Cool Stars 14 - Submitted Abstract # 158
This version created on 05 October 2006

Nonthermal Hard X-ray Emission and Iron Kalpha Emission from a
Superflare on II Pegasi


Rachel Osten, University of Maryland College Park
Stephen Drake, NASA Goddard Space Flight Center
Jack Tueller, NASA Goddard Space Flight Center
Jay Cummings, NASA Goddard Space Flight Center
Matteo Perri, ASI Science Data Center
Alberto Moretti, INAF-Osservatorio Astronomico di Brera
Stefano Covino, INAF-Osservatorio di Brera

We report on an X-ray flare detected on the active binary system II
Pegasi with the Swift telescope, out to energies of 200  keV.  This
flare was remarkable not only for its tremendous intensity -- at its
peak, the radiation in the 0.8--200  keV energy band represents ~28%
of the total bolometric luminosity of the subgiant and M dwarf
components -- but also for spectral evidence during the flare rise and
decay pointing to nonthermal hard X-ray emission and iron K alpha (6.4
keV) emission.  Superhot thermal plasma (T>80  MK) is evident
throughout the flare, as is an excess of emission at E>50  keV,
attributed to nonthermal thick-target bremsstrahlung emission from
accelerated electrons.  This marks the first occasion in which
evidence exists for nonthermal hard X-ray emission from a stellar
flare.  The amount of nonthermal energy (~10^40  erg) may only
slightly exceed the thermal energy radiated over all wavelengths.  We
investigate the emission mechanism responsible for producing the 6.4
keV feature, and find that collisional ionization from nonthermal
electrons appears to be more plausible than the photoionization
mechanism usually invoked on the Sun and pre-main sequence stars.  In
addition to the implications for stellar flare physics, we also
discuss the effect such large transient events would have on the
probable development and evolution of life on terrestrial-like planets
orbiting such a flaring star.

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