Fermi-LAT Detection of Extended GeV Emission toward the Middle-aged Supernova Remnant G156.2+05.7

Abstract

We analyse 17 years of Fermi-LAT $γ$-ray data toward the middle-aged supernova remnant (SNR) G156.2+05.7, supplemented by radio and CO observations and by published X-ray constraints. An extended $γ$-ray source is detected, well described by a 2D Gaussian template ($R_{68} \sim 0.84^\circ$). This description is preferred over a foreground CO template, whereas the Gaussian and a uniform disc are statistically indistinguishable under the AIC. The spectrum shows evidence for curvature, with a log-parabola preferred over a power law at the $\approx 3.4σ$ level. Multi-wavelength spectral energy distribution (SED) modelling, under the adopted assumptions, suggests a leptonic--hadronic joint scenario consistent with the data, with the Fermi-LAT spectrum favouring a hadronic contribution over a purely leptonic interpretation. However, the proton-dominated alternative remains competitive, so the partition of the GeV emission is model-dependent. In the adopted leptonic--hadronic model, the electron population accounts for the radio synchrotron emission and a leptonic $γ$-ray component, while a steep proton population contributes the remaining hadronic $γ$-rays. Adopting a reference distance $d = 2.7$ kpc, the fitted electron cutoff ($E_{e,\mathrm{cut}}\approx 1.2$ TeV) is compatible with the thermal-dominated X-ray emission, while the proton cutoff is fixed by assumption at $1$ TeV. The northeast and southwest quadrants show enhanced $γ$-ray emission, which may be related to a clumpy or asymmetric surrounding medium. A correspondence with the directions of optical H$α$ filaments is suggested by the quadrant orientation but is not yet established. No significant long-term flux variability is found, consistent with steady emission from an extended source.