The remarkable rapid X-ray, ultraviolet, optical and infrared variability in the black hole XTE J1118+480 |
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Authors: | R. I. Hynes &dagger ,C. A. Haswell,W. Cui,C. R. Shrader,K. O'Brien,S. Chaty,D. R. Skillman,J. Patterson, Keith Horne |
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Affiliation: | Astronomy Department, The University of Texas at Austin, 1 University Station C1400, Austin, Texas 78712-0259, USA;Department of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ;Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA;Department of Physics, Purdue University, 1396 Physics Building, West Lafayette, IN 47907-1396, USA;Laboratory for High-Energy Astrophysics, NASA Goddard Space Flight Centre, Greenbelt, MD 20771, USA;Sterrenkundig Instituut, Kruislaan 403, 1098 SJ Amsterdam, the Netherlands;School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS;European Southern Observatory, Casilla 19001, Santiago 19, Chile;UniversitéParis 7, Fédération APC, 2 place Jussieu, 75005 Paris, France;Service d'Astrophysique, DSM/DAPNIA/SAp, CEA-Saclay, Bat. 709, L'Orme des Merisiers F-91 191 Gif-sur-Yvette, Cedex, France;Centre for Backyard Astrophysics (East), 9517 Washington Avenue, Laurel, MD 20723, USA;Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027, USA |
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Abstract: | 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. |
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Keywords: | accretion, accretion discs binaries: close stars: individual: XTE J1118+480 ultraviolet: stars X-rays: stars |
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