XTE J1550−564:INTEGRALObservations of a Failed Outburst

The well known black-hole X-ray binary transient XTE J1550-564 underwent anoutburst during the spring of 2003 which was substantially underluminous incomparison to previous periods of peak activity in that source. In addition,our analysis shows that it apparently remained in the hard spectral state overthe duration of that outburst. This is again in sharp contrast to majoroutbursts of that source in 1998/1999 during which it exhibited an irregularlight curve, multiple state changes and collimated outflows. This leads us toclassify it as a "failed outburst." We present the results of our study of thespring 2003 event including light curves based on observations from bothINTEGRAL and RXTE. In addition, we studied the evolution of the high-energy3-300 keV continuum spectrum using data obtained with three main instruments onINTEGRAL. These spectra are consistent with typical low-hard-state thermalComptonization emission. We also consider the 2003 event in the context of amulti-source, multi-event period-peak luminosity diagram in which it is a clearoutlyer. We then consider the possibility that the 2003 event was due to adiscrete accretion event rather than a limit-cycle instablility. In thatcontext, we apply model fitting to derive the timescale for viscous propagationin the disk, and infer some physical characteristics.Comment: 22 pages, 8 figures, to be published in The Astrophysical Journa