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R. Soria (Mullard Space Science Laboratory (UCL)), D. C. Hannikainen (University of Southampton), K. Wu, D. Campbell-Wilson (University of Sydney), B. A. Harmon (NASA Marshall Space Flight Center), R. W. Hunstead, H. M. Johnston (University of Sydney), M. L. McCollough (NASA Marshall Space Flight Center), V. J. McIntyre (Australia Telescope National Facility (CSIRO))
We present observations of the September 1998 outburst of the black-hole candidate XTE J1550-564 in the radio, optical and X-ray bands. Each light-curve shows a sequence of rising and declining phases, before the system settled into a ``soft state'' characteristic of black-hole binaries. We discuss the properties of the various phases at each wavelength. We propose that the companion star in XTE J1550-564 is a low-mass, evolved magnetic star, with a surface field strong enough to influence the dynamics of mass transfer. The initial impulsive rise in the hard X-rays is explained by instantaneous transfer of matter with low angular momentum from the magnetic companion. The giant flare is due to the instability of accretion during the condensation of a disk.
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