Search for GeV Gamma-Ray Emission from SPT-SZ selected Galaxy Clusters with 15 years of Fermi-LAT data

Abstract: Galaxy clusters could produce gamma-rays from inverse Compton scattering of cosmic ray electrons or hadronic interactions of cosmic ray protons with the intracluster medium. It is still an open question on whether gamma-ray emission ($>$ GeV energies) has been detected from galaxy clusters. We carry out a systematic search for gamma-ray mission based on 300 galaxy clusters selected from the 2500 deg.$^2$ SPT-SZ survey after sorting them in descending order of $M_{500}/z^2$, using about 15 years of Fermi-LAT data in the energy range between 1-300 GeV. We were able to detect gamma-ray emission with significance of about $6.1\sigma$ from one cluster, viz SPT-CL J2012-5649. The estimated photon energy flux from this cluster is approximately equal to $1.3 \times 10^{-6}$ MeV cm$^{-2}$ s$^{-1}$. The gamma-ray signal is observed between $1-10$ GeV with the best-fit spectral index equal to $-3.61 \pm 0.33$. However, since there are six radio galaxies spatially coincident with SPT-CL J2012-5649 within the Fermi-LAT PSF, we cannot rule out the possibility this signal could be caused by some of these radio galaxies. Six other SPT-SZ clusters show evidence for gamma-ray emission with significance between $3-5\sigma$. None of the remaining clusters show statistically significant evidence for gamma-ray emission.