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

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

Corresponding author: R. Schwarz, rschwarz@aip.de

Received: 9 April 2002
Accepted: 29 May 2002

Abstract

We report the discovery of a new, bright () 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 ( 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 ~. 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.