  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 (2.5 +- 0.2) e10
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.

Astrophysics