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Transient supersoft X-ray emission from V751 Cyg during the optical low-state

J. Greiner·G. H. Tovmassian·R. DiStefano·A. Prestwich·R. González-Riestra·L. Szentasko·C. Chavarria
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Abstract

We have observed V751 Cyg with the ROSAT HRI during its recent optical low state and clearly detect it at a count rate of 0.015 cts/s. The X-ray intensity is a factor of ~20 higher than the upper limit obtained with the ROSAT PSPC during the optical high state. Spectral investigations suggest a very soft X-ray spectrum. We investigate archival IUE data of V751 Cyg and derive a distance of V751 Cyg of d~500 pc based on the extinction estimate of E(B-V)=0.25+-0.05. This implies that the X-ray emission is very luminous, on the order of 10^34-10^36 erg/s. The spectrum during the optical low-state is characterized by a very blue continuum and the presence of strong emission lines of the Balmer series and HeI. Also, HeII 4686 A is clearly detected. We establish that V751 Cyg is a transient supersoft X-ray source and speculate that other VY Scl stars may also be of similar type.

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J. Greiner, G. H. Tovmassian, R. DiStefano, A. Prestwich, R. González-Riestra, L. Szentasko, C. Chavarria. 1998-12-02. Transient supersoft X-ray emission from V751 Cyg during the optical low-state. https://arxiv.org/abs/astro-ph/9812054

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