Revision of the GeV $\gamma$-ray Emission in the Region of HESS J1813-178 with Fermi-LAT

Abstract

HESS J1813-178 is one of the brightest and most compact TeV $\gamma$-ray sources, and whether its $\gamma$-ray emission is associated with supernova remnant (SNR), pulsar wind nebula (PWN) or young stellar cluster (YSC) is still under debate. By analysing the GeV $\gamma$-ray data in the field of HESS J1813-178 using 14 years of PASS 8 data recorded by the Fermi Large Area Telescope (Fermi-LAT), we report the discovery of three different sources with different spectra in this region. The hard source with a power law spectral index of 2.11 $\pm$ 0.08 has a small size extension, which is spatially and spectrally coincident with the TeV $\gamma$-ray emission from HESS J1813-178. CO observations display the dense molecular clouds surrounding HESS J1813-178 in the velocity range of 45-60 km s$^{\rm -1}$. The possible origins of the $\gamma$-ray emission from HESS J1813-178 are discussed, including SNR G12.82-0.02, the PWN driven by the energetic X-ray pulsar PSR J1813-1749, and YSC Cl 1813-178. However, none of them can be ruled out clearly. Note that the maximum energy of protons in the hadronic model should exceed a few hundred TeV, which makes HESS J1813-178 to be a promising PeVatron. The detailed LHAASO data analysis about the morphology and spectrum would be helpful to investigate the origin of the $\gamma$-ray emission in this region and test its PeVatron nature.