| Name: Surajit Paul |
| Affiliation: Manipal Centre for Natural Sciences, Manipal Academy of Higher Education |
| Conference ID: ASI2026_1105 |
| Title: First Unambiguous Detection of Gamma Rays from the Intracluster Medium with 14 Years of Fermi-LAT Data |
| Abstract Type: Poster |
| Abstract Category: High Energy Phenomena, Fundamental Physics and Astronomy |
| Author(s) and Co-Author(s) with Affiliation: Gajanan D. Harale(Department of Physics, Savitribai Phule Pune University, Pune-411007, India), Surajit Paul(Manipal Centre for Natural Sciences, Manipal Academy of higher education, Manipal 576104, India) |
| Abstract: In the hierarchical structure formation, massive galaxy clusters (~10¹⁵ M⊙) form relatively late in the non-linear regime through major cluster mergers, the most energetic (~10⁶⁴ erg) phenomena in the post–Big-Bang era. This enormous energy dissipates via Mpc-scale shocks, analogous to supernova blast waves, as predicted by numerical simulations and observations of ring-like radio relics in merging systems (e.g. Abell3376). Merger-driven turbulence and shocks are expected to accelerate particles to ultra-high energies via Fermi mechanisms, making clusters promising reservoirs of high-energy cosmic rays. Despite strong theoretical motivation, conclusive detection of gamma-ray emission from the intracluster medium (ICM) has remained elusive, raising questions about our physical understanding or the limited sensitivity and localization capability of current gamma-ray telescopes. Additionally, gamma-ray interactions with intergalactic material along the line of sight restrict detectable signals to only a few very nearby, dynamically active clusters, further reducing the accessible test space.
Motivated by these challenges, we systematically shortlisted a sample of promising galaxy clusters and performed a comprehensive analysis of 14-years of Fermi-LAT data. In this talk, I will present our recent results reporting the first unambiguous detection of diffuse gamma-ray emission from the ICM of Abell 119 and other well-known systems. The analysis was carried out using Fermipy and the Fermi Science Tools, incorporating detailed spatial and spectral modeling of all potential gamma-ray sources. Using an optimized background estimation strategy, we successfully modeled and removed point-like sources, isolating the underlying diffuse emission. A rigorous statistical analysis demonstrates that the detected signal is physically associated with the ICM with high significance (>4σ). From the spatial morphology and spectral behavior, we conclude that the emission originates from non-thermal processes in the ICM, most plausibly hadronic interactions. Finally, our estimated neutrino flux of a few ×10⁻¹⁰ GeVcm⁻²s⁻¹sr⁻¹ provides strong motivation for future observations with next-generation neutrino facilities. |
