CompleteRXTESpectral Observations of the Black Hole X‐Ray Nova XTE J1550−564

We report on the X-ray spectral behavior of the exceptionally bright X-raynova XTE J1550-564 during its 1998-99 outburst. Our study is based on 209pointed observations using the PCA and HEXTE instruments onboard the RossiX-ray Timing Explorer spanning 250 days and covering the entire double-peakederuption that occurred from 1998 September until 1999 May. The spectra are fitto a model including multicolor blackbody disk and power-law components. Thesource is observed in the very high and high/soft outburst states of black holeX-ray novae. During the very high state, when the power-law component dominatedthe spectrum, the inner disk radius is observed to vary by more than an orderof magnitude; the radius decreased by a factor of 16 in one day during a 6.8Crab flare. If the larger of these observed radii is taken to be the laststable orbit, then the smaller observed radius would imply that the inner edgeof the disk is inside the event horizon! However, we conclude that the apparentvariations of the inner disk radius observed during periods of increasedpower-law emission are probably caused by the failure of the multicolordisk/power-law model; the actual physical radius of the inner disk may remainfairly constant. This interpretation is supported by the fact that the observedinner disk radius remains approximately constant over 120 days in the highstate, when the power-law component is weak, even though the disk flux andtotal flux vary by an order of magnitude. The mass of the black hole inferredby equating the approximately constant inner disk radius observed in thehigh/soft state with the last stable orbit for a Schwarzschild black hole isM_BH = 7.4 M_sun (D/6 kpc) (cos i)^{-1/2}.Comment: Submitted to ApJ, 20 pages including 6 figures + 4 large table