ASCA view of the supernova remnant gamma Cygni (G78.2+2.1): Bremsstrahlung X-ray spectrum from loss-flattened electron distribution

With the ASCA X-ray satellite, we perform spatial and spectral studies of the shell-type supernova remnant (SNR) gamma Cygni that is associated with the brightest EGRET unidentified source 3EG J2020+4017. At energies below 3 keV, the bulk of the X-ray flux from the remnant is well described by the emission of thermal plasma with characteristic temperature kT(e) similar or equal to 0.5-0.9 keV. In addition to this thermal emission, we found an extremely hard X-ray component from several clumps localized in the northern part of the remnant. This component, which dominates the X-ray emission from the remnant above 4 keV, is described by a power law with a photon index of Gamma similar or equal to 0.8-1.5. Both the absolute flux and the spectral shape of the nonthermal X-rays cannot be explained by the synchrotron or inverse-Compton mechanisms. We argue that the unusually hard X-ray spectrum can be interpreted naturally in terms of nonthermal bremsstrahlung from Coulomb-loss- flattened electron distribution in dense environs with the gas density about 10-100 cm(-3). For a given spectrum of the electron population, the ratio of the bremsstrahlung X- and gamma-ray fluxes depends on the position of the "Coulomb break" in the electron spectrum. Formally, the entire high-energy gamma-ray flux detected by EGRET from gamma Cygni could originate in the hard X-ray regions. However, it is more likely that the bulk of gamma-rays detected by EGRET comes from the radio-bright and X-ray-dim cloud at the southeast, where the very dense gas and strong magnetic field would illuminate the cloud in the radio and gamma-ray bands but suppress the bremsstrahlung X-ray emission due to the shift of the Coulomb break in the electron spectrum toward higher energies