X-Ray Off States and Optical Variability in CAL 83

Abstract

CAL 83 was one of the first supersoft X-ray binaries to be discovered and is considered to be the prototype of its class. In 15 X-ray observations between 1983-1997 it was observed to have nearly constant X-ray luminosity and temperature, with the exception of one off-state in 1996. We report on a second X-ray off-state, discovered with a Chandra ACIS-S observation in November 1999. Comparing the long-term X-ray and MACHO optical light curves, we find that, CAL 83 has exhibited distinct and well-defined low, intermediate, and high optical states. Transitions between states are not accompanied by color variations. Both X-ray off states were observed during optical high states and were followed by optical low states within ~50 days. We discuss possible explanations for the observed optical and X-ray variations.