Discovery of the Ultra-high energy gamma-ray source LHAASO J2108+5157

TL;DR

This paper reports the discovery of a new ultra-high energy gamma-ray source, LHAASO J2108+5157, with significant gamma-ray excess observed in the 25-200 TeV range, likely originating from a hadronic process linked to a molecular cloud.

Contribution

First detection of LHAASO J2108+5157 as an ultra-high energy gamma-ray source using LHAASO data, revealing its spectral properties and potential hadronic origin.

Findings

Significant gamma-ray excess at 25-100 TeV and >100 TeV with high sigma levels.

Energy spectrum follows a power-law with index -2.83±0.18.

Source position correlates with a giant molecular cloud, suggesting hadronic processes.

Abstract

We report the discovery of a UHE gamma-ray source, LHAASO J2108+5157, by analyzing the LHAASO-KM2A data of 308.33 live days. Significant excess of gamma-ray induced showers is observed in both energy bands of 25-100 TeV and $\gt$100 TeV with 9.5 sigma and 8.5 sigma, respectively. This source is not significantly favored as an extensive source with the angular extension smaller than the point-spread function of KM2A. The measured energy spectrum from 20 to 200 TeV can be approximately described by a power-law function with an index of -2.83$\pm$ 0.18stat. A harder spectrum is demanded at lower energies considering the flux upper limit set by Fermi-LAT observations. The position of the gamma-ray emission is correlated with a giant molecular cloud, which favors a hadronic origin. No obvious counterparts have been found, deeper multiwavelength observations will help to shed new light on this intriguing UHE source.