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.
Publication Source (Journal or Book title)
Monthly Notices of the Royal Astronomical Society
Hynes, R., Haswell, C., Cui, W., Shrader, C., O'Brien, K., Chaty, S., Skillman, D., Patterson, J., & Horne, K. (2003). The remarkable rapid X-ray, ultraviolet, optical and infrared variability in the black hole XTE J1118+480. Monthly Notices of the Royal Astronomical Society, 345 (1), 292-310. https://doi.org/10.1046/j.1365-8711.2003.06938.x