X-ray observations of SU UMa throughout six outbursts

TL;DR

This study presents RXTE X-ray observations of six outbursts of the dwarf nova SU UMa, revealing consistent X-ray behavior, delays between optical and X-ray changes, and spectral variations that challenge existing models.

Contribution

It provides detailed X-ray monitoring of SU UMa's outbursts, highlighting discrepancies with current accretion disk instability models and boundary layer expectations.

Findings

X-ray count rate drops by a factor of ten during outbursts

Half-day delay between optical rise and X-ray suppression

X-ray flux remains high during quiescence, contrary to models

Abstract

We present RXTE observations covering six normal outbursts of the dwarf nova SU UMa, the prototype of its class. The outbursts showed consistent X-ray behaviour with the X-ray count rate dropping suddenly by a factor of our, and with evidence for a half day delay between the optical rise and the X-ray suppression. In contrast to SS Cyg, an X-ray flux increase at the beginning of outburst was not observed, although it is expected from boundary layer models. The X-ray flux was high and decreasing during quiescence, in conflict with the disc instability model. The X-ray spectrum of SU UMa was softer in outburst than during quiescence, and it was consistent with constant reflection.