Optical observations of the black hole candidate XTE J1550-564 during reflare and quiescence

We report optical monitoring of the soft X-ray transient XTE J1550-564 during the 1999 season (1999 January 4-August 24). The first optical observations available in 1999 show that the peak "reflare" brightness had exceeded the peak brightness of the initial optical flare in 1998 September by over half a magnitude. We compare the optical reflare light curves with the total X-ray flux, the power-law flux, and disk flux light curves constructed from the spectral fits to RXT E/PCA data. During the first 60 days of the observed optical reflare, 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 anticorrelated with the optical dips, suggesting that the optical flux may be up scattered into the X-ray by the hot corona. Periodic modulations were discovered during the final stage of the outburst (May-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.