A Stacked Analysis of GeV Gamma-Ray Emission from SPT-SZ Galaxy Clusters with 16 Years of Fermi-LAT Data

Abstract: We report a statistically significant detection of cumulative $\gamma$-ray emission from a stacked sample of SPT-SZ selected galaxy clusters using 16.4 years of data from the Fermi Gamma-Ray Space Telescope's Large Area Telescope (LAT). By analyzing a population of clusters with individual Test Statistic (TS) values $<$ 9.0, we identify a robust cumulative signal with a TS of 75.2, which corresponds to approximately $8.4\sigma$ significance. In contrast, clusters with TS $<$ 4.0 yield a weaker cumulative signal of TS equal to 9.65 (2.65$\sigma$), consistent with background fluctuations. The derived $\gamma$-ray spectrum is well characterized by a power law model with a best-fit spectral index of $-2.59 \pm 0.20$ and an integrated flux of $1.67^{+1.35}_{-1.07} \times 10^{-11}$ ph cm$^{-2}$ s$^{-1}$. The high TS values in the full sample are likely driven by AGN-dominated clusters. However, when clusters with TS between 4 and 9 are excluded, the remaining low-TS population shows a stacked signal consistent with hadronic emission.