HR-5110 SUPERFLARE - AN INTERBINARY FLARE IDENTIFIED

A 73-ks pointed ROSAT observation revealed the RS CVn system HR 5110 to be in an outburst lasting over 3 d. Although the rise to outburst was not observed, over this 3-d period the PSPC count rate decayed to one-half of its initial value, which itself was some 4-5 times larger than the all-sky survey value. The count rate for the simultaneous WFC observations remained virtually constant throughout the pointing but was about a factor of 3.6 higher than its value during the all-sky survey. Calculations based on the spectral parameters show that a simple flare mechanism and standard two-ribbon flare models cannot explain the observations satisfactorily. This is the first time that such departures from a two-ribbon flare model have been identified unambiguously, and instead an interbinary flare is suggested as the cause of the outburst.