EDP Sciences
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Volume 456, Number 1, September II 2006
Page(s) 323 - 327
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20054752

A&A 456, 323-327 (2006)
DOI: 10.1051/0004-6361:20054752

The periodic variations of a white-light flare observed with ULTRACAM

M. Mathioudakis1, D. S. Bloomfield1, D. B. Jess1, V. S. Dhillon2 and T. R. Marsh3

1  Department of Physics and Astronomy, Queen's University Belfast, Belfast, BT7 1NN, Northern Ireland, UK
    e-mail: M.Mathioudakis@qub.ac.uk
2  Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH, UK
3  Department of Physics, University of Warwick, Coventry, CV4 7AL, UK

(Received 22 December 2005 / Accepted 19 April 2006)

High time resolution observations of a white-light flare on the active star EQ PegB show evidence of intensity variations with a period of $\approx$10 s. The period drifts to longer values during the decay phase of the flare. If the oscillation is interpreted as an impulsively-excited, standing-acoustic wave in a flare loop, the period implies a loop length of $\approx$3.4 Mm and $\approx$6.8 Mm for the case of the fundamental mode and the second harmonic, respectively. However, the small loop lengths imply a very high modulation depth making the acoustic interpretation unlikely. A more realistic interpretation may be that of a fast-MHD wave, with the modulation of the emission being due to the magnetic field. Alternatively, the variations could be due to a series of reconnection events. The periodic signature may then arise as a result of the lateral separation of individual flare loops or current sheets with oscillatory dynamics (i.e., periodic reconnection).

Key words: waves -- stars: activity -- stars: atmospheres -- stars: flare -- stars: individual: EQ Pegasi

© ESO 2006