Radio outbursts in HR 1099: An inhomogeneous gyrosynchrotron model

Radio emission is the only available diagnostic of nonthermal phenomena in stars. Both quiescent and flaring emission have been detected in more than 30 RS CVn type binaries /1/. A set of almost simultaneous observations of the RS CVn system HR 1099 at five microwave frequencies between 1.4 and 86 GHz was performed during a long-lasting period of activity in February 1978 /2/,/3/. During most of this period the radio emission is moderately polarized and has a broad-band flux spectrum with positive slope at low frequencies and a turnover around 15 GHz. These properties of the radiation resemble those of solar microwave bursts. Because of this analogy the flux spectrum was interpreted in terms of gyrosynchrotron radiation from mildly relativistic non thermal electrons in /4/. A uniform magnetic field and a strong density decrease of energetic electrons with height above the photosphere of the primary star was assumed in their study. Although this model perfectly fits the observed spectrum, the assumption of constant field seems to be rather unrealistic, especially for the large source size implied if the observed fluxes arise from a noncoherent mechanism. The case of a heterogeneous magnetic field is considered in this contribution with emphasis on the spatial distribution of the brightness.