Suzaku Detection of Superhard X-Ray Emission from the Classical Nova V2491 Cygni

We report the detection of superhard (>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 Kα line 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 1.4 × 10²³ cm^-2. The 15-70 keV luminosity at 10.5 kpc is 6 × 10³⁵ erg s^-1. The superhard 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 nonthermal origin for the emission, which suggests the presence of accelerated charged particles in the nova explosion.