Assessing Uncertainties in the Predicted Very-High-Energy Flux of Globular Clusters in the Cherenkov Telescope Array Era

TL;DR

This paper analyzes the uncertainties in predicting very-high-energy gamma-ray fluxes from globular clusters, emphasizing the importance of precise measurements to guide Cherenkov Telescope Array observations.

Contribution

It demonstrates how parameter uncertainties affect flux predictions and shows that certain parameter regions satisfy existing observational upper limits.

Findings

Uncertainty in model parameters causes large flux prediction spread.

Predicted fluxes can satisfy current upper limits with specific parameter choices.

Improved measurement accuracy is crucial for better flux predictions and observational planning.

Abstract

Terzan 5 is the only Galactic globular cluster that has plausibly been detected in the very-high-energy range. Stacking upper limits by H.E.S.S. on the integral $\gamma$-ray flux of a population of other globular clusters are very constraining for leptonic cluster emission models. We demonstrate that uncertainty in model parameters leads to a large spread in the predicted flux, and there are indeed regions in parameter space for which the stringent stacking upper limits are satisfied. We conduct two more case studies: we study the uncertainties in differential TeV flux for M15, showing that our model can satisfy the stringent MAGIC upper limits for this cluster, for typical cluster parameters. We also calculate the differential flux at TeV energies for $\omega$ Cen, from which five pulsars have recently been detected at radio energies. It is thus important to increase measurement accuracy on key model parameters in order to improve predictions of cluster fluxes so as to better guide the observational strategy of the Cherenkov Telescope Array.