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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]

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 2175-Å 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 fitted 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 normalization of our spectral fits we estimate a likely distance range of 4.6–8.0 kpc , although a value outside this range cannot securely be ruled out. [ABSTRACT FROM AUTHOR]