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Astron. Astrophys. 326, 195-202

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RX J2115.7-5840: a short-period, asynchronous polar*

A.D. Schwope1, D.A.H. Buckley2, D. O'Donoghue3, G. Hasinger1, J. Trümper4, and W. Voges4

1Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany
2South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town, RSA
3University of Capetown, Dept. Astronomy, Ronde Bosch, 7700, RSA
4Max-Planck-Institut für Extraterrestrische Physik, D-85740 Garching, Germany

Received 24 March 1997 / Accepted 25 April 1997


We report phase-resolved optical polarimetric, photometric and spectroscopic observations of RX J2115.7-5840 (= EUVE J2115-58.6, Craig 1996) which confirms the system to be a magnetic cataclysmic binary of the polar (AM Herculis) subclass. The optical light curve is sometimes flat and occasionally displays a pronounced bright phase, reminiscent of the self-eclipse of a small accretion spot by the revolving white dwarf, as seen in self-eclipsing polars. Our period search reveals ambiguous results only which can be interpret assuming that the white dwarf is not synchronously rotating with the binary orbit. We find circularly polarized cyclotron radiation with V/I ranging from 0% to FORMULA on one occasion, from 8% to FORMULA on another occasion. Compared with other polars, the self-occulted accretion region of RX J2115.7-5840 had an extreme red cyclotron spectrum. In addition, the system has an extreme hard X-ray colour during the ROSAT all-sky survey observation. Both properties suggest a low value of the magnetic field strength, and our best estimate gives FORMULA MG. Due to the absence of significant M-star features in our low-resolution spectra we estimate the minimum distance to RX J2115.7-5840 to be d > 250 pc (for an M5+ secondary star).

Key words: accretion - cataclysmic variables - AM Herculis binaries - stars: RX J2115.7-5840 )

*Based in part on observations at the European Southern Observatory La Silla (Chile) with the 2.2m telescope of the Max-Planck-Society

Send offprint requests to: A. Schwope (ASchwope@aip.de)

© European Southern Observatory (ESO) 1997

Online publication: September 9, 1997
Last change: April 20, 1998