A study of GeV gamma-ray emission towards supernova remnant G51.26+0.11 and its molecular environment

We reanalyse the Fermi-LAT (Large Area Telescope) GeV γ-ray emission in the region of supernova remnant (SNR) G51.26+0.11 and investigate its interstellar molecular environment with the CO line data. At GeV energies, based on 13.2 yr of Fermi-LAT data, the extended γ-ray emission observed in this region is resolved into a uniform-disc source ('Src A') with a significance of 19.5σ and a point source (4FGL J1924.3+1628) with a significance of 4.2σ in 0.2-500 GeV. With an angular radius of ~0.17^○, 'Src A' overlaps with SNR G51.26+0.11 significantly in the line of sight. On the other hand, the morphological coincidence between the SNR and the ~+54 km s^-1 molecular clouds (MCs) together with the asymmetric or broad ¹²CO line profiles near the SNR boundary provides evidence for the very likely SNR-MC interaction. The SNR-MC interaction and the H I absorption features indicate that SNR G51.26+0.11 is located at a kinematic distance of 6.2 ± 0.5 kpc. Combined with the results from the multiwavelength analysis, the γ-ray emission of the SNR ('Src A') can be naturally explained by a hadronic model with a soft power-law proton spectrum of index ~2.25.