Astron. Astrophys. 328, 571-578 (1997)

4. Summary

The basic results of our observational study can be summarized as follows:

1. Based on the nature of the X-ray spectrum, the synchronized WD spin and orbital period, and despite the moderate strength of He II relative to H , the ROSAT all-sky survey source RX J0719.2+6557 is identified as a new eclipsing polar. The orbital period is found to be 98.2 min. Narrow eclipses are observed which correspond to the occultation of the hot spot on the accretion stream by the companion. The eclipse period coincides with the radial velocity variations of the major emission lines.
2. The high amplitude of the radial velocity variations and its phasing relative to the eclipses allow us to conclude that the major line emission also comes from the frame of the accretion stream and hot spot. However, line emission requires different conditions (e.g. density) than continuums emission, so that the two emission regions are expected to be distinct. We were successful to retrieve also the weaker component of emission lines formed on the heated side of the secondary. The geometric interpretation fits well the usual picture of eclipsing polars in analogy with other similar magnetic cataclysmic variables.
3. From the absorption of the X-ray spectrum and the inferred absolute magnitude of the companion as compared to the observed one we estimate a lower limit of the distance to -150 pc. The phase-averaged, unabsorbed X-ray luminosity is erg/s in the 0.1-2.4 keV range.
4. The Doppler tomography localizes the source of strong He II line emission to the accretion stream/hot spot. Other lines with lower excitation levels are more spread out over the system.
5. A low magnetic field for the white dwarf is suggested by three facts: (a) The optical spectrum does not show hints for cyclotron lines. (b) The X-ray spectrum is observed to be rather hard. The soft-to-hard X-ray flux ratio for polars with measured magnetic field strengths has been shown to increase with magnetic field (Beuermann & Schwope 1994). (c) The stronger photometric modulation in red than in blue and its phasing also argues for a low magnetic field.

© European Southern Observatory (ESO) 1997

Online publication: March 26, 1998

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