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

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.