The Fall and the Rise of X-rays from Dwarf Novae in Outburst: RXTE Observations of VW Hydri and WW Ceti

TL;DR

This study uses RXTE observations to analyze X-ray and optical behavior in dwarf novae VW Hyi and WW Ceti, confirming anti-correlation during outbursts and highlighting diversity in X-ray responses among such systems.

Contribution

It provides new observational data on VW Hyi and WW Ceti, challenging the assumption that SS Cyg's behavior is typical among dwarf novae.

Findings

Confirmed optical/X-ray anti-correlation in VW Hyi and WW Ceti.

Did not observe increased hard X-ray flux during outburst rise or decline.

Showed SS Cyg's behavior is not representative of all dwarf novae.

Abstract

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