W. Wenzel

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.

Rapid searches for counterparts of GRB 930131

A fading counterpart to a gamma-ray burst (GRB) would appear as a point source inside a GRB error region soon after the burst which dims on a timescale from minutes to days. The favorable circumstances of the burst GRB 930131 allowed for an international campaign to search for fading counterparts starting 6.8 hr after the burst. We report observations from many optical sites, two radio telescopes, and archival ROSAT data, including deep Schmidt exposures 35, 44, and 64 hr after the burst. No fading counterparts were detected with our observations.

Simultaneous optical/gamma-ray observations of GRBs

We report on the on-going project to search for serendipitous time-correlated optical photographic observations of γ-ray bursts (GRBs). The regular photography at 12 observatories is used to look for plates which have been exposed simultaneously with a GRB detected by BATSE and contain its position. The results of this search for GRBs detected during the previous 4 yrs of BATSE operation are presented.

European Observation Network: first experience with BACODINE triggers

We present preliminary results obtained with the European Observation Network. This network consists of 9 observatories in the Czech Republic, Germany and Bulgaria and has been involved in the BACODINE activities since April 1, 1994. We also discuss related problems such as the background of unknown variable stars and suggest a strategy for work in this area.

Searches for optical counterparts to GRBs: EON and AIO

The recent results of the searches for optical counterparts provided by the programs operated by the Astronomical Institute Ondřejov (AIO) and under collaboration of several European observatories (European Observation Network, EON) are outlined and discussed, as well as new projects and approaches in development and/or in preparation. Most of the results have been obtained within the framework of the GRO/BATSE/BACODINE programs. We also briefly mention and discuss new results obtained in archival searches for optical activity inside the error boxes of precisely positioned GRBs.