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R. K. Jain, C. D. Bailyn (Yale University), J. A. Orosz (Pennsylvania State University), J. E. McClintock, G. J. Sobczak (Center for Astrophysics), R. A. Remillard, A. M. Levine, E. H. Morgan (Center for Space Research, MIT)
The transient XTE J1550-564 was discovered by the RXTE satellite on Sept. 7 1998 (IAUC 7008). Since that time we have monitored the source with the RXTE ASM and PCA detectors (IAUC 7019, 7025) and in the optical with the YALO 1m telescope in Chile (IAUC 7013, 7020). The X-ray and optical characteristics of the source strongly suggest that this object belongs to the growing class of transient black hole binary systems.
The ASM X-ray light curve rose to 3.2 Crab by Sept. 18, triggering our program of frequent observations with the PCA detector. On Sept. 19-20, a large flare peaked at 6.8 Crab, making XTE J1550-564 the brightest X-ray nova yet observed by RXTE. During the flare, PCA observations revealed quasi-periodic oscillations (QPOs) at 183.6hz. This source is thus the third black hole candidate to exhibit X-ray QPOs above 50hz, joining the ``micro-quasars'' GRS1915+105 (Morgan et al. 1997, ApJ 482, 993) and GRO~J1655-40 (Remillard et al. 1998 astro-ph 980649). The optical counterpart was discovered at V=16.7 (IAUC 7009), and has been falling steadily ever since, except for a mild flare of \approx 0.1 magnitudes which peaked \approx 1 day after the X-ray flare. The quiescent optical counterpart can be seen near the plate limit of the ESO sky survey plates, so the optical amplitude is \approx 4 magnitudes. Additional details of the optical and X-ray observations will be presented.