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Astron. Astrophys. 327, 245-251


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The eclipsing AM Herculis binary V2301 Ophiuchi

I. ROSAT & IUE observations

F.V. Hessman1, K. Beuermann1,2, V. Burwitz1, D. de Martino3,4, and H.-C. Thomas5

1Universitäts-Sternwarte, Geismarlandstr. 11, D-37083 Göttingen, Germany
2Max-Planck-Institut für extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany
3VILSPA, Villafranca, Apartado 50727, E-28080 Madrid, Spain
4Osservatorio Astronomico de Capodimonte, via Moiariello n. 16, I-80131 Napoli, Italy
5Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85740 Garching, Germany

Received 16 April 1997 / Accepted 19 June 1997

Abstract

We present X-ray and UV observations of the unique eclipsing cataclysmic variable V2301 Oph (1H1752+081). The X-ray eclipse of the primary star is total and corresponds in phase with the optical eclipse of the white dwarf. The X-ray light curve shows a quasi-sinusoidal orbital ''hump'' typical of AM Her systems. The orbital phase at which the hump has its peak has remained at FORMULA0.9 in all ROSAT observations taken over several years. We can rule out a 100% modulated spin-pulse signal with a period shorter than FORMULAorb. Thus, it is very unlikely that there is either a non-synchronously rotating white dwarf or an accretion disk in V2301 Oph. The relatively hard X-ray spectrum of this weakly-magnetic system does not change its shape during the orbit or around the eclipse and is consistent with that expected from a hot accretion column on a weakly magnetized white dwarf. The interstellar absorbing column density of FORMULA H-atoms cm-2 required by the X-ray spectral fits is capable of hiding any weak soft X-ray component. The lower limit to the relative contributions of a hard bremsstrahlung and a soft blackbody component is consistent with both the empirical relation for AM Hers and the theoretical expectations for a system with a low magnetic field strength. The irregular ''dips'' present in the light curve around phases 0.85-0.92 with depths up to 80% are not accompanied by significant changes in the hardness ratios and must be due to partial covering by a structured and blobby accretion stream which has been lifted out of the orbital plane by the magnetic field of the primary. The close agreement between the phasing of the dips and the peak of the X-ray light curves is expected in a synchronous magnetic accretor. The relative strengths of the NV, CIV, and HeII UV emission lines are typical for AM Her stars. Thus, the X-ray orbital light curves and the X-ray and UV spectra clearly identify V2301 Oph as an AM Her system.

Key words: stars: individual: V2301 Oph - stars: cataclysmic variables - stars: eclipsing binaries - stars: magnetic fields - X-rays: stars - UV: stars

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© European Southern Observatory (ESO) 1997

Online publication: October 1, 1997
Last change: April 8, 1998
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