Gamma-ray Emission from the S147 Region: Indication of Escaping Cosmic Rays Interacting with Molecular Clouds

Abstract

We present a detailed analysis of $\gamma$-ray emission from the middle-aged supernova remnant (SNR) S147 (G180.0$-$1.7) using approximately 16.5 years of Fermi-LAT data. Spatially, a new extended $\gamma$-ray component distinct from the emission associated with the H$\alpha$ filaments of the SNR shell is identified. This new component exhibits a strong spatial correlation with dense molecular clouds (MCs) identified in CO emission at Local Standard of Rest velocities of $0$-$5\,\mathrm{km\,s^{-1}}$. Spectrally, the cloud-associated emission implies an underlying cosmic-ray (CR) proton population described by a hard power law with an index of $\Gamma \approx 2.1$, compatible with the standard diffusive shock acceleration prediction. We interpret the $\gamma$-ray emission in this region with a hadronic scenario involving two distinct CR populations: trapped CRs reaccelerated within the radiative SNR shell as proposed in previous work, and escaping CRs illuminating the nearby MCs. The derived CR proton intensity in the MC region significantly exceeds the local Galactic background measured by AMS-02, strongly suggesting that the cloud is illuminated by particles accelerated by S147. These findings provide observational evidence for CR escape during the earlier evolutionary phases of this middle-aged SNR and highlight S147 as a promising candidate for detection at TeV energies by LHAASO.