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

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 in the hard X-ray band on December 16, 2005 at 11:21:52 UT with a 10-200 keV luminosity of 2.2×10 erg s — 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 ×10K. X-ray spectral analysis from 0.8–200 keV with the X-Ray Telescope and BAT in the next two orbits reveals evidence for a thermal component (T>80 ×10K) and Fe K 6.4 keV emission. A tail of emission out to 200 keV can be fit with either an extremely high temperature thermal plasma (T∼3×10K) or power-law emission. Based on analogies with solar flares, we attribute the excess continuum emission to nonthermal thicktarget bremsstrahlung emission from a population of accelerated electrons. We estimate the radiated energy from 0.01–200 keV to be ∼6×10 erg, the total radiated energy over all wavelengths ∼10 erg, the energy in nonthermal electrons Hubble Fellow Also USRA