Examining a hadronic γ-ray scenario for the radiative shell and molecular clouds of the old GeV supernova remnant G298.6-0.0

Based on the 13.7 yr Fermi-LAT data, Yeung, Bamba, and Sano (2023, PASJ, 75, 384) claimed detection of two gamma-ray sources (namely Src-NE and Src-NW) associated with the supernova remnant (SNR) G298.6-0.0, and interpreted it as an old GeV SNR interacting with molecular clouds (MCs). In this follow-up study, we refine the flux measurements below 2 GeV with Fermi-LAT event types of better angular reconstruction. Then, we report our gamma-ray spectral fittings and cosmic-ray phenomenology in a hadronic scenario, considering both the shell and MC regions of SNR G298.6-0.0. We confirm that the gamma-ray spectra of both Src-NE and Src-NW exhibit spectral breaks, at 1.50(-0.50)(+0.60) and 0.68(-0.11)(+0.32) GeV, respectively. Src-NW has a harder broad-band photon index than Src-NE, suggesting an appreciable difference between the physical separations of their respective emission sites from SNR G298.6-0.0. The cosmic-ray spectrum responsible for Src-NE starts with a minimum energy E-CR,E-min = 1.38(-0.16)(+0.47) GeV, and has a proton index Gamma(CR) = 2.57(-0.21)(+0.18) below the exponential cutoff energy E-CR,E-max = 240(-150)(+240) GeV. Accordingly, we argue that Src-NE is dominated by the SNR shell, while only a minor portion of lower-energy emission is contributed by the MCs interacting with the SNR. The cosmic-ray population for Src-NW starts at a higher energy such that the E-CR,E- min ratio of Src-NW to Src-NE is greater than or similar to 2. The high E-CR,E- min, as well as the high cosmic-ray energy density required (similar to 26 eV cm(-3)), supports the interpretation that Src-NW is predominantly the gamma-ray emission from the farther MCs being bombarded by protons that had earlier escaped from SNR G298.6-0.0. By comparing the high-energy features of G298.6-0.0 with those of analogical SNRs, especially SNR W 28 and SNR W 44, we further constrain the age of SNR G298.6-0.0 to be 10-30 kyr, under the assumption of a purely hadronic scenario.