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R. K. Jain, C. D. Bailyn (Yale), J. A. Orosz (Utrecht), J. E. McClintock, G. J. Sobczak (CFA), R. A. Remillard (M.I.T.)
We report the detection of periodic modulation from the black hole candidate XTE J1550-564 late in its outburst cycle and also during quiescence. Data were collected using the Yale 1-m telescope located in Chile as part of an extensive multi-wavelength monitoring campaign of XTE J1550-564. We have searched for periodicities in the optical light curve and find a clear signal at P=1.546±.038 with peak-to-peak amplitude of roughly 0.15 magnitudes during the final stage of the outburst when the overall light curve was still decaying at a rate of 0.05 mags/day. >From the quiescence data, we find a double peaked ellipsoidal modulation with P=1.540±.008 days with nearly double the peak-to-peak amplitude ~0.36 mags. The large amplitude suggests the system is at a high inclination. Consequently, if the compact object is indeed a black hole, the mass function is likely to exceed 3M\sun, which is the maximum stable limit of the neutron star.