Transient supersoft X-ray emission from V 751 Cygni during the optical low-state
J. Greiner 1,
G.H. Tovmassian 2,
R. DiStefano 3,
A. Prestwich 3,
R. González-Riestra 4,
L. Szentasko and
C. Chavarría 2
Received 27 July 1998 / Accepted 23 November 1998
We have observed V 751 Cyg with the ROSAT HRI in a target-of-opportunity mode during its recent optical low state and clearly detect it at a count rate of 0.015 cts s-1. The X-ray intensity is a factor of 7-19 (depending on the exact X-ray spectral shape) higher than the upper limit obtained with the ROSAT PSPC during the optical high state, thus suggesting an anti-correlation of X-ray and optical intensity. Spectral investigations suggest a very soft X-ray spectrum. We investigate archival IUE data of V 751 Cyg and derive a distance of V 751 Cyg of d500 pc based on the extinction estimate of =0.250.05. This implies that the X-ray emission is very luminous, on the order of 1034-1036 erg s- 1.
We have obtained quasi-simultaneous optical photometry and spectroscopy. 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 Å is clearly detected.
We establish that V 751 Cyg is a transient supersoft X-ray source and speculate that other VY Scl stars may also be of similar type.
Key words: accretion, accretion disks stars: individual: HD 199178 stars: individual: V 751 Cyg stars: novae, cataclysmic variables X-rays: stars
Send offprint requests to: J. Greiner (firstname.lastname@example.org)
© European Southern Observatory (ESO) 1999
Online publication: March 1, 1999