Issue A&A Volume 387, Number 3, June I 2002 Page(s) 944 - 954 Section Formation, structure and evolution of stars DOI http://dx.doi.org/10.1051/0004-6361:20020463

A&A 387, 944-954 (2002)
DOI: 10.1051/0004-6361:20020463

X-ray off states and optical variability in CAL 83

J. Greiner^{1, 2} and R. Di Stefano<sup>3, 4

1</sup>  Astrophysical Institute Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
²  Max-Planck-Institute for extraterrestrial Physics, 85741 Garching, Germany
³  Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
⁴  Department of Physics and Astronomy, Tufts University, Medford, MA 02155, USA

(Received 30 July 2001 / Accepted 13 March 2002)

Abstract
CAL 83 was one of the first supersoft X-ray binaries (SSBs) 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 (Kahabka et al. 1996). We report on a second X-ray off-state, discovered with a Chandra ACIS-S observation in November 1999. We consider the long-term X-ray and MACHO optical light curves. We find that, during more than 7 years of monitoring by the MACHO team, CAL 83 has exhibited distinct and well-defined low, intermediate, and high optical states. Transitions between states are not accompanied by color variations. We also find that 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. While photospheric adjustments might account for the variations in soft X-ray flux, optical variations can be explained only by invoking changes in the accretion disk, which is the primary source of optical radiation.

Key words: X-ray: stars -- accretion, accretion disks -- stars: binaries: close -- stars: individual: CAL 83

Offprint request: J. Greiner, jcg@mpe.mpg.de

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