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

We report the discovery of a new, bright (V 17 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 ob- servations 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 dier 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 dierent epochs display emission from two equally bright accretion regions separated by160. Both spots pos- sess 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.