The nature of the X-ray spectrum of VW Hydri.

Abstract

We analyze the EXOSAT data of the October 1984 superoutburst of the dwarf nova VW Hydri. We use the upper limit to the flux at 650 A, measured by VOYAGER, to argue that the spectral component that dominates at 1000 A does not significantly contribute to the counts in any of the EXOSAT filters. This is confirmed by analysis of a flux-ratio diagram, which shows that the EXOSAT data of three superoutbursts observed in 1983, 1984, and 1985 can be explained with a single optically thin spectrum with temperature varying from just above 10 exp 5 K to approximately greater than 10 exp 6 K. This implies that the X-ray spectrum remains optically thin during superoutbursts. An alternative possibility is a combination of a hot (greater than 10 exp 6 K), optically thin component with an optically thick, cooler (80,000 - 100,000 K) component. Both components satisfy the upper limit at 650 A; neither component contributes significantly to the flux at 1000 A. The surface area of the cool component is small with respect to the white dwarf surface.