Discovery of a new $\gamma$-ray source LHAASO J0341+5258 with emission up to 200TeV

Abstract

We report the discovery of a new unidentified extended $\gamma$-ray source in the Galactic plane named LHAASO J0341+5258 with a pre-trial significance of 8.2 standard deviations above 25 TeV. The best fit position is R.A.$=55.34^{\circ }\pm 0.11^{\circ }$ and Dec$=52.97^{\circ }\pm 0.07^{\circ }$. The angular size of LHAASO J0341+5258 is $0.29^{\circ }\pm 0.06_{stat}^{\circ }\pm 0.02_{sys}^{\circ }$. The flux above 25 TeV is about $20\%$ of the flux of Crab Nebula. Although a power-law fit of the spectrum from 10 TeV to 200 TeV with the photon index $\alpha =2.98\pm 0.19_{stat}\pm 0.02_{sys}$ 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 %($\alpha \le 1.75$) spectrum with a cutoff at $\approx 50$ TeV. We briefly discuss the origin of UHE gamma-rays. The lack of an energetic pulsar and a young SNR inside or in the vicinity of LHAASO J0341+5258 challenge, but do not exclude both the leptonic and hadronic scenarios of gamma-ray production.

Accepted by APJL

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• A discussion on the origin of the sub-PeV Galactic gamma-ray emission
• Detection of molecular clouds in the PeVatron candidate source LHAASO J0341$+$5258 by the Nobeyama 45-m radio telescope
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