2023ApJ...943...46X

CTB 109 is a middle-aged shell-type supernova remnant (SNR) with bright thermal X-ray emission. We reanalyze the GeV γ-ray emission from CTB 109 using 13 yr of Pass 8 data recorded by the Fermi Large Area Telescope. The γ-ray emission of CTB 109 shows a center bright morphology, which is well consistent with its thermal X-ray emission rather than the shell-type structure in the radio band. The spectral analysis shows an evident spectral curvature at ∼several GeV for the GeV γ-ray spectrum, which can naturally explain the lack of TeV γ-ray emission from CTB 109. Although either a leptonic or a hadronic model could fit the multiwavelength observations of CTB 109, the hadronic model is favored considering its γ-ray morphology and the spectral curvature of the GeV spectrum. The unusual γ-ray spectrum of CTB 109 with other SNRs and the luminosity-diameter squared relation make CTB 109 distinguished both from the young-aged SNRs with hard GeV γ-ray spectra and several old-aged SNRs interacting with molecular clouds.