Astronomy and Astrophysics – Astronomy
Scientific paper
Feb 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010head...11.1711o&link_type=abstract
American Astronomical Society, HEAD meeting #11, #17.11; Bulletin of the American Astronomical Society, Vol. 41, p.684
Astronomy and Astrophysics
Astronomy
Scientific paper
The Swift satellite has now detected several flares from nearby, magnetically active stars which were bright enough to be mistaken for gamma-ray bursts. Due to their large "superflare" flux levels ( >10 -9 erg/cm2/s above 15 keV), variations in these flares can be studied on timescales of a few hundred seconds. The detection by Swift of Iron Kα (6.4 keV) emission and evidence for nonthermal hard X-ray emission mark two firsts in stellar flare astronomy. Understanding the formation of the 6.4 keV emission line is important because it allows for a relatively model-independent determination of coronal scale sizes. The flare temperatures are within the range of other less luminous flares, and hydrodynamic modelling of the response of coronal loops to heating and cooling reveals the need for compact (loop height < 0.3 stellar radii) loops. In general the flux levels of Kα emission are consistent to within factors of a few of the expected contribution from a fluorescence process, where hot X-ray continuum emission from an overlying coronal loop illuminates the stellar photosphere, resulting in fluorescence of neutral or nearly neutral photospheric iron. However, in the cases of superflares from EV Lac and CC Eri, the variable amount of Kα flux requires an additional mechanism beyond fluorescence to account for the observed Kα emission. Following discussions of the origin of iron Kα emission in solar flares, we investigate the possibility of impulsive collisional ionization by accelerated electrons to produce the needed extra Kα flux. We will discuss the time-resolved analysis of the soft and hard X-ray spectra of these flares and present plausible arguments for the need to include the action of nonthermal electrons.
Cummings Jay
Drake Stephen
Godet Olivier
Krimm Hans
Osten Rachel Ann
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