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Astron. Astrophys. 328, 565-570

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A large X-ray flare on HU Virginis

M. Endl1, K.G. Strassmeier1, and M. Kürster1,2*

1Institut für Astronomie, Universität Wien, Türkenschanzstraße 17, A-1180 Wien, Austria
(endl@venus.ast.univie.ac.at, strassmeier@astro.ast.univie.ac.at)
2Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, D-85740 Garching, Germany

Received 24 June 1997 / Accepted 5 August 1997


We present two ROSAT-HRI observations of the RS CVn-type binary HU Virginis (=HD 106225). The first observation covers almost three consecutive rotation cycles of HU Virginis or a total of 26.7 days and an exposure time of 35 ksec. On JD 2,449,544 a long duration flare was detected and observed for 1.5 days. This event was releasing a total energy of $\approx 7.7^{+3.3}_{-1.9} \times 10^{36}$ erg in the 0.1-2.4 keV bandpass. The good coverage of the onset and maximium phase of the flare light curve allowed a detailed comparison with two solar flare models. We derived an estimate for the size of the active region responsible for the flare. The resulting loop size is of the order of one stellar radius. One year later, in 1995, HU Virginis was again observed by ROSAT continuously for 8 days and a total exposure time of 69 ksec. The X-ray flux shows variability on time scales shorter than the rotational period.

Key words: stars: activity - stars: coronae - stars: flare - stars: individual: HU Vir - X-rays: stars

*Current address: ESO, Casilla 19001, Vitacura, Santiago 19, Chile, mkurster@eso.org

Send offprint requests to: K.G. Strassmeier

© European Southern Observatory (ESO) 1997

Online publication: November 24, 1997
Last change: March 26, 1998