Gamma-ray properties of globular clusters and the "fundamental planes"

TL;DR

This paper reports the discovery of gamma-ray emissions from multiple globular clusters, revealing correlations with cluster properties and proposing 'fundamental planes' that link gamma-ray luminosity to underlying pulsar populations.

Contribution

It presents the first detailed correlation study between gamma-ray luminosities and cluster parameters, introducing the concept of 'fundamental planes' for globular clusters.

Findings

Strong correlation between gamma-ray luminosity and stellar encounter rate.

Gamma-ray luminosity correlates with cluster metallicity [Fe/H].

Energy densities of soft photons are linked to gamma-ray emission.

Abstract

We report on the discovery of gamma-ray emission from several globular clusters (GCs), including Terzan 5, the second known gamma-ray GCs. By now, more than a dozen GCs are known to emit gamma-rays of energies above 100 MeV, thus enabling us to carry out the first detailed correlation study with several cluster properties. We found strong correlations between the observed gamma-ray luminosities and four cluster parameters: stellar encounter rate, metallicity [Fe/H], and energy densities of the soft photons at the cluster locations. These "fundamental planes" of gamma-ray GCs put an intimate relation of the observed gamma-rays to the underlying millisecond pulsar population and have important implications on the origin of the gamma-ray emission of GCs.