GALEXhigh time-resolution ultraviolet observations of dMe flare events

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 GALEX satellite. Methods. Taking advantage of the time-tagged events recorded with the GALEX 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 ≈10 9 cm for an assumed plasma temperature of 5−20 × 10 6 K. This suggests a loop length for these M-dwarf flares of less than 1/10th of the stellar radii. We believe that this is the first detection of non-solar coronal loop flare oscillations observed at ultraviolet wavelengths.

Stellar flare oscillations: evidence for oscillatory reconnection and evolution of MHD modes

“…In fact, a straight line fit of τ = 1.62P 0.99 fitted QPPs ranging in period from ∼20 s to 60 min, suggesting a similar mechanism. Here, we extend work first presented by Welsh et al (2006) which used data from GALEX. The GALEX satellite (Martin et al 2005) was launched in 2003 and operated for just over 10 yr.…”

“…1. Since gPhoton is still experimental, we compared these flare light curves with those published in Welsh et al (2006), using the same aperture (10 arcsec aperture) and background annulus (extending from ∼15 to 20 arcsec from the central position of the star). Excellent agreement was obtained for AF Psc, CR Dra (epoch 5), and SDSS J084425+513830, while GJ 3685A differed by ∼20 per cent.…”

“…It operated in two ultraviolet wavebands: the far-UV (FUV: 1350-1750 Å) and/or near-UV (NUV: 1750-2800 Å) and was capable of providing sub-second time resolution data of flares. In addition to looking again at the oscillations reported by Welsh et al (2006), we include additional objects, plus we searched for evidence of additional periods in the data as done by Van Doorsselaere et al (2011). For this, we used two different techniques, wavelet transformation plus the empirical mode decomposition addressing different issues; how many of these UV flares on M dwarfs show oscillations, when during the flare do we see them, do we see more than one period during the flare and is there evidence for period evolution during the flare?…”

“…More specifically, some QPPs exhibit periodic and decaying variation of the flux in a particular waveband after the flare peak, while others are only present in the impulsive phase. For solar flares, these range from milliseconds to a few minutes, while stellar flares can have oscillations of up to several tens of minutes (Welsh et al 2006;Simões, Hudson & Fletcher 2015;Cho et al 2016). If QPPs are generated by quasi-periodic processes, then this provides information on the ongoing physical processes during the flare, e.g.…”

“…For this, we used two different techniques, wavelet transformation plus the empirical mode decomposition addressing different issues; how many of these UV flares on M dwarfs show oscillations, when during the flare do we see them, do we see more than one period during the flare and is there evidence for period evolution during the flare? Welsh et al (2006) looked at the line and continuum contribution in the NUV filter (for a non-flaring atmosphere). The dominant line emitters were Mg II, Fe II, Al III, C III, etc.…”