GALEX high time-resolution ultraviolet observations of dMe flare events

¹ Experimental Astrophysics Group, Space Sciences Laboratory, University of California, 7 Gauss Way, Berkeley, CA 94720, USA e-mail: bwelsh@ssl.berkeley.edu
² Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland
³ California Institute of Technology, MC 405-47, 1200 East California Boulevard, Pasadena, CA 91125, USA
⁴ Solar Physics and Upper-Atmosphere Research Group, Department of Applied Mathematics, University of Sheffield, Sheffield, S3 7RH, USA

Received: 29 March 2006
Accepted: 10 August 2006

Abstract

Aims.We present near ultraviolet (NUV: 1750–2800 Å) and far ultraviolet (FUV: 1350–1750 Å) light-curves for flares on 4 nearby dMe-type stars (GJ 3685A, CR Dra, AF Psc and SDSS J084425.9+513830.5) observed with the satellite.

Methods.Taking advantage of the time-tagged events recorded with the photon counting detectors, we present high temporal resolution (<0.01 s) analysis of these UV flare data.

Results.A statistical analysis of 700 s of pre-flare quiescence data for both CR Dra and SDSS J084425.9+513830.5 failed to reveal the presence of significant micro-flare activity in time bins of 0.2, 1 and 10 s intervals. Using an appropriate differential emission measure for both the quiescent and flaring state, it is possible to reproduce the observed FUV:NUV flux ratios. A major determinant in reproducing this flux ratio is found to be the value of plasma electron density during the flare. We also searched the count rate data recorded during each of the four flare events for periodicity associated with magneto-hydrodynamic oscillations in the active region coronal loops. Significant oscillations were detected during the flare events observed on all 4 stars, with periodicities found in the 30 to 40 s range. Flare oscillations with this periodicity can be explained as acoustic waves in a coronal loop of length of cm for an assumed plasma temperature of  K. This suggests a loop length for these M-dwarf flares of less than of the stellar radii. We believe that this is the first detection of non-solar coronal loop flare oscillations observed at ultraviolet wavelengths.