HAWC, VERITAS, Fermi-LAT and XMM-Newton follow-up observations of the unidentified ultra-high-energy gamma-ray source LHAASO J2108+5157

TL;DR

This paper presents multi-instrument observations of the ultra-high-energy gamma-ray source LHAASO J2108+5157, revealing its spectral properties, possible pulsar wind nebula origin, and constraining its magnetic field through combined gamma-ray and X-ray data.

Contribution

First comprehensive multi-wavelength follow-up of LHAASO J2108+5157, combining VERITAS, HAWC, Fermi-LAT, and XMM-Newton data to characterize its emission and origin.

Findings

HAWC detected extended emission between 3 and 146 TeV with high significance.

Fermi-LAT identified a soft-spectrum point source consistent with the region.

XMM-Newton did not detect X-ray emission, constraining the magnetic field to less than 1.5 μG.

Abstract

We report observations of the ultra-high-energy gamma-ray source LHAASO J2108$+$5157, utilizing VERITAS, HAWC, Fermi-LAT, and XMM-Newton. VERITAS has collected $\sim$ 40 hours of data that we used to set ULs to the emission above 200 GeV. The HAWC data, collected over $\sim 2400$ days, reveal emission between 3 and 146 TeV, with a significance of $7.5~\sigma$, favoring an extended source model. The best-fit spectrum measured by HAWC is characterized by a simple power-law with a spectral index of $2.45\pm0.11_{stat}$. Fermi-LAT analysis finds a point source with a very soft spectrum in the LHAASO J2108+5157 region, consistent with the 4FGL-DR3 catalog results. The XMM-Newton analysis yields a null detection of the source in the 2 - 7 keV band. The broadband spectrum can be interpreted as a pulsar and a pulsar wind nebula system, where the GeV gamma-ray emission originates from an unidentified pulsar, and the X-ray and TeV emission is attributed to synchrotron radiation and inverse Compton scattering of electrons accelerated within a pulsar wind nebula. In this leptonic scenario, our X-ray upper limit provides a stringent constraint on the magnetic field, which is $\lesssim 1.5\ \mu$G.