Cosmic ray electrons and the diffuse gamma ray spectrum
Abstract
The bulk of the diffuse galactic gamma ray emission above a few tens of GeV has been conventionally ascribed to the decay of neutral pions produced in cosmic ray interactions with interstellar matter. This is based in part on the observed electron spectrum being relatively steep, implying a source spectrum with index 2.4. However, this steep spectrum may arise due to severe energy losses if the nearest acceleration region is about 50 pc or more away. Thus, flatter injection spectra are possible, and we investigate the consequences of this for the diffuse galactic gamma ray spectrum. We perform a propagation calculation for cosmic ray electrons, and use the resulting interstellar electron spectrum to obtain the gamma ray spectrum due to inverse Compton and bremsstrahlung interactions consistently from MeV to PeV energies. We find that inverse Compton scattering can contribute significantly to the high energy diffuse galactic gamma ray spectrum, and may dominate over neutral pion decay above about 10 GeV. We compare our results with available observations from satellite-borne telescopes, optical Cherenkov telescopes and air shower arrays. With future observations at TeV-PeV energies it should be possible to determine the average interstellar spectrum of cosmic ray electrons, and hence estimate their spectrum on acceleration.Keywords
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