On the X-ray properties of V Sge and its relation to the supersoft X-ray binaries

TL;DR

This paper studies the X-ray behavior of V Sge, linking its optical states to X-ray properties and comparing it to supersoft X-ray binaries, revealing state-dependent spectral changes and possible absorption effects.

Contribution

It provides the first detailed analysis of V Sge's X-ray states and proposes a model explaining optical/X-ray variability through variable absorption and persistent hard X-ray emission.

Findings

V Sge is a faint hard X-ray source during optical bright states.

During optical faint states, V Sge exhibits supersoft X-ray emission.

A persistent hard X-ray component is necessary to explain the observations.

Abstract

We investigate the ROSAT X-ray properties of V Sge, which has been proposed to be related to supersoft X-ray binaries. During optical bright states, V Sge is a faint hard X-ray source, while during optical faint states (V >=12 mag), V Sge is a `supersoft' X-ray source. Spectral fitting confirms that V~Sge's X-ray properties during its soft X-ray state may be similar to those of supersoft X-ray binaries, although a much lower luminosity cannot be excluded. It is possible to explain the different optical/X-ray states by a variable amount of extended uneclipsed matter, which during the optical bright states contributes significantly to the optical flux and completely absorbes the soft X-ray component. An additional, perhaps permanent, hard X-ray component, such as a bremsstrahlung component with a 0.1-2.4 keV luminosity of ~10^30 erg/s, must be present to explain the X-ray properties during the optical bright/hard X-ray state.