Observation of the Galactic Center PeVatron Beyond 100 TeV with HAWC

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 (dN/dE= (E/26 \, TeV) ^), where =-2. 88 0. 15ₒₓ₀ₓ - 0. 1ₒₘₒ and =1. 5 10^-15 (TeV cm^2s) ^-1 \, 0. 3ₒₓ₀ₓ\, ^+0. 08ₒₘₒ-₀. ₁₃_ₒₘₒ 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^* (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) 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.