Nonthermal Emission from a Supernova Remnant in a Molecular Cloud

In evolved supernova remnants (SNRs) interacting with molecular clouds, a highly inhomogeneous structure consisting of a forward shock of moderate Mach number, a cooling layer, a dense radiative shell and an interior region filled with hot tenuous plasma is expected. We present a model of nonthermal electron injection, acceleration and propagation in that environment and find that these SNRs are efficient electron accelerators and sources of hard Xand γ-ray emission. A forward shock of velocity vs >∼ 100 km s−1 with an ionized precursor propagating into the molecular cloud accompanied by magnetohydrodynamic turbulence provides a spatially inhomogeneous distribution of nonthermal electrons. The energy spectrum of the nonthermal electrons is shaped by the joint action of first and second order Fermi acceleration in a turbulent plasma with substantial Coulomb losses. Bremsstrahlung, synchrotron, and inverse Compton radiation of the nonthermal electrons produce multiwavelength photon spectra in quantitative agreement with the radio and the hard emission observed by ASCA and EGRET from IC 443. We distinguish interclump shock wave emission from molecular clump shock wave emission; particles reach higher energies in the interclump shock and that is the likely source of γ-ray emission and radio synchrotron emission. Spatially resolved Xand γ-ray spectra from the supernova remnants IC 443, W44, and 3C391 as might be observed with BeppoSAX, Chandra XRO, XMM, INTEGRAL and GLAST would distinguish the contribution of the energetic lepton component to the γ-rays observed by EGRET, constraining the cosmic ray nuclear component spectra in these SNRs. These data would provide a valuable tool for studying the complex structure of molecular clouds where SNR radiative shocks interact with dense molecular clumps. Subject headings: acceleration of particles — cosmic rays — ISM: Supernova Remnants — radiation mechanisms: nonthermal