2000ApJ...544..993S

We report on the X-ray spectral behavior of XTE J1550-564 during its 1998-1999 outburst. XTE J1550-564 is an exceptionally bright X-ray nova and is also the third Galactic black hole candidate known to exhibit quasi-periodic X-ray oscillations above 50 Hz. Our study is based on 209 pointed observations using the PCA and HEXTE instruments on board the Rossi X-Ray Timing Explorer (RXTE) spanning 250 days and covering the entire double-peaked eruption that occurred from 1998 September until 1999 May. The spectra are fitted to a model including multicolor blackbody disk and power-law components. The spectra from the first half of the outburst are dominated by the power-law component, whereas the spectra from the second half are dominated by the disk component. The source is observed in the very high and high/soft outburst states of black hole X-ray novae. During the very high state, when the power-law component dominated the spectrum, the inner disk radius is observed to vary by more than an order of magnitude; the radius decreased by a factor of 16 in one day during a 6.8 crab flare. If the larger of these observed radii is taken to be the last stable orbit, then the smaller observed radius would imply that the inner edge of the disk is inside the event horizon! However, we conclude that the apparent variations of the inner disk radius observed during periods of increased power-law emission are probably caused by the failure of the multicolor disk/power-law model; the actual physical radius of the inner disk may remain fairly constant. This interpretation is supported by the fact that the observed inner disk radius remains approximately constant over 120 days in the high state, when the power-law component is weak, even though the disk flux and total flux vary by an order of magnitude. The mass of the black hole inferred by equating the approximately constant inner disk radius observed in the high/soft state with the last stable orbit for a Schwarzschild black hole is M_BH=7.4 M_☉(D/6 kpc)(cosi)^–1/2.