Abstract The origin of Galactic Cosmic Rays (CRs), particularly at sub-GeV energies, remains uncertain due to their difficulty in being traced. One possible tracer of low-energy CRs is ionised carbon (C II), which may be enhanced in regions affected by CR-induced ionisation. We present observations of C II 158 μm line emission across the γ-ray-bright supernova remnant (SNR) RX J1713.7-3946 obtained with SOFIA. These data are compared with molecular and atomic gas from Mopra (12CO), Nanten (12CO) and SGPS (HI), and high-energy emission from TeV γ-ray H.E.S.S. and X-ray XMM-Newton. We find C II emission follows the atomic gas more closely than molecular gas, particularly in regions near the SNR shock front. Ratios of C II intensity to gas column density vary across the remnant, with peak values coinciding with regions of enhanced TeV γ-ray emission. To put the C II in a wider context, we examined pointings from the Herschel GOT C+ survey. Our analysis of the GOT C+ data shows no significant difference in the IC II/I12CO ratio for observations towards star formation regions, HII regions and SNRs. To assess the origin of carbon ionisation, we model C II emissivity using the photoionisation code CLOUDY. We find UV photons and CRs can produce comparable levels of C II emission, assuming low-energy CRs ( 1 GeV) are accelerated and trapped within the SNR shock. These results highlight the potential of C II as a tracer of CR ionisation and the importance of combining multi-wavelength observations to probe CR interactions in SNRs.
Thakur et al. (2026) studied this question.