2006MNRAS.371.1975Y

We study the emission from an old supernova remnant (SNR) with an age of around 10⁵yr and that from a giant molecular cloud (GMC) encountered by the SNR. When the SNR age is around 10⁵yr, proton acceleration is efficient enough to emit TeV γ-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 γ-rays and X-rays are dominated by hadronic processes, π⁰-decay and synchrotron radiation from secondary electrons, respectively. However, if the SNR is older than several 10⁵yr, there are few high-energy particles emitting TeV γ-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 γ-ray emitting particles, we calculated the ratio of TeV γ-ray (1-10TeV) to X-ray (2-10keV) energy flux and found that it can be more than ∼10². Such a source showing large flux ratio may be a possible origin of recently discovered unidentified TeV sources.