A Gamma-ray Stacking Survey of Fermi-LAT Undetected Globular Clusters

TL;DR

This study uses 13 years of Fermi-LAT data to detect cumulative gamma-ray emission from a stacked group of 39 undetected high-latitude globular clusters, suggesting millisecond pulsars as the source.

Contribution

It provides the first evidence of gamma-ray emission from a stacked population of undetected globular clusters using long-term Fermi-LAT data.

Findings

Significant gamma-ray signal detected from stacked GCs (~4σ)

No correlation found between gamma-ray luminosity and cluster properties

Method distinguishes cluster emission from background using control fields

Abstract

We present evidence for $\gamma$-ray emission from a stacked population of 39 high-latitude globular clusters (GCs) not detected in the Fermi Point Source Catalog, likely attributable to populations of millisecond pulsars within them. In this work, we use 13 years of data collected by the Large Area Telescope aboard the Fermi Gamma-Ray Space Telescope to search for a cumulative signal from undetected GCs and compared them to control fields (CFs), selected to match the celestial distribution of the target clusters so as to distinguish the $\gamma$-ray signal from background emission. The joint likelihood distribution of the GCs has a significant separation ($\sim4\sigma$) from that of the CFs. We also investigate correlations between detected cluster luminosities and other cluster properties such as distance, the number of millisecond pulsars associated with each cluster, and stellar encounter rate but find no significant relationships.