Letter to the Editor

Doppler imaging an X-ray flare on the ultrafast rotator BO Mic^*,**

A contemporaneous multiwavelength study using XMM-Newton and VLT

¹ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: [uwolter;jrobrade;jschmitt]@hs.uni-hamburg.de
² Arizona State Universtity, PO Box 871404, Tempe, AZ, 85287-1404, USA e-mail: Jan-Uwe.Ness@asu.edu

Received: 11 October 2007
Accepted: 21 November 2007

Abstract

We present an analysis of contemporaneous photospheric, chromospheric and coronal structures on the highly active K-dwarf star BO Mic (Speedy Mic). We concentrate on a moderate flare that we localize in the stellar atmosphere and study its energetics, size and thermal behavior. The analysis is based on strictly simultaneous X-ray, UV- and optical observations carried out by XMM-Newton and the VLT. We use Doppler imaging and related methods for the localization of features. Based on X-ray spectroscopy we study the the coronal plasma in and outside the flare. The flare emits in X-rays and UV, but is not detected in white light; it is located at intermediate latitude between an extended spot group and the weakly spotted pole. We estimate its height to be below 0.4 stellar radii, making it clearly distinct in longitude and height from the prominences found about two stellar radii above the surface. In contrast to BO Mic's photospheric brightness, neither its chromospheric nor its X-ray emission show a pronounced rotational modulation.