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Astron. Astrophys. 327, 183-190 (1997)

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4. Discussion and conclusions

Our optical and X-ray observations of the soft X-ray sources 2022-39 and 0132-65 revealed that both objects are AM Herculis systems with comparable high magnetic field strengths ([FORMULA] 67 MG and [FORMULA] 68 MG, respectively). Both systems have orbital periods of only [FORMULA] 78 min. In the case of 2022-39, it is well documented by the radial velocity curve of the Balmer and He II emission lines that the photometric period is indeed the orbital period. In 0132-65, the orbital period could in principle be twice the photometric period, but we consider this unlikely as the shape of the optical lightcurve repeats very well with the 78 min period and there is no indication that the system might show two photometric maxima per orbit.

We can confine the accretion geometry of 2022-39 as follows: The dip at phase [FORMULA] seen in the optical, and probably also in the X-ray lightcurve is a feature which is similarly present in the lightcurves of other polars, e.g. EF Eri and EK UMa (Beuermann et al. 1987 , Beuermann & Thomas 1993 ). It is probably caused when the accretion stream crosses our line-of-sight and occults part of the accretion region on the white dwarf. This implies that the orbital inclination i must be larger than the co-latitude [FORMULA] of the accretion spot ([FORMULA]). This picture is supported by the phasing of the radial velocity curve of the emission lines which reach maximum redshift at [FORMULA]. As we do not resolve the broad and narrow line components which are frequently seen in higher-resolved spectra of polars, our Doppler-shift measurements certainly do not only include the magnetically confined part of the stream but also the free-falling trajectory leaving the secondary, and the narrow emission line from the heated face of the secondary. Maximum redshift, therefore, expectedly occurs shortly after the dip. During the optical maxima at phases [FORMULA] and [FORMULA] the cyclotron harmonics are most pronounced which is expected as the cyclotron emission is beamed orthogonally to the field lines and thus best visible when we view it perpendicular to the field lines in the accretion region. Except for the dip phase, X-ray emission is observed at all orbital phases with enhanced flux occurring between [FORMULA] and [FORMULA]. This implies that the accretion region does not completely disappear behind the horizon during the rotation of the white dwarf and gives the constraint [FORMULA]. As there is no eclipse seen of the white dwarf by the secondary star, we conclude that [FORMULA] and [FORMULA] for 2022-39.

A similar picture emerges from our observations of 0132-65. The large-amplitude variation of the optical flux is probably due to cyclotron beaming. This is supported by the average bright-phase and faint-phase optical spectra which show cyclotron harmonics only in the bright phase. This means that the accretion region is oriented towards the observer at [FORMULA] (i.e. optical minimum) and is located at the rim of the white dwarf at [FORMULA] (i.e. optical maximum). Consequently, the maximum of the soft X-ray flux occurs during the optical faint phase. The X-ray flux vanishes for [FORMULA] 0.1 of the orbit just around the optical maximum ([FORMULA]) which may be due to self-eclipse by the white dwarf. There may also be an X-ray dip at [FORMULA], shortly before the X-ray maximum. Like in 2022-39, this could be caused by the accretion stream crossing our line-of-sight.

0132-65 and 2022-39 have the shortest two orbital periods found so far in polars. Together with EV UMa (= RE 1307+535, Osborne et al. 1994 , Hakala et al. 1994 ), RX J1015.5+0904 (Burwitz et al. 1996 ), and FH UMa (=RX J1047.1+6335, Singh et al. 1995 ) they form a new group of systems with orbital periods below 80 min. Prior to the ROSAT soft X-ray and the WFC EUV surveys, EF Eri ([FORMULA] min has been the only system at the short-end of the period distribution (Ritter & Kolb 1992 ). The existence of a peak of systems with [FORMULA] min) has been predicted by evolutionary scenarios (e.g. Kolb & de Kool 1993 ).

At [FORMULA] min, the two new magnetic CVs are found at or close to the theoretical minimum orbital period where the companion star moves more or less rapidly towards degeneracy (Paczynski & Sienkiewicz 1981 , Rappaport et al. 1982 , Nelson et al. 1985 , and references therein). Depending on the opacity in its atmosphere, the temperature may drop substantially, and the star becomes practically invisible. Our assumptions on its spectral type and the resulting distance estimate should be considered with caution, therefore. The long-term mass transfer rate, on the other hand stays at [FORMULA] /yr for some time after the minimum period is passed and the system should remain a moderately luminous X-ray source. If the observed fluxes are not substantially below any long-term mean values, the expected luminosity places the systems at distances comparable to those estimated above, i.e. a few hundred parsecs.

Finally, we comment on the fact that the field strengths of the two new polars are among the largest reported so far for members of their class (see e.g. Beuermann 1997 ). Most polars seem to crowd at field strengths below approximately 70 MG independent of orbital period (with the exception of AR UMa which has 230 MG, Schmidt et al. 1996 ). Contrary to neutron stars (Romani 1990 ), accretion seems to be ineffective in reducing the polar field strengths of white dwarfs, a situation which can be understood as a consequence of differences in the two dominant time scales, the flow time scale which carries the field into the interior of the star along with the accreted matter and the Rayleigh-Taylor time scale (among others) which tends to reestablish the field structure. In neutron stars the flow into the star is rapid and causes a secular reduction in surface field strength while in white dwarfs the flow is comparatively slow and such reduction is prevented (see Beuermann 1997 ).

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998