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Suzaku Detection of Super-hard X-ray Emission from the Classical Nova V2491 Cygni

We report the detection of super-hard (>10 keV) X-ray emission extending up to 70 keV from the classical nova V2491 Cygni using the Suzaku observatory. We conducted two ~20 ks target-of-opportunity observations 9 and 29 days after the outburst on 2008 April 11, yielding wide energy range spectra by combining the X-ray Imaging Spectrometer and the Hard X-ray Detector. On day 9, a spectrum was obtained at 1.0-70 keV with the Fe XXV Kαline feature and a very flat continuum, which is explained by thermal plasma with a 3 keV temperature and power-law emission with a photon index of 0.1 attenuated by a heavy extinction of 1.4x10^{23} cm^{-2}. The 15-70 keV luminosity at 10.5 kpc is 6x10^{35} ergs s^{-1}. The super-hard emission was not present on day 29. This is the highest energy at which X-rays have been detected from a classical nova. We argue a non-thermal origin for the emission, which suggests the presence of accelerated charged particles in the nova explosion.