A Search for Stellar Flares in Gamma-Ray Bursts Observed by BATSE and ULYSSES

Abstract

We have searched for gamma-ray bursts during stellar flares using Ulysses and BATSE/CGRO data and for a correlation between flare star positions and burst positions. More than 3000 stellar flares have been identified optically, but neither BATSE nor Ulysses observed any gamma-ray bursts which could be attributed to them. Using the BATSE trigger threshold, and the known distances to these flare stars, upper limits to the gamma-ray luminosity were obtained. We have also studied the conditions under which stellar flares could be detected by BATSE as weak GRBs. We found that if some weak events (10^-7 ergs cm^-2 s^-1) are to be explained by flares as recently suggested, the X-ray emission (>25 keV) must be comparable to the stellar optical emission, and the ratio of L_x/L_opt Must be higher than that for solar flares by at least four orders of magnitude. The stellar flare log N-log S and spatial distributions are studied and their implications for gamma-ray bursts are discussed.