Observation of the Galactic Center PeVatron Beyond 100 TeV with HAWC

Abstract

We report an observation of ultra-high energy (UHE) gamma rays from the Galactic Center region, using seven years of data collected by the High-Altitude Water Cherenkov (HAWC) Observatory. The HAWC data are best described as a point-like source (HAWC J1746-2856) with a power-law spectrum ($\mathrm{d}N/\mathrm{d}E=\ddot{I}•(E/26\text{TeV}{)}^3$), where ${\hat{I}}^3=-2.88\pm 0.15_{\text{stat}}-0.1_{\text{sys}}$ and $\ddot{I}•=1.5\times 10^{-15}$ (TeV cm${}^2$s)${}^{-1}$ $\pm 0.3_{\text{stat}}{}_{-0.13_{\text{sys}}}^{+0.08_{\text{sys}}}$ extending from 6 to 114 TeV. We find no evidence of a spectral cutoff up to $100$ TeV using HAWC data. Two known point-like gamma-ray sources are spatially coincident with the HAWC gamma-ray excess: Sgr A${}^{\ast }$ (HESS J1745-290) and the Arc (HESS J1746-285). We subtract the known flux contribution of these point sources from the measured flux of HAWC J1746-2856 to exclude their contamination and show that the excess observed by HAWC remains significant ($>$5$\ddot{I}ƒ$) with the spectrum extending to $>$100 TeV. Our result supports that these detected UHE gamma rays can originate via hadronic interaction of PeV cosmic-ray protons with the dense ambient gas and confirms the presence of a proton PeVatron at the Galactic Center.