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We report an observation of ultrahigh-energy (UHE) gamma rays from the Galactic center (GC) region, using 7 yr 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 ( d N / d E = ϕ E / 26 TeV γ ), where γ = −2.88 ± 0.15stat − 0.1sys and ϕ = 1.5 × 10−15 (TeV cm2 s)−1 ± 0.3 stat − 0.13 sys + 0.08 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* (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 (>5σ), 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 GC.
We report an observation of ultrahigh-energy (UHE) gamma rays from the Galactic center (GC) region, using 7 yr 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 ( d N / d E = ϕ E / 26 TeV γ ), where γ = −2.88 ± 0.15stat − 0.1sys and ϕ = 1.5 × 10−15 (TeV cm2 s)−1 ± 0.3 stat − 0.13 sys + 0.08 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* (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 (>5σ), 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 GC.
The recent observation of high-energy neutrinos from the Galactic plane implies an abundant population of hadronic cosmic-ray sources in the Milky Way. We explore the role of the coronae of accreting stellar-mass black holes as such astroparticle emitters. We show that the particle acceleration and interaction timescales in the coronal region are tied to the compactness of the X-ray source. Thus, neutrino emission processes may similarly happen in the cores of active galactic nuclei and black hole X-ray binaries (XRBs), despite their drastically different masses and physical sizes. We apply the model to the well-measured XRB Cygnus X-1 and find that the cascaded gamma rays accompanying the neutrino emission naturally explain the GeV emission that only presents during the source’s hard state, while the state-averaged gamma-ray emission explains the LHAASO observation above 20 TeV. We show that XRB coronae could contribute significantly to the Galactic cosmic-ray and Galactic plane neutrino fluxes. Our model predicts variable high-energy neutrino emission from bright Galactic XRBs that may be observed by IceCube and future neutrino observatories.
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