Asca View of the Supernova Remnant Γ 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) γ 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 kTe ≃ 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 Γ ≃ 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 naturally interpreted in terms of nonthermal bremsstrahlung from Coulomb-loss-flattened electron distribution in dense environs with the gas density about 10 to 100 cm. For given spectrum of the electron population, the ratio of the bremsstrahlung Xand γ-ray fluxes depends on the position of the “Coulomb break” in the electron spectrum. Formally, the entire high energy γ-ray flux detected by EGRET from γ Cygni could originate in the hard X-ray regions. However, it is more likely that the bulk of γ-rays detected by EGRET come from the radio-bright and X-ray dim cloud at southeast, where the very dense gas and strong magnetic field would illuminate the cloud in the radio and γ-ray bands, but suppress the bremsstrahlung X-ray emission due to the shift of the “Coulomb break” in the electron spectrum towards higher energies. Subject headings: acceleration of particles—cosmic rays— radiation mechanisms:non-thermal—shock waves— supernova remnants