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

TL;DR

This study analyzes long-term optical and X-ray variability of CAL 83, revealing anti-correlated brightness changes and supporting the limit-cycle model of white dwarf accretion, with refined orbital periods for CAL 83 and RX J0513.9-6951.

Contribution

It provides the most extensive optical and X-ray analysis of CAL 83, demonstrating anti-correlated variability and supporting the limit-cycle model of supersoft X-ray sources.

Findings

Optical brightness varies by ~1 mag on ~450-day cycles.

X-ray and optical light curves are anti-correlated.

Refined orbital periods for CAL 83 and RX J0513.9-6951.

Abstract

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 MACHO and 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-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 anti-correlated 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 timescale of ~168 days, and also anti-correlate 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 & Kato (2003a), which provides further support for these systems containing massive (~1.3 Msun) 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.