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Astron. Astrophys. 338, 465-478 (1998)

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7. Discussion

7.1. The nature of the object

In the previous sections we collected ample evidence that RX J0203 is a bona fide member of the AM Herculis sub-class of CVs. The tightest proof (while polarimetry is missing) is the identification of cyclotron humps in our high-state spectra confining the magnetic field in the accretion spot to be [FORMULA] MG. Our classification is supported by the presence of only one period in the optical photometric and spectroscopic data, regarded as the orbital period of the binary. The Doppler maps show no indication of a disk, the X-ray emission is highly instationary, the ROSAT X-ray spectrum has two components with the major flux being released as super-soft emission in the blackbody component. Taking all these facets together, we regard the classification as a polar as certain. The orbital period of 276.1 min is longer than that of any system known before ROSAT. Meanwhile, after intensive identification work of soft X-ray emitters detected with ROSAT and EUVE, two systems with longer orbital periods have been identified, V 1309 Ori (Shafter et al. 1995, [FORMULA] min), and V 895 Cen (Stobie et al. 1996, [FORMULA] min). With the exception of V 1309 Ori all polars, including in particular RX J0203 , host non-evolved secondaries.

The source is at a galactic latitude of [FORMULA] and a distance of [FORMULA] pc. This means that it is at a height [FORMULA] pc above the galactic plane and, with a scale height [FORMULA] pc, probably belongs to the galactic disk.

7.2. Spin-orbit synchronism and mass accretion rate

Observationally we found no clear evidence for an asynchronism between spin and orbital period. If we regard the photometric period [FORMULA] s as the spin period [FORMULA] of the white dwarf and the spectroscopic period [FORMULA] s as the orbital period [FORMULA], the degree of spin-orbit synchronism is [FORMULA]. Taking the uncertainty [FORMULA] s into account, one arrives at a more conservative limit of [FORMULA] for the likely degree of synchronism. This estimate rests on the assumption, that almost all observed morphological changes of the light curve are due to rather short-term changes of the accretion geometry.

In the high state most of the accretion energy is released as blackbody (EUV/soft X-rays), bremsstrahlung (hard X-rays), and cyclotron (optical, IR) radiation. These three components contribute as [FORMULA]3:1:1 parts to a total accretion luminosity of [FORMULA]L[FORMULA] (assuming [FORMULA]pc), which implies a mass accretion rate of [FORMULA]yr-1 (assuming [FORMULA]M[FORMULA]). Due to the uncertainty of [FORMULA], the difficulty to separate the proper cyclotron spectrum from the other radiation components and the uncertainties of the distance estimate, the given mass accretion rate might be inaccurate by as much as one order of magnitude. It is found only in rough agreement with Patterson's (1984) empirical [FORMULA]-P relation for CVs, which yields [FORMULA]M[FORMULA]yr-1 for the period of RX J0203 ([FORMULA] min). It is interesting to note that the two systems with orbital periods even longer than that of RX J0203 (V 1309 Ori and V895 Cen) also have estimated mass accretion rates lying a similar factor below Patterson's prediction. However, within the model outlined by Beuermann & Burwitz (1995, their Fig. 2) even a factor of 10 increase of the mass accretion rate would not break up synchronism at the field strength, orbital period and mass accretion rate determined in the present work.

7.3. The system geometry

From fits to the radial velocity and intensity variation of the NEL in high-resolution spectra we found the orbital inclination likely to be larger than 40o. X-rays are released in a highly instationary manner. The absence of any clear faint phase suggests that the accretion region undergoes no self-eclipse or that several accretion regions are active which are alternatively in view. In the first case the colatitude [FORMULA] of the accretion spot is constrained to [FORMULA], in the second case no constraint about the likely value(s) of [FORMULA] can be derived from the X-ray variability. Solutions with high orbital inclination predict a mass of the white dwarf in the vicinity of the canonical value of 0.6 M[FORMULA] and appear therefore more likely than solutions with low inclination (if our [FORMULA] correction is applicable). On the other hand, a high inclination will unevitably produce selfeclipses of the accretion region (if there is only a single region). Hence, high orbital inclination angles make multiple active regions more likely. However, no clear conclusions can be derived presently with the large allowed range of i.

Due to the lack of repeatability of the X-ray light curves we cannot give a likely value for the longitude of the accretion region. Such an estimate can be extracted from the phase of the optical minimum with respect to the line joining both stars if one assumes that the major impact on the optical light curves comes from cyclotron beaming. If so, the spectroscopic phase of the optical minimum indicates the longitude of the accretion region, [FORMULA], in the regular mode of accretion. The regular shape of the optical light curves and the smooth photometric variations seen at most occasions support the idea of an accretion region without selfeclipses (no flat-bottom photometric minima). Two pronounced deviations from the simple pattern of the photometric variation are obvious, firstly, the occurence of a strong colour-dependence of the photometric minimum and, secondly, the occurence of what we have referred to as non-regular mode of accretion. While the former probably may be explained in terms of radiative transfer effects in a structured accretion region, the latter is suggestive of an episode of a re-arrangement of the whole accretion region on a large scale. This might include (as in several other AM Her binaries) a switch between a simple one-pole to a more complicated two-pole geometry.

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© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998