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 $\sim 4\sigma$ excess of diffuse $\gamma$-ray emission offset by $\sim 0.25^{\circ }$ from the cluster center, plausibly associated with the cluster halo. An extended model provides the best fit, yielding luminosity bounds of $\sim 12.21_{-3.95}^{+2.74}\times 10^{42}\mathrm{erg}{\mathrm{s}}^{-1}$ and a particle spectral index of $\sim 2.25_{-0.13}^{+0.38}$, consistent with earlier expectations for cluster-scale non-thermal emission. These results suggest a hadronic origin for the detected signal. Although the $\sim 4\sigma$ 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 $E^2{\varphi }_{\nu }\approx 3\times 10^{-10}\mathrm{GeV}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}\mathrm{s}{\mathrm{r}}^{-1}$ motivates future observations with upcoming neutrino telescopes.

20 pages, 9 figures, Published in Physical Review D, This is the accepted manuscript. The final published version is available at https://doi.org/10.1103/gn1q-pzx3

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