1995A&A...296..509H

We present ROSAT observations of an X-ray flare on the G8III-type giant β Boo, which seems to be the first known flare on a single, low-activity, late-type giant. The flare occured on 8 August 1993. The PSPC count rate increased within ∼5 minutes nearly by a factor of 10. Most of the decay phase could be traced, before the observation interval ended. We derive plasma temperatures and emission measures for both the quiescent and the flare phases, and determine the integrated X-ray energy release of the flare to be ≃1.7x10³²erg. The flare can be modeled by the quasi-static cooling loop approximation, and characteristic loop properties like plasma density, flaring volume and loop length are estimated. In comparison to well studied solar and stellar flares, the β Boo flare seems to be similar to solar compact flares or their counterparts in M-type dwarfs, although the emission measure and energy release are much larger than in the Sun. The possibility of an undetected M-type dwarf companion of β Boo being a source of the flare is discussed.