A&A 436, 1041-1047 (2005)
DOI: 10.1051/0004-6361:20052834

The first observed stellar X-ray flare oscillation: Constraints on the flare loop length and the magnetic field

U. Mitra-Kraev, L. K. Harra, D. R. Williams and E. Kraev

Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
    e-mail: umk@mssl.ucl.ac.uk

(Received 7 February 2005 / Accepted 12 March 2005)

Abstract
We present the first X-ray observation of an oscillation during a stellar flare. The flare occurred on the active M-type dwarf AT Mic and was observed with XMM-Newton. The soft X-ray light curve (0.2-12 keV) is investigated with wavelet analysis. The flare's extended, flat peak shows clear evidence for a damped oscillation with a period of around 750 s, an exponential damping time of around 2000 s, and an initial, relative peak-to-peak amplitude of around 15%. We suggest that the oscillation is a standing magneto-acoustic wave tied to the flare loop, and find that the most likely interpretation is a longitudinal, slow-mode wave, with a resulting loop length of  cm. The local magnetic field strength is found to be 105  50 G. These values are consistent with (oscillation-independent) flare cooling time models and pressure balance scaling laws. Such a flare oscillation provides an excellent opportunity to obtain coronal properties like the size of a flare loop or the local magnetic field strength for the otherwise spatially-unresolved star.

Key words: stars: coronae -- stars: flare -- stars: magnetic fields -- stars: oscillations -- X-rays: stars -- stars: individual: AT Mic

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