The Fall and the Rise of X-Rays from Dwarf Novae in Outburst:<i>RXTE</i>Observations of VW Hydri and WW Ceti

Abstract

In a dwarf nova, the accretion disk around the white dwarf is a source of\nultraviolet, optical, and infrared photons, but is never hot enough to emit\nX-rays. Observed X-rays instead originate from the boundary layer between the\ndisk and the white dwarf. As the disk switches between quiescence and outburst\nstates, the 2-10 keV X-ray flux is usually seen to be anti-correlated with the\noptical brightness. Here we present RXTE monitoring observations of two dwarf\nnovae, VW Hyi and WW Cet, confirming the optical/X-ray anti-correlation in\nthese two systems. However, we do not detect any episodes of increased hard\nX-ray flux on the rise (out of two possible chances for WW Cet) or the decline\n(two for WW Cet and one for VW Hyi) from outburst, attributes that are clearly\nestablished in SS Cyg. The addition of these data to the existing literature\nestablishes the fact that the behavior of SS Cyg is the exception, rather than\nthe archetype as is often assumed. We speculate on the origin of the diversity\nof behaviors exhibited by dwarf novae, focusing on the role played by the white\ndwarf mass.\n