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

TL;DR

This work analyzes 14 years of Fermi-LAT data for Abell 119 to search for diffuse gamma-ray emission from the intracluster medium. By employing point-source, morphological, and CRp-based templates against multi-wavelength maps (galaxy density, Planck tSZ, X-ray), the authors identify a ~4σ local excess offset from the cluster center, with the extended CRp model providing the best fit. They constrain the CRp population to a normalization of about $X_{\mathrm{CRp}}\approx0.07$–$0.08$ relative to the thermal energy and a spectral slope $\alpha_{\mathrm{CRp}}\approx2.2$, yielding a 100 MeV–1 TeV flux around $(11$–$13)\times10^{-10}$ ph cm$^{-2}$ s$^{-1}$ and a neutrino flux expectation of $E^{2}\phi_{\nu}\approx3\times10^{-10}$ GeV cm$^{-2}$ s$^{-1}$ sr$^{-1}$. While the local signal is compelling, the global significance is modest due to look-elsewhere effects, so a definitive detection remains elusive; nonetheless, the results support a hadronic origin for cluster high-energy emission and motivate deeper observations with upcoming neutrino and gamma-ray facilities.

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 $γ$-ray detection from them has been achieved. This non-detection may stem from limited sensitivity and source localization of current $γ$-ray instruments, as well as strong interactions of $γ$-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 $γ$-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 $\sim4σ$ excess of diffuse $γ$-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 $\sim4σ$ 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 $E^{2}φ_ν\approx3\times10^{-10}\,\mathrm{GeV\,cm^{-2}\,s^{-1}\,sr^{-1}}$ motivates future observations with upcoming neutrino telescopes.

Figures (9)

• Figure 1: Residual excess map in the vicinity of the A119 cluster for the total energy band $100$ MeV - $1$ TeV, $100$ MeV - $1$ GeV, and $1$ GeV - $1$ TeV. The dotted green circle shows the extension of the $\gamma$-ray emission, the white cross indicates the center of the cluster, and the black $+$ symbols represent 4FGL-DR4 sources within the region.
• Figure 2: Above images describe the $\gamma$-ray emission morphology within the galaxy cluster. From left to right and top to bottom: (1) density of galaxies, (2) thermal Sunyaev-Zel'dovich (tSZ) signal, (3) X-ray emission (ROSAT) , (4) X-ray emission (XMM Newton) ($s^{-1}arcmin^{-2}$). Each template provides a distinct observational perspective on cluster structure, aiding in the identification of features potentially correlated with $\gamma$-ray emission.
• Figure 3: Comparison of MINOT model predictions for the cosmic-ray proton (CRp) distribution and associated $\gamma$-ray emission within a A119 cluster, under various assumptions about the CRp distribution. All models here are normalized to $X_{CRp}(R_{500})=10^{-2}$. Top left panel shows the radial profile of the CRp distribution (density). Top right panel shows ratio of enclosed CRp energy to thermal energy as function of radius. Bottom left panel shows the radial $\gamma$-ray surface brightness profile. Bottom right panel shows the integrated $\gamma$-ray spectrum within the $R_{500}$
• Figure 4: Left Panel: shows disc model and Right panel: shows extended model $(n_{CRp}\propto n_{gas}^{\frac{1}{2}})$: Localization map of the $\gamma$-ray source obtained with Fermipy. The color scale represents the likelihood surface in terms of $2 \times \Delta \ln L$, with warmer colors indicating regions of higher likelihood. The red cross marks the cluster position, while the black diamond denotes the best-fit position derived from the localization analysis. The solid and dashed contours correspond to the 68% and 99% confidence regions, respectively. For reference, the red cross indicates the center of the Abell 119 cluster providing spatial context for assessing the association between the localized $\gamma$-ray emission and the cluster environment.
• Figure 5: Left panel: Histogram of Test Statistic (TS) values from null simulations (blue) compared with injected source simulations (orange). The red vertical dashed line marks the observed TS ($\mathrm{TS_{obs}}=18$). Dotted lines denote empirical local significance thresholds corresponding to $3\sigma$ ($\mathrm{TS}\approx10.3$), $4\sigma$ ($\mathrm{TS}\approx17.3$), and $5\sigma$ ($\mathrm{TS}\approx25.0$). The green dash-dotted line indicates the FDR-corrected threshold ($\alpha=0.05$, $\mathrm{TS}\approx9.2$). Right panel: Cumulative distribution function (survival probability) of TS values under the background-only hypothesis (blue) and with an injected source (orange). The red vertical dashed line shows the observed TS, while dotted and dash-dotted lines again indicate the empirical local significance thresholds and the FDR-corrected threshold.