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J. L. Sokoloski (CfA), P. A. Charles, W. I. Clarkson (U. Southampton)
The nature of rapid variability from different types of accreting white-dwarf (WD) binaries may be a function of both the energy source in these systems and the accretion geometry. Cataclysmic variables (CVs) generally show large-amplitude stochastic variations (flickering) from the accretion disk or stream impact region on time scales as short as minutes or seconds. In most symbiotic stars, on the other hand, any rapid, accretion-related photometric variations appear to be overwhelmed by nebular emission that is powered by quasi-steady nuclear shell burning on the accreting WD. Supersoft X-ray binaries (SSXBs) also have quasi-steady nuclear burning on the WD surface. Yet in at least some SSXBs, stochastic variations are observed. To investigate the nature and origin of optical flickering in SSXBs, we performed high time resolution white-light and BVR observations of the galactic SSXB MR Vel with the SAAO 1.9-m telescope during January, 2001. The observations reveal flickering with time scales as short as several minutes, although with smaller amplitude than is usually measured for CVs. We believe these to be the fastest variations reported for MR Vel, or any SSXB. Such variations impose constraints on current models which associate the variable optical emission with reprocessing of soft X-rays sited at the disk rim. The constraints arise because on these time scales, it would be difficult for the size of the disk rim to change significantly, and the nuclear-burning flux is constant. It is therefore possible that we have detected CV-like flickering associated with viscous dissipation in the disk. We observed correlated variations in a 4-hr series of cyclic B-, V-, and R-band observations, with a slightly larger variability amplitude in B. The V- and R-band fluctuations look almost identical, but there is at least one flare-like event in the B light curve that is muted or absent at V and R. We compare our results for MR Vel with two other SSXBs, Cal 87 and RX J0513.9-6951.
This work was supported in part by NSF grant INT-9902662.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.