X-ray and UV observations of V751 Cyg in an optical high state

TL;DR

This study presents X-ray and UV observations of V751 Cyg during its optical high state, revealing orbital modulation, spectral characteristics, and variability, challenging previous super-soft spectrum claims and suggesting absorption-related flux changes.

Contribution

First detailed X-ray and UV analysis of V751 Cyg in high state, showing orbital modulation and clarifying its spectral nature as multi-temperature optically thin emission.

Findings

Detected X-ray modulation at the 3.467 hr orbital period.

X-ray spectrum was not super-soft but multi-temperature optically thin.

Observed flux variability linked to absorption, possibly from wind structures.

Abstract

Aims: The VY Scl system (anti-dwarf nova) V751 Cyg is examined following a claim of a super-soft spectrum in the optical low state. Methods: A serendipitous XMM-Newton X-ray observation and, 21 months later, Swift X-ray and UV observations, have provided the best such data on this source so far. These optical high-state datasets are used to study the flux and spectral variability of V751 Cyg. Results: Both the XMM-Newton and Swift data show evidence for modulation of the X-rays for the first time at the known 3.467 hr orbital period of V751 Cyg. In two Swift observations, taken ten days apart, the mean X-ray flux remained unchanged, while the UV source brightened by half a magnitude. The X-ray spectrum was not super-soft during the optical high state, but rather due to multi-temperature optically thin emission, with significant (10^{21-22} cm^-2) absorption, which was higher in the observation by Swift than that of XMM-Newton. The X-ray flux is harder at orbital minimum, suggesting that the modulation is related to absorption, perhaps linked to the azimuthally asymmetric wind absorption seen previously in H-alpha.