechanism. The optical/ultraviolet light curve behaved very differently from that of the X-rays; this might suggest that the soft X-ray light curve is actually a composite of the two known spectral components, one gradually increasing with the optical/ultraviolet emission (accretion disc) and the other following the behaviour of the hard X-rays (jet and/or corona). [ABSTRACT FROM AUTHOR]

We report multiwavelength observations of the soft X-ray transient XTE J1118+480, which we observed with UKIRT, Hubble Space Telescope ( HST), RXTE, Extreme Ultraviolet Explorer ( EUVE) and many other instruments and facilities. Adding radio (Ryle Telescope, VLA), submillimetre (JCMT) and X-ray ( Chandra and SAX) data from the literature, we assembled the most complete spectral energy distribution (SED) of this source yet published. We followed the evolution of this source for 1 yr, including six observations performed during the outburst, and one observation at the end of the outburst. Because of the unusually high galactic latitude of XTE J1118+480, it suffers from very low extinction, and its SED is nearly complete, including extreme ultraviolet observations. XTE J1118+480 exhibits an unusually low low/hard state (estimated inner radius of ) and a strong non-thermal contribution in the radio to optical domain, which is likely to be due to synchrotron emission. We discuss the interstellar column density and show that it is low, between 0.80 and . We analyse the evolution of the SED during the outburst, including the contributions from the companion star, the accretion disc, the outflow, and relating irradiation and variability of the source in different bands to the SED. We find no significant spectral variability during the outburst evolution, consistent with the presence of a steady outflow. An analysis of its outflow to accretion energy ratio suggests that the microquasar XTE J1118+480 is analogous to radio-quiet quasars. This, combined with the inverted spectrum from radio to optical, makes XTE J1118+480 very similar to other microquasar sources, e.g. GRS 1915+105 and GX 339−4 in their low/hard state. We model the high-energy emission with a hot disc model, and discuss different accretion models for the broad-band spectrum of XTE J1118+480. [ABSTRACT FROM AUTHOR]

The transient black-hole binary XTE J1118+480 exhibited dramatic rapid variability at all wavelengths which were suitably observed during its 2000 April–July outburst. We examine time-resolved X-ray, ultraviolet, optical and infrared data spanning the plateau phase of the outburst. We find that both X-ray and infrared bands show large amplitude variability. The ultraviolet and optical variability is more subdued, but clearly correlated with that seen in the X-rays. The ultraviolet, at least, appears to be dominated by the continuum, although the lines are also variable. Using the X-ray variations as a reference point, we find that the ultraviolet (UV) variability at long wavelengths occurs later than that at short wavelengths. Uncertainty in the Hubble Space Telescope timing prohibits a determination of the absolute lag with respect to the X-rays, however. The transfer function is clearly not a delta-function, exhibiting significant repeatable structure. For the main signal we can rule out an origin in reprocessing on the companion star – the lack of variation in the lags is not consistent with this, given a relatively high orbital inclination. Weak reprocessing from the disc and/or companion star may be present, but is not required, and another component must dominate the variability. This could be variable synchrotron emission correlated with X-ray variability, consistent with our earlier interpretation of the infrared (IR) flux as due to synchrotron emission rather than thermal disc emission. In fact, the broad-band energy distribution of the variability from IR to X-rays is consistent with expectations of optically thin synchrotron emission. We also follow the evolution of the low-frequency quasi-periodic oscillation in X-rays, UV, and optical. Its properties at all wavelengths are similar, indicating a common origin. [ABSTRACT FROM AUTHOR]