CAL 83: a 1-day period low-mass X-ray binary in the LMC

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Abstract
From the analysis of extensive V-band CCD photometry obtained in an international campaign in 1984 December, the orbital period of CAL83 is unambiguously determined to be 1.0436±0.0044 day, with a sinusoidal modulation of full amplitude 0.22 mag about a mean of V = 16.87. Superimposed on this modulation we find evidence for occasional variability on a shorter time-scale of ∼2 hr. Data obtained in 1984 August showed that the source was in a brighter state at this time, with a mean brightness of V = 16.32 and a maximum of V = 16.20; overall we have observed CAL83 over a brightness range of 0.9 mag. Optical spectroscopy obtained contemporaneously from AAO and ESO is consistent with the photometric period, with a probable K-velocity of ∼40 km s−1. We find no evidence for an X-ray photoionized nebula surrounding CAL83. The low Lx/Lopt ratio of CAL83 indicates that it contains an accretion disc corona, and the intensity of its He II line implies a luminosity in X-rays which intrinsically is extremely high; therefore CAL83 and LMC X-2 are the two most luminous low mass X-ray binaries known at both optical and X-ray energies.

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