Flaring and quiescent X-rays from Castor.

Using data obtained with the Low Energy (LE) and Medium Energy instruments aboard Exosat, the first detection of both flaring and quiescent X-ray emission from the A-type visual binary Castor (alpha Gem) is reported. In the LE, Castor was clearly resolved from the nearby star YY Gem, which was also observed to flare some hours after Castor. After verifying that the Castor flare was indeed an X-ray as opposed to a UV event, physical parameters of the flaring source are derived. The detection of the quiescent emission led to reevaluation of the previous X-ray observations by the Einstein Observatory showing that, contrary to earlier reports, Castor was strongly detected in the IPC. Possible interpretations of the results are discussed by devoting some attention to the multiplicity of the Castor system, suggesting that the X-rays originate from an unseen late-type companion rather than from the A-type primaries.