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

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6. Discussion

There are two reasons why V2301 Oph has been considered to be a potential intermediate polar candidate: (1) most (but not all) AM Hers are very soft X-ray sources while most (but not all) intermediate polars have hard spectra like V2301 Oph; and (2) given the observed very low mean magnetic field, it should be difficult to prevent the formation of a disk and, in turn, an intermediate polar.

The total number of known AM Her stars and intermediate polars has increased dramatically within the last 5 years, due largely to the RASS (Beuermann & Burwitz 1995), and it is now clear that the class of an object cannot be derived solely from its X-ray hardness. Beuermann & Schwope (1994) have shown that there is a tight, empirical relationship between the flux ratio of the optically thick, soft X-ray component and the optically thin, hard X-ray component and the magnetic field strength in all AM Her systems (in the sense that higher magnetic fields imply more flux in the soft component). This effect is easily explained as being due to the increasing importance of cyclotron cooling in systems with higher magnetic field strengths (Beuermann 1997). The fitted spectral parameters for this "Standard AM Her Model" ([FORMULA] keV, [FORMULA] =25 ev) using our data and the H column density derived above are also shown in Table 2. No significant soft-component could be detected: the resulting 2- [FORMULA] lower limit to the ratio of the hard Bremsstrahlung and the soft Blackbody component ROSAT fluxes [FORMULA] is 1.4. Thus, the X-ray emission from V2301 Oph fits the empirical relation and our theoretical expectations quite nicely: the bremsstrahlung component is dominant in the system with the smallest measured magnetic field (Ferrario et al. 1995).

The X-ray light curves clearly show that V2301 Oph is not an intermediate polar. Although we can only rule out a spin-pulse with nearly 100% amplitude, empirically, no intermediate polar has a mean quasi-sinusoidal orbital light curve which peaks near phase 0.9. While X-ray "dips" in the form of substantial phase-dependent absorption of the underlying X-ray flux are seen in a wide range of CVs with accretion disks, including a large number of intermediate polars (Hellier, Garlick & Mason 1993), the absorption is always strongest at orbital phases 0.6-0.8, whereas the dips in V2301 Oph occur later within a much narrower phase range (0.8-0.95). This latter type of dipping behavior is common in AM Her stars with large inclinations (Watson 1995) and is attributable to the passage of the accretion stream along the line-of-sight between the underlying X-ray source on the white dwarf and the observer. Since this can occur only if the stream has been vertically deflected from its trajectory in the orbital plane, the phase of the absorption dips roughly indicates where the magnetic threading of the stream occurs. The maximum dipping in V2301 Oph occurs at the same phase as the peak of the X-ray light curves, a situation expected for a synchronously rotating magnetic accretor.

The phasings of the PSPC and HRI light curves already suggest that [FORMULA] must be greater than about 150 yr. If the white dwarf in V2301 Oph accretes from a Keplerian disk, it must be spinning up on a timescale (e.g. Wang 1987)


where [FORMULA] is the torque function of the fastness parameter [FORMULA], and where a white dwarf mass [FORMULA] of 0.9 M [FORMULA], an inner disk radius [FORMULA] of 52% of the spherical Alven radius [FORMULA], a moment of inertia I of [FORMULA] g cm2, and a surface field strength of 7 MG have been assumed (yielding [FORMULA] and [FORMULA]). Large variations in the mass-accretion rate on timescales of [FORMULA] yr are certainly possible (King et al. 1996), so that V2301 Oph might have suffered a long period of nearly zero mass-transfer in the past, and have slowed down to nearly synchronous rotation. However, given the still small number of known magnetic CV systems, it seems very unlikely that we would catch a system like V2301 Oph just at the point where a major change in the accretion has occurred, even taking possible selection effects into consideration. Thus, the lack of a measureable spin period makes it very unlikely that an accretion disk is currently present in V2301 Oph.

The only remaining reason for believing that V2301 Oph could still be an intermediate polar - in spite of all the other evidence - is our expectation, that the very low mean magnetic field may not be strong enough to prevent the formation of a disk. The position and kinematics of the magnetospheric boundary layer - the region where the accretion stream threads the magnetic field of the white dwarf - provides us with a direct measure of the strength of the magnetic field. We will study this region in great detail and so address this final question in Paper II (Hessman et al., in preparation).

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998