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Astron. Astrophys. 327, 245-251 (1997)

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1. Introduction

Cataclysmic variables (hereafter CV's) are close binary systems containing a white dwarf primary and a late-type secondary main-sequence star. The secondary fills its Roche lobe and transfers material via the inner Lagrangian point to the primary. If the primary is only weakly magnetic (a DQ Her system or Intermediate Polar) or non-magnetic, the specific angular momentum of the accretion stream results in the formation of an accretion disk, with a warm, shocked region - the "hot-spot" - where the stream hits the outer disk. In magnetic systems with large primary surface magnetic fields of typically 10-60 MG (the AM Her variables), the material in the accretion stream is eventually directed along the magnetic field lines of the primary, producing a very hot shock near the surface of the white dwarf.

The HEAO-1 Modulation Collimator Survey source V2301 Oph (1H1752+081) was identified as a deeply eclipsing CV with an orbital period of 113 minutes by Remillard et al. (1992). Silber (1992; Silber et al. 1994) obtained CCD photometry of the system which showed features reminiscent of the eclipses of a classical disk "hot spot" and a white dwarf. The derived disk radius was unusually small, with a radius roughly corresponding to the specific angular momentum of the accretion stream. However, no features attributable to a disk eclipse were seen, suggesting that V2301 Oph might be an AM Her. The main argument against the latter model was that V2301 Oph is a very hard X-ray source whereas most AM Her's are observed to be quite soft.

Barwig, Ritter & Bärnbantner (1994; hereafter BRB) obtained high-speed photoelectric photometry and higher-resolution time-resolved spectroscopy of V2301 Oph (their ephemeris will be used in all analyses which follow). The variable ingress times of the "hot-spot" eclipse and the high amplitude radial velocity variations of the Balmer emission lines led them to identify V2301 Oph as an AM Her system. The duration of the "white dwarf" eclipse ingress lasted twice as long as the egress, implying that the accretion spot had some longitudinal structure on the face of the white dwarf. Despite the difference in the eclipse times, the amplitude of the eclipse was the same. This effect went away during the "low-state" seen in 1995, during which the eclipse times were the same (Barwig, private communication).

The magnetic nature of V2301 Oph was finally determined by Ferrario et al. (1995), who observed the Zeeman splitting of the Balmer absorption lines from the white dwarf during a low state. They determined a mean magnetic field strength of only 7 MG, significantly smaller than the next lowest magnetic fields in AM Hers (Beuermann & Burwitz 1995): about 12-14 MG in AM Her itself (Bailey et al. 1991), EF Eri (Östreicher et al. 1990), and RXJ1957-57 (Thomas et al. 1996). Thus, the polarized optical cyclotron lines seen in the intermediate-to-high states of other AM Hers are located in the near infrared in V2301 Oph, explaining not only why the classical signs of a magnetic accretor weren't seen previously, but why Ramsay & Cropper (1994) and Ferrario et al. (1995) could only find an upper limit of about 1% to the optical broad-band circular polarisation - usually a sign of an intermediate polar. However, imi et al. (1997) show that the Balmer emission line and optical continuum light curves are more easily explained as begin due to emission from the accretion stream and the accreting poles rather than from a bright spot at the edge of a disk.

As the first part of a multi-wavelength study of V2301 Oph, we present X-ray and UV observations of V2301 Oph which enable us to shed further light on the nature of this unusual object. In a second paper, we will present phase-resolved spectroscopy of the optical emission lines and a tomographic analysis of the magnetospheric boundary layer.

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998