ASCAView of the Supernova Remnant γ Cygni (G78.2+2.1): Bremsstrahlung X‐Ray Spectrum from Loss‐flattened Electron Distribution

We perform X-ray studies of the shell-type supernova remnant (SNR)gamma-Cygni associated with the brightest EGRET unidentified source 3EGJ2020+4017. In addition to the thermal emissions with characteristictemperature of kT = 0.5-0.9 keV, we found an extremely hard X-ray componentfrom several clumps localized in the northern part of the remnant. Thiscomponent is described by a power-law with a photon index of 0.8-1.5. Both theabsolute flux and the spectral shape of the nonthermal X-rays cannot beexplained by the synchrotron or inverse-Compton mechanisms. We argue that theunusually hard X-ray spectrum can be naturally interpreted in terms ofnonthermal bremsstrahlung from Coulomb-loss-flattened electron distribution indense environs with the gas density about 10 to 100 cm^-3 . For given spectrumof the electron population, the ratio of the bremsstrahlung X- and gamma-rayfluxes depends on the position of the ``Coulomb break'' in the electronspectrum. The bulk of gamma-rays detected by EGRET would come from theradio-bright and X-ray dim cloud at southeast, where very dense gas and strongmagnetic field would illuminate the cloud in the radio and gamma-ray bands, butsuppress the bremsstrahlung X-ray emission due to the shift of the ``Coulombbreak'' in the electron spectrum towards higher energies.Comment: 14 pages, 8 figures, emulateapj5, accepted for publication in the Astrophysical Journa