HR-5110 SUPERFLARE - AN INTERBINARY FLARE IDENTIFIED

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