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 deg2 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 6.1σ from one cluster, viz. SPT-CL J2012-5649. The estimated photon energy flux from this cluster is approximately equal to 1.3 × 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 ± 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 3–5σ. None of the remaining clusters show statistically significant evidence for gamma-ray emission.