Discovery of Interacting Molecular Gas toward the TeV Gamma-Ray Peak of the SNR G 347.3–0.5

Abstract

Abstract Supernova remnants ($=$ SNR) are suggested to be sites of cosmic-ray acceleration. In particular, it has been an issue of keen interest whether cosmic ray protons are being accelerated in a SNR which emits TeV $\gamma$-rays. A crucial observational test for this is to find dense molecular gas towards the SNR, because such molecular gas can best verify the existence of cosmic-ray protons via pion decay to $\gamma$-rays. Here, we show that new high-resolution mm-wave observations of interstellar CO molecule have revealed molecular gas at 1 kpc distance interacting with the TeV $\gamma$-ray SNR G 347.3$-$0.5, and that a molecular cloud of $\sim 200$ solar masses is clearly associated with the TeV $\gamma$-ray peak, providing strong evidence for proton acceleration. We have estimated the total energy of accelerated protons to be $\sim 1048 \,\mathrm{erg}$, which corresponds to an acceleration efficiency of $\sim 0.001$, posing an observational constraint on the proton acceleration.