Simultaneous X-ray and optical monitoring of the flare star YZ CMi.

Abstract

Broad-band photometric observations of YZ CMi show a 1.2 mag U-band flare, however, simultaneous X-ray observations from EXOSAT show no detectable increase. It is suggested that the event could have occurred low down in a part of the chromosphere, which may not have been coupled to the transition region and corona via magnetic fields. However, a more plausible explanation may have been absorption of the X-rays by an over-lying dense prominence. Several short-lived enhancements (micro-flares) were detected in both the Johnson U-band and the Balmer emission lines.