Discovery of a New Gamma-Ray Source, LHAASO J0341+5258, with Emission up to 200 TeV

Abstract

We report the discovery of a new unidentified extended γ-ray source in the Galactic plane named LHAASO J0341+5258 with a pretrial significance of 8.2 standard deviations above 25 TeV. The best-fit position is R.A. = 55. 34 0. 11 and decl. = 52. 97 0. 07. The angular size of LHAASO J0341+5258 is 0. 29 0. 06stat 0. 02sys. The flux above 25 TeV is about 20% of the flux of the Crab Nebula. Although a power-law fit of the spectrum from 10 to 200 TeV with the photon index α = 2.98 0.19stat 0.02sys is not excluded, the LHAASO data together with the flux upper limit at 10 GeV set by the Fermi-LAT observation, indicate a noticeable steepening of an initially hard power-law spectrum with a cutoff at ≈50 TeV. We briefly discuss the origin of ultra-high-energy gamma rays. The lack of an energetic pulsar and a young supernova remnant inside or in the vicinity of LHAASO J0341+5258 challenge, but do not exclude, both the leptonic and hadronic scenarios of gamma-ray production.