Optical Observations of the Black Hole Candidate XTE J1550-564 During Re-Flare and Quiescence

Abstract

We report optical monitoring of the soft X-ray transient XTE J1550-564 during the 1999 season (4 January 1999 to 24 August 1999). The first optical observations available in 1999 show that the peak ``re-flare'' brightness had exceeded the peak brightness of the initial optical flare in September 1998 by over half a magnitude. We compare the optical re-flare light curves with the total X-ray flux, the power-law flux and disk flux light curves constructed from the spectral fits to RXTE/PCA data made by Sobczak et al. (1999, 2000). During the first 60 days of the observed optical re-flare, we find no correspondence between the thermal component of the X-rays often associated with a disk and the optical flux -- the former remains essentially flat whereas the latter declines exponentially and exhibits three substantial dips. However, the power law flux is anti-correlated with the optical dips, suggesting that the optical flux may by up-scattered into the X-ray by the hot corona. Periodic modulations were discovered during the final stage of the outburst (May to June), with P=1.546+/-0.038 days, and during quiescence (July and August), with P=1.540+/-0.008 days. The analysis of the combined data set reveals a strong signal for a unique period at P=1.541+/-0.009 days, which we believe to be the orbital period.