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.

A pilot search for MeV gamma-ray emission from five galaxy clusters using archival COMPTEL data

“…From this catalog, we were able to detect gamma-ray emission from one galaxy cluster with significance > 5σ, viz. SPT-CLJ 2012-5649 with six other clusters having significance between 3−5σ [18]. However we could not discern if this emission is from the intra-cluster medium or because of radio galaxies in the cluster.…”

“…Crucially, under the assumption that the background model adequately captures the data, the TS follows a χ 2 distribution with n degrees of freedom, where n denotes the number of free parameters within the source model component. The same statistics is also used in IceCube and Fermi-LAT analysis [18,35].…”

“…Another possible source of gamma-rays in clusters could be from dark matter annihilations if the dark matter is a WIMP [17]. All possible mechanisms of gamma-ray production in clusters have been recently reviewed in [18].…”

“…Most recently, we carried out a systematic search for gamma-ray emission between 1-300 GeV using 15 years of Fermi-LAT data. [18], from a sample of 300 galaxy clusters selected from the South Pole Telescope 2500 square degree cluster survey [19,20], whose redshifts were estimated from optical/NIR follow-ups [21,22]. The clusters in the SPT-SZ survey have been detected using the Sunyaev-Zeldovich effect [23] and hence represent a mass limited sample.…”

“…Given some of the intriguing hints for GeV gamma-ray emission from galaxy clusters [18], it would be interesting to look for detections at MeV energies from the same clusters. Therefore, as a pilot search, we analyze the archival COMPTEL data to look for detections at MeV energies.…”