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

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 <sup>2

1</sup> 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

Abstract

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 (jgreiner@aip.de)

SIMBAD Objects

Contents

• 1. Introduction
• 2. Observations
  • 2.1. X-ray observations
  • 2.2. Archival IUE observations
  • 2.3. Optical observations
    • 2.3.1. Photometric observations
    • 2.3.2. Spectroscopic observations
• 3. Results
  • 3.1. Identification of the X-ray source with V 751 Cyg
  • 3.2. Relation of optical and X-ray intensity
  • 3.3. The extinction towards and distance of V 751 Cyg
  • 3.4. The inclination of the V 751 Cyg binary
  • 3.5. The X-ray spectrum and luminosity of V 751 Cyg
  • 3.6. Re-assessment of the HEAO-1 detection of V 751 Cyg
  • 3.7. The optical emission
• 4. V 751 Cyg
  • 4.1. V 751 Cyg as a supersoft X-ray binary
  • 4.2. Comparison of V 751 Cyg to RX J0513.9-6951
• 5. Are other VY Scl systems also SSBs?
  • 5.1. X-ray emission of VY Scl stars at optical high state
  • 5.2. Are all VY Scl stars supersoft X-ray binaries?
    • 5.2.1. Group properties of VY Scl stars and how they fit the supersoft X-ray binary scenario
  • 5.3. Accretion disk during optical state transitions
  • 5.4. Detectability of VY Scl systems in UV/EUV surveys
• 6. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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