No evidence yet for hadronic TeV gamma - ray emission from SNR RX J 1713 . 7 − 3946

Recent TeV-scale γ-ray observations with the CANGAROO II telescope have led to the claim that the multi-band spectrum of RX J1713.7−3946 cannot be explained as the composite of a synchrotron and an inverse Compton component emitted by a population of relativistic electrons. It was argued that the spectrum of the high-energy emission is a good match to that predicted by pion decay, thus providing observational evidence that protons are accelerated in SNR to at least TeV energies. In this Letter we discuss the multi-band spectrum of RX J1713.7−3946 under the constraint that the GeV-scale emission observed from the closely associated EGRET source 3EG J1714−3857 is either associated with the SNR or an upper limit to the gamma-ray emission of the SNR. We find that the pion-decay model adopted by Enomoto et al. is in conflict with the existing GeV data. We have examined the possibility of a modified proton spectrum to explain the data, and find that we cannot do so within any existing theoretical framework of shock acceleration models. 1. Evidence for particle acceleration in SNR Supernova remnants (SNR) are considered the most likely sources of cosmic rays, either as individual accelerators or by their collective effect in superbubbles. Yet observational evidence in favor of this scenario has been found only for cosmic-ray electrons, not for the nucleons. Three shell-type SNR have been detected at TeV γ-ray energies so far, all of which show non-thermal X-ray emission, which presumably is synchrotron radiation. It is known that the synchrotron radiating electrons would inverse-Compton scatter the microwave background and the ambient far-infrared photon field to TeV γ-ray energies with a flux depending mainly on the X-ray flux and the magnetic field strength within the remnant (Pohl 1996), provided both are measured at photon energies corresponding to the same electron energy and escape from the compression region at the SNR shock is inefficient. The X-ray and TeV spectra should then be similar, which would permit one to discriminate between a hadronic and a leptonic origin of the radiation. Indeed, the observed TeV γ-ray spectrum of SN 1006 (Tanimori et al. 1998, 2001) is consistent with synchrotron/inverse Compton models A significant contribution of γ-rays from hadronic interactions appears unlikely on account of the low density environment in which the remnant resides. Recently, TeV γ-rays have been detected from Cassiopeia A (Aharonian et al. 2001), if with 0.03 Crab above 1 TeV at a …