Revised the $\gamma$-ray emission from SNR CTB 109 with Fermi-LAT

TL;DR

This paper reanalyzes the gamma-ray emission from SNR CTB 109 using 13 years of Fermi-LAT data, revealing a center-bright morphology and spectral curvature that favor a hadronic origin, distinguishing it from other supernova remnants.

Contribution

The study provides a detailed gamma-ray spectral analysis of CTB 109 with updated Fermi-LAT data, highlighting its unique morphology and spectral features that support a hadronic emission model.

Findings

Gamma-ray emission is center-bright, matching thermal X-ray morphology.

Spectral curvature observed at several GeV explains the lack of TeV emission.

Hadronic model is favored over leptonic based on morphology and spectral analysis.

Abstract

CTB 109 is a middle-aged shell-type SNR with bright thermal X-ray emission. We reanalyze the GeV $\gamma$-ray emission from CTB 109 using thirteen years of Pass 8 data recorded by the Fermi Large Area Telescope (Fermi-LAT). The $\gamma$-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 $\sim$ several GeV for the GeV $\gamma$-ray spectrum, which can naturally explain the lack of TeV $\gamma$-ray emission from CTB 109. Although either a leptonic or a hadronic model could fit the multi-wavelength observations of CTB 109, the hadronic model is favored considering its $\gamma$-ray morphology and the spectral curvature of GeV spectrum. The unusual $\gamma$-ray spectrum of CTB 109 with other SNRs and the luminosity-diameter squared relation make CTB 109 to be distinguished both from the young-aged SNRs with hard GeV $\gamma$-ray spectra and several old-aged SNRs interacting with molecular clouds.