Primary versus Secondary Leptons in the EGRET Supernova Remnants

The EGRET supernova remnants (SNR's) are all expanding into nearby densemolecular clouds, powering a shock at the interface where protons and electronsaccelerate to relativistic energies. A viable mechanism for the emission ofgamma$-rays in these sources is the decay of neutral pions created incollisions between the relativistic hadrons and protons in the ambient medium.But neutral pion decay alone cannot reproduce the whole high-energy spectrum,particularly below 100 MeV. A pion-decay scenario thus requires a leptonpopulation to "fill in" the lower part of the spectrum via bremsstrahlungemission. This population, however, is constrained by the SNR radio spectrum.Taking our cue from the behavior of Sgr A East, an energetic EGRET SNR at theGalactic center, we here examine the role played in these sources by secondaryleptons--electrons and positrons produced in proton-proton scattering eventsand the ensuing particle cascades. We show that while secondary leptons cannotaccount for the gamma-rays below 100 MeV, they can account for the hard radiospectra observed from the EGRET SNR's. Thus, it appears that both primary andsecondary leptons may be important contributors to the overall broadbandemission from these sources, but if so, must radiate most of their energy indifferent parts of the SNR-cloud environment. We show that shock accelerationin dense cores being overtaken by the expanding SNR shell can naturally lead tosuch a scenario.Comment: 32 pages, 16 figures. Submitted to Ap