2001ApJ...546.1086J

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 RXTE/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.