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~Pegasiwith the Swift telescope. The trigger had a 10-200 keV luminosity of2.2$\times10^{32}$ erg s$^{-1}$-- a superflare, by comparison with energies oftypical stellar flares on active binary systems. The trigger spectrum indicatesa hot thermal plasma with T$\sim$180 $\times10^{6}$K. X-ray spectral analysisfrom 0.8--200 keV with the X-Ray Telescope and BAT in the next two orbitsreveals evidence for a thermal component (T$>$80 $\times10^{6}$K) and Fe K 6.4keV emission. A tail of emission out to 200 keV can be fit with either anextremely high temperature thermal plasma (T$\sim3\times10^{8}$K) or power-lawemission. Based on analogies with solar flares, we attribute the excesscontinuum emission to nonthermal thick-target bremsstrahlung emission from apopulation of accelerated electrons. We estimate the radiated energy from0.01--200 keV to be $\sim6\times10^{36}$ erg, the total radiated energy overall wavelengths $\sim10^{38}$ erg, the energy in nonthermal electrons above 20keV $\sim3\times10^{40}$ erg, and conducted energy $<5\times10^{43}$ erg. Thenonthermal interpretation gives a reasonable value for the total energy inelectrons $>$ 20 keV when compared to the upper and lower bounds on the thermalenergy content of the flare. This marks the first occasion in which evidenceexists for nonthermal hard X-ray emission from a stellar flare. We investigatethe emission mechanism responsible for producing the 6.4 keV feature, and findthat collisional ionization from nonthermal electrons appears to be moreplausible than the photoionization mechanism usually invoked on the Sun andpre-main sequence stars.Comment: 41 pages, 7 figures, accepted for publication in the Astrophysical Journa