Inverse Compton Emission from Galactic Supernova Remnants: Effect of the Interstellar Radiation Field

The evidence for particle acceleration in supernova shells comes fromelectrons whose synchrotron emission is observed in radio and X-rays. Recentobservations by the HESS instrument reveal that supernova remnants also emitTeV gamma-rays; long awaited experimental evidence that supernova remnants canaccelerate cosmic rays up to the ``knee'' energies. Still, uncertainty existswhether these gamma-rays are produced by electrons via inverse Comptonscattering or by protons via neutral pion decay. The multi-wavelength spectraof supernova remnants can be fitted with both mechanisms, although a preferenceis often given to neutral pion decay due to the spectral shape at very highenergies. A recent study of the interstellar radiation field indicates that itsenergy density, especially in the inner Galaxy, is higher than previouslythought. In this paper we evaluate the effect of the interstellar radiationfield on the inverse Compton emission of electrons accelerated in a supernovaremnant located at different distances from the Galactic Centre. We show thatcontribution of optical and infra-red photons to the inverse Compton emissionmay exceed the contribution of cosmic microwave background and in some casesbroaden the resulted gamma-ray spectrum. Additionally, we show that if asupernova remnant is located close to the Galactic Centre its gamma-rayspectrum will exhibit a ``universal'' cutoff at very high energies due to theKlein-Nishina effect and not due to the cut-off of the electron spectrum. As anexample, we apply our calculations to the supernova remnants RX J1713.7-3946and G0.9+0.1 recently observed by HESS.Comment: 4 pages, 4 figures. Uses emulateapj.cls. Accepted by ApJ