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

The evidence for particle acceleration in supernova shells comes from electrons whose synchrotron emission is observed in radio and X-rays. Recent observations by the HESS instrument reveal that supernova remnants also emit TeV γ-rays, long-awaited experimental evidence that supernova remnants can accelerate cosmic rays up to the ``knee'' energies. Still, uncertainty exists whether these γ-rays are produced by electrons via inverse Compton scattering or by protons via π<sup>0</sup>-decay. The multiwavelength spectra of supernova remnants can be fitted with both mechanisms, although a preference is often given to π<sup>0</sup>-decay due to the spectral shape at very high energies. A recent study of the interstellar radiation field indicates that its energy density, especially in the inner Galaxy, is higher than previously thought. In this Letter we evaluate the effect of the interstellar radiation field on the inverse Compton emission of electrons accelerated in a supernova remnant located at different distances from the Galactic center. We show that contribution of optical and infrared photons to the inverse Compton emission may exceed the contribution of cosmic microwave background and in some cases broaden the resulting γ-ray spectrum. In addition, we show that if a supernova remnant is located close to the Galactic center, its γ-ray spectrum will exhibit a ``universal'' cutoff at very high energies due to the Klein-Nishina effect and not due to the cutoff of the electron spectrum. As an example, we apply our calculations to the supernova remnants RX J1713.7-3946 and G0.9+0.1, recently observed by HESS.