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

Abstract

We report on an X-ray flare detected on the active binary system II~Pegasi with the Swift telescope. The trigger had a 10-200 keV luminosity of 2.2$\times 10^{32}$ erg s${}^{-1}$-- a superflare, by comparison with energies of typical stellar flares on active binary systems. The trigger spectrum indicates a hot thermal plasma with T$\sim$180 $\times 10^6$K. 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 $\times 10^6$K) 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$\sim 3\times 10^8$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--200 keV to be $\sim 6\times 10^{36}$ erg, the total radiated energy over all wavelengths $\sim 10^{38}$ erg, the energy in nonthermal electrons above 20 keV $\sim 3\times 10^{40}$ erg, and conducted energy $<5\times 10^{43}$ erg. 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 pre-main sequence stars.

41 pages, 7 figures, accepted for publication in the Astrophysical Journal

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