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Astron. Astrophys. 343, 183-196 (1999)

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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

1 Astrophysical Institute Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany
2 Observatorio Astronomico Nacional, Instituto de Astronomía, UNAM, AP 877, 22860, Ensenada, B.C., México Harvard-Smithonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3 Laboratorio de Astrofísica Espacial y Física Fundamental, P.O. Box 50727, E-28080 Madrid, Spain
4 Budapest, Hungary

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 d[FORMULA]500 pc based on the extinction estimate of [FORMULA]=0.25[FORMULA]0.05. 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 (jgreiner@aip.de)

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999