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

TL;DR

This study probes the ISM around two middle-aged gamma-ray SNRs, W41 and G22.7–0.2, by combining high-resolution $^{12}$CO($J=1-0$) and HI data with radio and TeV gamma-ray observations. It identifies the dominant target gas in the velocity ranges +$50$ to +$80$ km s$^{-1}$ (W41) and +$76$ to +$110$ km s$^{-1}$ (G22.7–0.2), and shows HI self-absorption within CO clouds, establishing a molecular-dominated proton reservoir with mean densities of about $1.2\times10^{3}$ cm$^{-3}$ and $5.3\times10^{2}$ cm$^{-3}$, respectively. From gamma-ray luminosities, the total energies in accelerated protons are $W_\mathrm{p} \sim 3.2\times10^{47}$ erg (W41) and $W_\mathrm{p} \sim 1.2\times10^{48}$ erg (G22.7–0.2), i.e., 0.03–0.1% of a canonical $\sim10^{51}$ erg SN energy, consistent with the age–$W_\mathrm{p}$ relation for middle-aged SNRs. The results support a scenario in which CR protons escape and diffuse into surrounding clouds over $\sim10^4$ years, and highlight the importance of accurate gas mass estimates and ISM structure for interpreting hadronic gamma-ray emission from SNRs.

Abstract

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.

Figures (8)

• Figure 1: Maps of (a) 20 cm radio continuum from the MAGPIS survey Helfand2006AJ....131.2525H, (b) TeV gamma-ray significance map obtained from H.E.S.S. hess2018AA...612A...1H toward the SNRs W41 and G22.7–0.2. The superposed contours are 20 cm radio continuum. The contour levels are 1.40, 1.75, 2.80, 4.55, 7.00, 10.15, and 14.00 mJy beam$^{-1}$. The circles indicates the positions of the 1720 MHz OH maser frail2013ApJ...773L..19F. The positions of TeV gamma-ray sources are indicated by the white crosses in Figure \ref{['fig:faceimage']}(b).
• Figure 2: Velocity channel distributions of the $^{12}$CO($J$ = 1–0) and H i brightness temperatures towards W41 superposed with the same radio continuum contours as in Figure \ref{['fig:faceimage']}(a). Each panel shows intensity distributions averaged every 5 km s$^{-1}$ in a velocity range from +45 to +120 km s$^{-1}$. The color bar is shown on top of the set of panels.
• Figure 3: Same velocity channel maps as shown in Figure \ref{['fig:channel_map_CO_1']}, but it is centered on G22.7–0.2.
• Figure 4: Longitude–Velocity diagrams of (a) $^{12}$CO($J$ = 1–0) and (b) H i . The integrated range is from $-$0$.\!\!^\circ$3 to $-$0$.\!\!^\circ$1 in the Galactic Latitude. The double-headed arrows indicate the diameter of each SNR shell. The dashed black curves show the expanding gas motion for each SNR (see the text). Horizontal dashed and one-dot chain lines indicate the integration velocity ranges for each SNR. The superposed contours in each panel indicate $^{12}$CO($J$ = 1–0), whose contour levels are 1.2, 1.5, 1.8, 2.1, 2.4, 2.7, 3.0, and 3.3, K degree. The white filled circles represent the positions of the 1720 MHz OH masers frail2013ApJ...773L..19F.
• Figure 5: Integrated intensity maps of the $^{12}$CO($J$ = 1–0) and H i emission toward (a), (b) W41 in the velocity range $v_\mathrm{LSR}$ = +50–+80 km s$^{-1}$, and (c), (d) G22.7–0.2 in the range $v_\mathrm{LSR}$ = +76–+110 km s$^{-1}$. Black boxes mark the positions where line profiles were extracted. Line profiles shown below indicate $^{12}$CO($J$ = 1–0) (Blue) and H i (orange) at six positions: A ($l$, $b$) = (23$.\!\!^\circ$43, $-$0$.\!\!^\circ$21), B (23$.\!\!^\circ$26, $-$0$.\!\!^\circ$30), C (23$.\!\!^\circ$04, $-$0$.\!\!^\circ$32), D (22$.\!\!^\circ$74, $-$0$.\!\!^\circ$25), E (22$.\!\!^\circ$62, $-$0$.\!\!^\circ$10), and F (22$.\!\!^\circ$58, $-$0$.\!\!^\circ$22). The shaded area and vertical dashed lines in Position B indicates an expected background H i profile and a velocity range from +55 to +80 km s$^{-1}$.