EDP Sciences
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Volume 392, Number 2, September III 2002
Page(s) 505 - 514
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20021193

A&A 392, 505-514 (2002)
DOI: 10.1051/0004-6361:20021193

A new two-pole accretion polar: RX J1846.9+5538

R. Schwarz1, J. Greiner1, 2, G. H. Tovmassian3, S. V. Zharikov3 and W. Wenzel4

1  Astrophysical Institute Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
2  Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85740 Garching, Germany
3  Instituto de Astronomía, UNAM, Apdo.Post. 877, 22860 Ensenada, B.C., México
4  Sonneberg Observatory, 96515 Sonneberg, Germany

(Received 9 April 2002 / Accepted 29 May 2002)

We report the discovery of a new, bright ( $V\sim 17^{\rm m}$) AM Her system as the optical counterpart of the soft ROSAT All-Sky-Survey source RX J1846.9+5538 ( =1RXS J184659.4+553834). Optical photometric and spectroscopic follow-up observations reveal a single period of 128.7 min, consistent with a high degree of spin-orbit synchronization, and a low polar field strength ( B<20 MG) of the primary accretion region. The system was observed in optical intermediate and high states that differ by about 1 mag. These brightness variations were accompanied by a correlated change of the optical light curve, which we interpret as a switch between one- and two-pole accretion. This explanation is also supported by the X-ray light curves, which at two different epochs display emission from two equally bright accretion regions separated by ~ $160^{\circ}$. Both spots possess distinct spectral X-ray properties as seen from the X-ray hardness ratio, where the secondary accretion region appears significantly softer, thus probably indicating a higher field strength compared to the primary region. In all ROSAT pointings a deep dip is present during the primary flux maxima, very likely caused by absorption in one of the accretion streams.

Key words: X-rays: stars -- cataclysmic variables -- accretion -- stars: magnetic fields -- stars: individual: RX J1846.9+5538

Offprint request: R. Schwarz, rschwarz@aip.de

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