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Astron. Astrophys. 331, 262-270 (1998)

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A new ROSAT discovered polar near the lower period limit: RX J1015.5+0904 in Leonis *

V. Burwitz 1, K. Reinsch** , 1, A.D. Schwope** , 2, P.J. Hakala 3, K. Beuermann 1, 4, Th. Rousseau 1, H.-C. Thomas 5, B.T. Gänsicke 1, V. Piirola 6 and O. Vilhu 7

1 Universitäts-Sternwarte Göttingen, Geismarlandstr. 11, D-37083 Göttingen, Germany
2 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany
3 Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
4 MPI für Extraterrestrische Physik, Giessenbachstr. 1, D-85740 Garching, Germany
5 MPI für Astrophysik, Karl-Schwarzschild-Str. 1, D-85740 Garching, Germany
6 Tuorla Observatory, FIN-21500 Piikkio, Finland
7 Observatory and Astrophysics Laboratory, Box 14, FIN-00014 Helsinki, Finland

Received 28 August 1997 / Accepted 24 October 1997

Abstract

We have identified the bright, soft, X-ray source RX J1015.6 [FORMULA] 0904 detected during the ROSAT All-Sky Survey with a new AM Herculis binary (polar). Its soft X-ray light curve is typical for a system with a one-pole accretion geometry and shows a bright phase lasting about 0.75 of the binary orbit with ROSAT PSPC countrates of up to 10 cts/s. The bright phase is interrupted by a 3 min long dip with zero countrate where the X-ray emission from the accretion region must be absorbed by the accretion stream crossing our line-of-sight. From the X-ray dip times we derive an orbital period Porb  = 79.87946(7) min making this system another member of the new group of polars discovered near the theoretical lower period limit for magnetic CVs. In the optical, the light curve is double-humped and the system varies between V = 16-17 mag in the high state. The optical light is highly polarized with the degree of circular polarization varying between -5% and +30% in R and -5% and 22% in I which is a strong indication for the magnetic nature of the system. The shape of the optical light curve and the broad optical continuum component seen as additional light during the bright-phase can be explained by cyclotron emission in a magnetic field of [FORMULA] 23 MG in the accretion region. At this field strength only the higher cyclotron harmonics are present in the optical and are smeared to an almost featureless continuum at a viewing angle [FORMULA]. The orbital inclination of the new AM Her binary can be constrained to [FORMULA] and the mass ratio must be [FORMULA] where the mass of the secondary star is estimated to [FORMULA] [FORMULA]. No features of the late M-type secondary are present in our low-state optical spectra, providing a lower limit for the distance to RX J1015.6+0904 of [FORMULA] pc

Key words: accretion, accretion disks – stars: individual (RX J1015.6+0904) – stars: novae, cataclysmic variables – X-rays: stars – polarization

* Based on observations collected at the European Southern Observatory, La Silla, Chile with the 2.2-m telescope of the Max-Planck-Society in MPI time and with various telescopes in ESO time (ESO [FORMULA] 50.6-021 and 54.D-0698).
** Visiting Astronomer, German-Spanish Astronomical Center, Calar Alto, operated by the Max-Planck-Institute für Astronomie Heidelberg jointly with the Spanish National Commission for Astronomy.

Send offprint requests to: V. Burwitz (burwitz@mpe.mpg.de)

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© European Southern Observatory (ESO) 1998

Online publication: February 4, 1998
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