Abstract
Based on the 13.7 yr Fermi-LAT data, we claimed detection of two γ-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). We reported our γ-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 γ-ray spectra of both Src-NE and Src-NW exhibit spectral breaks, at ~1.50 and ~0.68 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 of ~1.38 GeV, and has a proton index of ~2.57 below the exponential cutoff energy of ~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 ECR, min ratio of Src-NW to Src-NE is ≳2. The high ECR, min, as well as the high cosmic-ray energy density required (~26 eV cm-3), supports the interpretation that Src-NW is predominantly the γ-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.
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