Complete RXTE spectral observations of the black hole X-ray nova XTE J1550-564

We report on the X-ray spectral behavior of XTE J1550-564 during its 1998-1 999 outburst. XTE J1550-564 is an exceptionally bright X-ray nova and is al so the third Galactic black hole candidate known to exhibit quasi-periodic X-ray oscillations above 50 Hz. Our study is based on 209 pointed observati ons using the PCA and HEXTE instruments on board the Rossi X-Ray Timing Exp lorer (RXTE) spanning 250 days and covering the entire double-peaked erupti on that occurred from 1998 September until 1999 May. The spectra are fitted to a model including multicolor blackbody disk and power-law components. T he spectra from the first half of the outburst are dominated by the power-l aw 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 outb urst 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 obser ved to vary by more than an order of magnitude; the radius decreased by a f actor of 16 in one day during a 6.8 crab Rare. If the larger of these obser ved radii is taken to be the last stable orbit, then the smaller observed r adius would imply that the inner edge of the disk is inside the event horiz on! However, we conclude that the apparent variations of the inner disk rad ius observed during periods of increased power-law emission are probably ca used by the failure of the multicolor disk/power-law model; the actual phys ical radius of the inner disk may remain fairly constant. This interpretati on is supported by the fact that the observed inner disk radius remains app roximately constant over 120 days in the high state, when the power-law com ponent is weak, even though the disk flux and total flux vary by an order o f magnitude. The mass of the black hole inferred by equating the approximat ely constant inner disk radius observed in the high/soft state with the las t stable orbit for a Schwarzschild black hole is M-BH = 7.4 M.(D/6 kpc)(cos i)(-1/2).