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

TL;DR

This work targets the origin of gamma-ray emission around the middle-aged SNR S147, testing hadronic scenarios with two CR populations: CRs trapped in the SNR shell and escaping CRs illuminating nearby MCs. Leveraging ~16.5 years of Fermi-LAT data (100 MeV–1 TeV) and CO-based gas templates, the authors identify a distinct extended component spatially aligned with MCs in the 0–5 km s^{-1} interval, best described by a CO template over geometric models or a catalog point source. The CO component has a hard spectrum with Γ_CO ≈ 2.1 and significant TS, and its inferred CR proton intensity greatly exceeds the local Galactic background, indicating CR escape from S147 into surrounding clouds. This provides observational evidence for CR escape during the remnant’s earlier evolution and positions S147 as a viable TeV candidate for LHAASO, with implications for CR propagation and SNR feedback on the ISM.

Abstract

We present a detailed analysis of $γ$-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 $γ$-ray component distinct from the emission associated with the H$α$ 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 $Γ\approx 2.1$, compatible with the standard diffusive shock acceleration prediction. We interpret the $γ$-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.

Figures (5)

• Figure 1: Panels (a)--(f) show the hydrogen column density distribution in different LSR velocity intervals. Panel (a) covers the full velocity range from $-15$ to $+10~\mathrm{km\,s^{-1}}$, while panels (b)--(f) present consecutive $5~\mathrm{km\,s^{-1}}$-wide velocity bins within the same range. White circles denote the disk template adopted in the Third Catalog of Hard Fermi-LAT Sources (3FHL; Fermi-LAT:2017sxyFermi-LAT:2017mzh). Cyan contours outline the H$\alpha$ template proposed by Katsuta:2012zz. Green crosses indicate the $\gamma$-ray sources listed in the 4FGL catalog.
• Figure 2: Residual TS maps of the $5^\circ \times 5^\circ$ region in the $1~\mathrm{GeV}$--$1~\mathrm{TeV}$ energy band. Green crosses indicate the $\gamma$-ray sources in the 4FGL-DR4 catalog, and white contours trace the MC distribution derived from the CO($0$--$5~\mathrm{km\,s^{-1}}$). Panel (a) shows the residual TS map obtained with the baseline model. Panel (b) shows the residual TS map calculated after removing the unassociated source 4FGL J0534.2+2751 from the baseline Model, revealing the extended excess. Panel (c) shows the residual TS map for the best-fit model that includes the CO component while excluding 4FGL J0534.2+2751. See Section\ref{['sec:morphology']} for more details.
• Figure 3: SEDs of the $\gamma$-ray emission in the S147 region. The blue data points and shaded band represent the SNR H$\alpha$ emission derived from the Baseline model (without adding the CO component). The red and green data points represent the SNR H$\alpha$ and CO($0$--$5\,\mathrm{km\,s^{-1}}$) components, respectively, derived from the simultaneous fit in the optimal model. The solid lines and shaded bands indicate the global best-fit spectral functions (LogParabola for H$\alpha$, Power Law for CO) and their $1\sigma$ uncertainties. Vertical error bars represent $1\sigma$ statistical uncertainties, and arrows indicate 95% confidence level upper limits. Note that the first data point represents the merged low-energy interval analyzed using exclusively PSF3 events.
• Figure 4: SEDs of the $\gamma$-ray emission from S147 region fitted with hadronic models. The black data show the Fermi-LAT measurements for the SNR shell (H$\alpha$ component) and the grey data represent the emission associated with the MC, i.e. the CO ($0$--$5\,\mathrm{km\,s^{-1}}$) component. The solid black and gray curves show the best-fit hadronic model for $\gamma$-ray spectra associated with the SNR shell and the MC, respectively. For comparison, the blue dashed curve represents the expected $\gamma$-ray emission produced by the local Galactic CR proton 10.1093/mnras/sty2235 interacting with the same MC.
• Figure 5: Comparison between the inferred CR proton spectra in the vicinity of S147 and local Galactic CR measurements. The blue and green curves represent the converted CR proton intensities for the SNR shell and the MC regions, respectively. The shaded bands indicate the 16th–84th percentile ranges derived from the spectral fitting. The black data points with error bars show the locally measured CR proton spectrum reported by AMS-02 AGUILAR20211, retrieved from the CRDB database refId0universe6080102. The gray band corresponds to a fit to CR proton data combining Voyager 1 measurements at low energies and AMS observations at high energies, as presented in 10.1093/mnras/sty2235.