RX J0515.6- AN UNUSUAL, ECLIPSING, MAGNETIC CATACLYSMIC VARIABLE(0105)

We report multiwavelength observations of the optical counterpart of t he ROSAT X-ray source RX J0515.6+0105. Optical photometric data spanni ng similar to 1 yr reveal eclipse features that are deep, total, and v ariable in profile. The eclipse timings give an orbital ephemeris of J D. similar or equal to 2449087.8173(3)+0.332612(7) E, with considerabl e phase Jitter caused by the variable eclipse profiles. The system is characterized by a soft thermal spectrum (kT(bb) similar or equal to 3 3 eV), strong He II lambda 4686 emission, large-amplitude radial veloc ity variations consistent with emission from an accretion stream, and, most significantly, significant circular polarization. Taken together , these characteristics strongly suggest that RX J0515.6+0105 is a mag netic cataclysmic variable, most likely an AM Herculis system in which the accretion column/stream is eclipsed. However, the possibility tha t the white dwarf rotates asynchronously, and that RX J0515.6+0105 is a DQ Her system, has not yet been ruled out unequivocally. The optical light curve is characterized by a strong asymmetric brightness modula tion at twice the orbital frequency, and by quasi-periodic oscillation s with a characteristic timescale of 6-15 minutes. The orbital modulat ion is likely caused by a combination of cyclotron beaming and by the varying aspect of the accretion stream and secondary star, both of whi ch are heated by radiation from the accretion column and are seen with the largest projected area at orbital quadrature. The cause of the qu asi-periodic oscillations is unknown, but may be related to radiation- driven oscillations near the inner Lagrangian point. Spectroscopic obs ervations suggest the possibility of cyclotron features near 8500 and 5900 Angstrom. These wavelengths are roughly consistent with those exp ected for the second and third harmonics of cyclotron radiation origin ating from an similar to 61 MG held. Observations obtained during ecli pse reveal the secondary star to be a slightly evolved M0-M1 dwarf, an d allow us to place a lower limit on the distance of RX J0515.6+0105 o f similar to 500 pc.