Search for best globular cluster candidates for H.E.S.S. and CTA

Abstract

Globular clusters (GCs) are multi-wavelength objects. About 20 of these systems have been detected by the Fermi Large Area Telescope in the GeV band, whilst the ground- based Cherenkov telescope, the High Energy Stereoscopic System (H.E.S.S.), which is operated in a pointing mode at energies above 100 GeV, has only seen a single GC within our Galaxy, i.e., Terzan 5. H.E.S.S. has accumulated more data since their pre- vious published analysis and thus I reanalysed the H.E.S.S. data for 15 non-detected GCs as well as for Terzan 5. I confirmed the detection of Terzan 5 (with a significance of 7.1σ for hard cuts) and that no individual GC or stacking analysis shows any signif- icant excess emission. I found flux upper limits for the non-detected sources that are consistent with the published results. I furthermore present results from an emission code that models the spectral energy density of GCs by solving a Fokker-Planck-type transport equation for leptons and calculate inverse Compton and synchrotron radi- ation to make predictions for the flux expected from GCs. I performed a parameter study and constrained model parameters for three GCs using γ-ray and X-ray data. I accumulated parameters for all 16 Galactic GCs and used the code to study their detectability with H.E.S.S. and the Cherenkov Telescope Array (CTA), ranking them according to their predicted TeV fluxes. The spectrum of each cluster and therefore the detectability ranking is very sensitive to the choice of parameters. I expect H.E.S.S. to detect two more GCs, i.e., 47 Tucanae and NGC 6388, if the 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. I lastly expect CTA to detect more than half of the known Galactic population, depending on observation time and model parameters.