Optical and X-ray Properties of CAL 83: I. Quasi-periodic Optical and Supersoft Variability

We have studied the long-term (∼ years) temporal variability of the prototype supersoft X-ray source (SSS) CAL 83 in the LMC, using data from the MACHOand OGLE projects. The CAL 83 light curve exhibits dramatic brightness changes of ∼ 1 mag on timescales of ∼ 450 days, and spends typically ∼ 200 days in the optical low state. Combined with archival XMM-NewtonX-ray observations these represent the most extensive X-ra y/optical study to date of this system, and reveal in much greater detail that th e X-ray light curve is anticorrelated with the optical behaviour. This is remarkably s imilar to the behaviour of the “transient” SSS, RX J0513.9-6951, where the SSS outbursts recur on a timescale of ∼ 168 days, and also anti-correlate with the optical flux. We performed s imple blackbody fits to both high and low state X-ray spectra, and find that the blackbody tempe rature and luminosity decrease when the optical counterpart brightens. We interpret these long-term variations in terms of the limit-cycle model of Hachisu & Kato (2003a), which provides further support for these systems containing massive (∼ 1.3 M⊙) white dwarfs. In addition, we have refined their orbital periods in the MACHO and OGLE-III light curves to values of 1.047529(1) days and 0.762956(5) days for CAL 83 and RX J0513.9-6951, respectively.