Energy-Dependent Analyses of the Gamma-Ray Emission from HESS J1857+026 with Fermi-LAT

Abstract: We report the discovery of energy-dependent morphology for the GeV gamma-ray emission from HESS J1857+026 with more than 13 years of {\it Fermi} Large Area Telescope (LAT) data. The GeV gamma-ray emission from this region is composed of two extended components. The hard component with an index of $1.74 \pm 0.07$ in the energy range of 0.5-500 GeV is spatially coincident with HESS J1857+026, and its 68\% containment radius varies from $\sim 0.44^\circ$ below 40 GeV to $\sim 0.30^\circ$ above 140 GeV. The hard GeV gamma-ray spectrum and the energy-dependent morphology of HESS J1857+026 make it favor a PWN origin, which is associated with the energetic pulsar, PSR J1856+0245. The soft component with an index of $2.70 \pm 0.16$ and another extended gamma-ray source detected in this region, 4FGL J1857.9+0313e with an index of $2.55 \pm 0.07$, are spatially coincidence with two molecular clumps in the northeast and southwest of HESS J1857+026, which favors the hadronic process, and the protons could be accelerated by the hypothetical SNR associated with PSR J1856+0245.