Astron. Astrophys. 326, 195-202 (1997)

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

A.D. Schwope ¹, D.A.H. Buckley ², D. O'Donoghue ³, G. Hasinger ¹, J. Trümper ⁴ and W. Voges <sup>4

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

Received 24 March 1997 / Accepted 25 April 1997

Abstract

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 ranging from 0% to on one occasion, from to 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 . 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 (for an 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)

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Contents

• 1. Introduction
• 2. Observations and analysis
  • 2.1. The ROSAT all-sky survey (RASS) observations
  • 2.2. Spectroscopic observations
  • 2.3. Photometry
  • 2.4. Polarimetry
• 3. Results and discussion
  • 3.1. Period search
  • 3.2. The cyclotron spectrum of the far pole
  • 3.3. A distance estimate
  • 3.4. RX J2115.7-5840 as X-ray and EUV-emitter
  • 3.5. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998
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