Excess of diffuse gamma-ray emission detected from the galaxy cluster Abell 119 from 14-year Fermi-LAT Data

TL;DR

This study reports a potential diffuse gamma-ray excess from galaxy cluster Abell 119 using 14-year Fermi-LAT data, suggesting possible non-thermal emission linked to cosmic-ray interactions, though not conclusively detected.

Contribution

First detailed analysis of gamma-ray emission from Abell 119, revealing a potential diffuse excess and providing constraints on non-thermal processes in galaxy clusters.

Findings

Detected a ~4σ gamma-ray excess offset from cluster center.

Estimated gamma-ray luminosity consistent with hadronic models.

Suggested potential neutrino flux motivating future observations.

Abstract

Galaxy clusters are among the most massive gravitationally bound systems in the Universe and are considered major reservoirs of high-energy cosmic rays, yet no conclusive $\gamma$-ray detection from them has been achieved. This non-detection may stem from limited sensitivity and source localization of current $\gamma$-ray instruments, as well as strong interactions of $\gamma$-rays with intervening material that restrict detectable signals to only a few nearby and dynamically active clusters. Motivated by these constraints, we selected a sample of nearby ($z<0.05$) merging clusters and analyzed 14 years of \textit{Fermi}-LAT data. In this work, we present a detailed study of Abell 119 (A119), a merging cluster with significant X-ray luminosity and complex dynamics. Using \textit{Fermipy} and \textit{Fermi} Science Tools, we modeled all potential $\gamma$-ray sources and confirmed the 4FGL point sources 4FGL J0059.3$-$0152, 4FGL J0101.0$-$0059, and 4FGL J0059.2+0006 with significant TS values.s. It further reveals, a $\sim4\sigma$ excess of diffuse $\gamma$-ray emission offset by $\sim0.25^\circ$ from the cluster center, plausibly associated with the cluster halo. An extended model provides the best fit, yielding luminosity bounds of $\sim 12.21^{+2.74}_{-3.95}\times10^{42}\,\mathrm{erg\,s^{-1}}$ and a particle spectral index of $\sim2.25^{+0.38}_{-0.13}$, consistent with earlier expectations for cluster-scale non-thermal emission. These results suggest a hadronic origin for the detected signal. Although the $\sim4\sigma$ excess is compelling, uncertainties in localization and instrumental limitations prevent a definitive detection. Nonetheless, the results highlight the potential for deeper cluster studies, and the estimated neutrino flux $E^{2}\phi_{\nu}\approx3\times10^{-10}\,\mathrm{GeV\,cm^{-2}\,s^{-1}\,sr^{-1}}$ motivates future observations with upcoming neutrino telescopes.