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 Large Magellanic Cloud, using data from the massive compact halo object (MACHO) and Optical Gravitational Lensing Experiment (OGLE) projects. The CAL 83 light curve exhibits dramatic brightness changes of ∼1 mag on time-scales of ∼450 d, and spends typically ∼200 d in the optical low state. Combined with archival XMM-Newton X-ray observations these represent the most extensive X-ray/optical study to date of this system, and reveal in much greater detail that the X-ray light curve is anticorrelated with the optical behaviour. This is remarkably similar to the behaviour of the `transient' SSS, RX J0513.9-6951, where the SSS outbursts recur on a time-scale of ∼168 d, and also anticorrelate with the optical flux. We performed simple blackbody fits to both high- and low-state X-ray spectra, and find that the blackbody temperature and luminosity decrease when the optical counterpart brightens. We interpret these long-term variations in terms of the limit cycle model of Hachisu and Kato, 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) d and 0.762956(5) d for CAL 83 and RX J0513.9-6951, respectively.