Discovery of non-thermal emission from the classical nova V2491 Cygni

摘要

We report the long-awaited discovery of non-thermal X-ray emission from classical nova explosions with a target-of-opportunity observation of V2491 Cygni using the Suzaku observatory. Two 20 ks observations were conducted 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 onboard Suzaku. As a result, super-hard continuum emission extending up to 70 keV was detected successfully on day 9, but it was not present on day 29. This is the highest energy at which X-rays have been detected from classical novae. The spectrum is well explained by an extremely flat power-law emission with a photon index of 0.1 attenuated by a heavy extinction. The power-law emission indicates the presence of an accelerated population of electrons with a non-thermal energy distribution. The extremely flat photon index is too hard for standard diffusive shock acceleration, suggesting that other mechanisms such as acceleration by multiple shocks or magnetic reconnections might take place.