TeV -rays from old supernova remnants

Abstract

We study the emission from an old supernova remnant (SNR) with an age of around 10 5 yr and that from a giant molecular cloud (GMC) encountered by the SNR. When the SNR age is around 10 5 yr, proton acceleration is efficient enough to emit TeV -rays both at the shock of the SNR and that in the GMC. The maximum energy of primarily accelerated electrons is so small that TeV -rays and X-rays are dominated by hadronic processes, 0 -decay and synchrotron radiation from secondary electrons, respectively. However, if the SNR is older than several 10 5 yr, there are few high-energy particles emitting TeV -rays because of the energy-loss effect and/or the wave-damping effect occurring at low-velocity isothermal shocks. For old SNRs or SNR-GMC interacting systems capable of generating TeV -ray emitting particles, we calculated the ratio of TeV -ray (1-10 TeV) to X-ray (2-10 keV) energy flux and found that it can be more than 10 2 . Such a source showing large flux ratio may be a possible origin of recently discovered unidentified TeV sources.