Gas Kinematics and Cosmic-Ray Acceleration in the Gamma-ray SNRs W41 and G22.7-0.2

We present a study of the interstellar medium associated with the two middle-aged supernova remnants (SNRs) W41 and G22.7-0.2, both detected in TeV gamma-rays. Using high-angular-resolution $^{12}$CO($J$ = 1-0) data from the Nobeyama 45-m telescope and HI data from the VLA, we investigated the spatial and kinematic properties of molecular and atomic gas that interact with the SNRs. We identified associated clouds in the velocity ranges of +50-+80 km s$^{-1}$ for W41 and +76-+110 km s$^{-1}$ for G22.7-0.2. Column density analysis indicates that target protons are dominated by molecular hydrogen, while atomic hydrogen contributes less than $\sim$10-15% even after correction for self-absorption. The mean proton densities are $\sim$1.2$\times$10$^{3}$ cm$^{-3}$ for W41 and $\sim$5.3$\times$10$^{2}$ cm$^{-3}$ for G22.7-0.2. From the gamma-ray luminosities, we estimate the total energy of accelerated cosmic-ray protons as $W_\mathrm{p}$ $\sim$3$\times$10$^{47}$~erg for W41 and $\sim$1$\times$10$^{48}$ erg for G22.7-0.2, corresponding to 0.03-0.1% of the canonical supernova explosion energy. hese $W_\mathrm{p}$ values agree with the decreasing trend in $W_\mathrm{p}$ observed in the middle-aged SNRs within the previously reported SNR age-$W_\mathrm{p}$ relation.

💡 Research Summary

This paper presents a comprehensive investigation of the interstellar medium (ISM) surrounding two middle‑aged supernova remnants (SNRs), W41 and G22.7‑0.2, both of which are detected in TeV gamma‑rays. Using high‑resolution 12CO(J=1‑0) data from the Nobeyama 45‑m telescope (FUGIN survey) with an effective angular resolution of ~20″ and VLA H I data from the Galactic Plane Survey (60″ resolution), the authors map the spatial and kinematic distribution of molecular and atomic gas that interacts with the SNR shells. Complementary 20 cm radio continuum (MAGPIS) and H.E.S.S. TeV gamma‑ray maps are employed to compare the gas morphology with the non‑thermal emission.

The analysis identifies distinct CO clouds associated with each remnant: for W41, CO emission in the +55–+60 km s⁻¹ range outlines the southern to western shell, while a brighter component at +70–+80 km s⁻¹ (the GMC G23.3‑0.4) fills the interior and peaks near the gamma‑ray maximum. Corresponding H I shows weak (~30–60 K) emission that spatially coincides with the shell over +50–+80 km s⁻¹, and a pronounced H I depression on the northern side, interpreted as self‑absorption by dense molecular gas. For G22.7‑0.2, CO is distributed diffusely across +50–+115 km s⁻¹, with notable clumps at +75–+80 km s⁻¹ that match the eastern rim and southern shell. H I in the same velocity intervals exhibits faint (~60 K) emission and localized depressions (~30 K) that align with the CO clumps, reinforcing the physical association.

Column density calculations employ the standard CO‑to‑H₂ conversion factor (X_CO ≈ 2 × 10²⁰ cm⁻² (K km s⁻¹)⁻¹) and corrections for H I optical depth. The resulting mean proton densities are ≈1.2 × 10³ cm⁻³ for W41 and ≈5.3 × 10² cm⁻³ for G22.7‑0.2. Atomic hydrogen contributes only 10–15 % of the total proton budget even after self‑absorption correction, indicating that molecular hydrogen dominates the target material for hadronic interactions.

Using the measured TeV gamma‑ray luminosities from H.E.S.S., the authors estimate the total energy in accelerated cosmic‑ray protons (Wₚ) under a hadronic p‑p collision model. W41 yields Wₚ ≈ 3 × 10⁴⁷ erg, while G22.7‑0.2 gives Wₚ ≈ 1 × 10⁴⁸ erg. These values correspond to only 0.03–0.1 % of the canonical supernova explosion energy (10⁵¹ erg), consistent with the decreasing trend of Wₚ with SNR age reported in previous works (e.g., Sano et al. 2021). The ages of the remnants (≈60 kyr for W41 and ≈20 kyr for G22.7‑0.2) place them firmly in the “middle‑aged” regime where energy‑dependent diffusion of high‑energy protons out of the shell is expected to reduce the observable gamma‑ray output.

A notable ancillary result is the detection of a 1720 MHz OH maser at +74 km s⁻¹ toward W41, a classic sign of shock‑excited C‑type interactions between the SNR blast wave and dense molecular gas. This maser provides independent confirmation of the physical contact inferred from the CO/H I morphology.

In summary, the paper demonstrates that high‑resolution CO and H I observations are essential for accurately quantifying the target proton density in SNR environments, which in turn allows a reliable determination of the cosmic‑ray proton energy content from gamma‑ray data. By adding two well‑characterized middle‑aged SNRs to the sample, the study strengthens the empirical age–Wₚ relation and supports the picture that, as SNRs evolve beyond a few thousand years, the efficiency of cosmic‑ray acceleration declines due to diffusion and escape of the highest‑energy particles. The authors suggest that extending this methodology to a larger set of remnants, possibly incorporating higher‑J CO transitions and isotopologue data, will further refine our understanding of Galactic cosmic‑ray origins.