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We report on an X-ray flare detected on the active binary system II Pegasi with the Swift telescope. The event triggered the Burst Alert Telescope ( BAT ) in the hard X-ray band on 2005 December 16 at 11:21:52 UT with a 10Y200 keV luminosity of 2:2 ; 10 32 ergs s À1 -a superflare, by comparison with energies of typical stellar flares on active binary systems. The trigger spectrum indicates a hot thermal plasma with T $ 180 ; 10 6 K. X-ray spectral analysis from 0.8 to 200 keV with the X-Ray Telescope and BAT in the next two orbits reveals evidence for a thermal component (T > 80 ; 10 6 K) and Fe K 6.4 keVemission. A tail of emission out to 200 keV can be fit with either an extremely high temperature thermal plasma (T $ 3 ; 10 8 K) or power-law emission. Based on analogies with solar flares, we attribute the excess continuum emission to nonthermal thick-target bremsstrahlung emission from a population of accelerated electrons. We estimate the radiated energy from 0.01 to 200 keV to be $6 ; 10 36 ergs, the total radiated energy over all wavelengths $10 38 ergs, the energy in nonthermal electrons above 20 keV $3 ; 10 40 ergs, and conducted energy <5 ; 10 43 ergs. The nonthermal interpretation gives a reasonable value for the total energy in electrons >20 keV when compared to the upper and lower bounds on the thermal energy content of the flare. This marks the first occasion in which evidence exists for nonthermal hard X-ray emission from a stellar flare. 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 preYmain-sequence stars.

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