A Search for Hard X-ray Emission from Active Stars Using CGRO/BATSE

We report the results of a search for > 20 keV photons from active stars using CGRO/BATSE Earth-occultation observations. Twelve of the \usual suspects" together with 12 \placebo" locations have been analyzed using the BATSE software for occultation analysis developed at NASA/MSFC. There are four detections at the nominal 5 level, and eight at the 3 level. However the strongest detection (that of AB Dor) shows clear evidence for contamination from the nearby strong source LMC X{4. 18 of the 24 elds yield positive uxes, indicating a clear bias in the results, and possibly indicating the presence of weak background hard X{ray sources detectable by BATSE in long{term studies. So far, no star showing solar{like activity has been detected to emit photons at energies above 20 keV, i.e., in the hard X{ray and-ray range. Yet there is no question that such active stars must be sources of nonthermal >20 keV photons at some level, since they are strong soft X-ray sources, they show large ares in which we believe (by the solar analogy) that energetic nonthermal electrons are produced, and they exhibit nonthermal MeV-energy electrons in the form of steady nonthermal radio uxes. The high{energy uxes of active stars are potentially important for a number of branches of astrophysics: (i) in order to understand the nature and properties of the high{energy emission. In particular, is it largely connned to ares, as on the Sun? Or does it occur in a 1