Identifying the brightest Galactic globular clusters for future observations by H.E.S.S. and CTA

TL;DR

This study models gamma-ray emissions from Galactic globular clusters assuming millisecond pulsars as sources, predicting their detectability with H.E.S.S. and CTA, and identifying the most promising clusters for future observations.

Contribution

It introduces a detailed emission model for globular clusters based on MSPs, constrains parameters with observational data, and ranks clusters by their predicted gamma-ray flux for upcoming telescopes.

Findings

H.E.S.S. could detect 2 additional GCs with 100-hour observations.

CTA is expected to detect over half of known GCs, depending on parameters.

The detectability is highly sensitive to model assumptions.

Abstract

We present results from an emission code that assumes millisecond pulsars (MSPs) to be sources of relativistic particles in globular clusters (GCs) and models the resulting spectral energy distribution (SED) of Galactic GCs due to these particle's interaction with the cluster magnetic and soft-photon fields. We solve a transport equation for leptons and calculate inverse Compton (IC) and synchrotron radiation (SR) to make predictions for the flux expected from Galactic GCs. We perform a parameter study and also constrain model parameters for three GCs using gamma-ray and X-ray data. We next study the detectability of 16 Galactic GCs for the High Energy Stereoscopic System (H.E.S.S.) and the Cherenkov Telescope Array (CTA), ranking them according to their predicted TeV flux. The spectrum of each cluster and therefore the detectability ranking is very sensitive to the choice of parameters. We expect H.E.S.S. to detect two more GCs (in addition to Terzan 5), i.e., 47 Tucanae and NGC 6388, if these clusters are observed for 100 hours. The five most promising GCs for CTA are NGC 6388, 47 Tucanae, Terzan 5, Djorg 2, and Terzan 10. We lastly expect CTA to detect more than half of the known Galactic GCs population, depending on observation time and model parameters.