VLBI observations of a strong radio flare in HR 1099

Abstract

In 1991 July, we studied a strong radio flare in the RS CVn system HR 1099 with VLBI techniques. This system is one in a list of stars which we regularly monitor with the Noto 32-m antenna at 6 cm. The angular size of the radio source was found to increase at the beginning of the flare. It reached about 4 mas, which corresponds to a linear size comparable to that of the entire binary system. An upper limit of 80 G for the magnetic field was derived. The flare decay reflects the change of the power spectrum of the emitting electrons towards lower energies, due to radiative losses and interaction with the local plasma