Issue A&A Volume 436, Number 1, June II 2005 Page(s) 241 - 251 Section Stellar structure and evolution DOI http://dx.doi.org/10.1051/0004-6361:20042054

A&A 436, 241-251 (2005)
DOI: 10.1051/0004-6361:20042054

Flares observed with XMM-Newton and the VLA

K. Smith¹, M. Güdel² and M. Audard<sup>3

1</sup>  MPIfR, Auf dem Hügel 69, 53121 Bonn, Germany
    e-mail: ksmith@stsci.edu
²  Paul Scherrer Institut, Würenlingen und Villigen, 5232 Villigen PSI, Switzerland
    e-mail: guedel@astro.phys.ethz.ch
³  Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027, USA
    e-mail: audard@astro.columbia.edu

(Received 22 September 2004 / Accepted 16 December 2004)

Abstract
We present lightcurves obtained in X-ray by the XMM-Newton EPIC cameras and simultaneous radio lightcurves obtained with the VLA for five active M-type flare stars. A number of flare events were observed, and by comparing radio with X-ray data, we consider various possible flare mechanisms. In cases where there seems to be a clear correlation between radio and X-ray activity, we use an energy budget argument to show that the heating which leads to the X-ray emission could be due to the same particles emitting in the radio. In cases where there is radio activity without corresponding X-ray activity, we argue that the radio emission is likely to arise from coherent processes involving comparatively few particles. In one case, we are able to show from polarization of the radio emission that this is almost certainly the case. Cases for which X-ray activity is seen without corresponding radio activity are more difficult to explain. We suggest that the heating particles may be accelerated to very high energy, and the resulting synchrotron radio emission may be beamed in directions other than the line of sight.

Key words: stars: activity -- stars: coronae -- stars: flare

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© ESO 2005

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