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

• Published in: Astronomy and astrophysics (Berlin) 2005-06, Vol.436 (3), p.1041-1047
• Main Authors: Mitra-Kraev, U., Harra, L. K., Williams, D. R., Kraev, E.
• Format: Article
• Language: English
• Subjects: stars: coronae; stars: flare; stars: individual: AT Mic; stars: magnetic fields; stars: oscillations; X-rays: stars
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361:20052834

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 $(2.5 \pm 0.2)\times10^{10}$ 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.