A Large Millimeter Flare on the RS CVn Binary σ Geminorum

Abstract

The flaring behavior of active RS CVn binaries has been studied extensively at centimeter wavelengths, but very little has been published concerning RS CVn flares in the millimeter region, where the nonthermal gyrosynchrotron emission is much more likely to be optically thin. We present 3 mm (99 GHz) observations of a large flare on the active binary σ Gem using the Owens Valley Millimeter Array. Two 3 hr observations in 2004 February showed only 3 σ upper limits of 3 mJy, while the third observation on 2004 April 22 showed a large flare outburst that filled the 6 hr observation and reached a peak flux density of 90 mJy. The strongest brightening was preceded by at least a 4 hr period of significantly elevated emission at 20-40 mJy. These observations demonstrate the feasibility of detailed multifrequency studies of nonthermal electron distributions in active star coronae with the next generation of millimeter and centimeter radio arrays.