Search for Very-high-energy γ-ray emission from Galactic globular clusters with H.E.S.S

Abstract

Globular clusters (GCs) are established emitters of high-energy (HE, 100 MeV<E<100 GeV) γ-ray radiation which could originate from the cumulative emission of the numerous millisecond pulsars (msPSRs) in the clusters' cores or from inverse Compton (IC) scattering of relativistic leptons accelerated in the GC environment. These stellar clusters could also constitute a new class of sources in the very-high-energy (VHE, E>100 GeV) γ-ray regime, judging from the recent detection of a signal from the direction of Terzan 5 with the H.E.S.S. telescope array. We searched for point-like and extended VHE γ-ray emission from 15 GCs serendipitously covered by H.E.S.S observations and also performed a stacking analysis combining the data from all GCs to investigate the hypothesis of a population of faint emitters. Assuming IC emission as the origin of the VHE γ-ray signal from the direction of Terzan 5, we calculated the expected γ-ray flux from each of the 15 GCs, based on their number of millisecond pulsars, their optical brightness and the energy density of background photon fields. We did not detect significant VHE γ-ray emission from any of the 15 GCs in either of the two analyses. Given the uncertainties related to the parameter determinations, the obtained flux upper limits allow to rule out the simple IC/msPSR scaling model for NGC 6388 and NGC 7078. The upper limits derived from the stacking analyses are factors between 2 and 50 below the flux predicted by the simple leptonic scaling model, depending on the assumed source extent and the dominant target photon fields. Therefore, Terzan 5 still remains exceptional among all GCs, as the VHE γ-ray emission either arises from extra-ordinarily efficient leptonic processes, or from a recent catastrophic event, or is even unrelated to the GC itself.

8 pages, 1 figure, accepted for publication in Astronomy and Astrophysics

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• Formation and evolution of compact binaries in globular clusters: II. Binaries with neutron stars
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Cited by in corpus (14)

• The H.E.S.S. Galactic plane survey
• TeV gamma-ray emission initiated by the population or individual millisecond pulsars within globular clusters
• Gamma-ray emission from globular clusters
• Cherenkov Telescope Array sensitivity to the putative millisecond pulsar population responsible for the Galactic center excess
• Misaligned TeV gamma-ray sources in the vicinity of globular clusters
• Deep observations of the globular cluster M15 with the MAGIC telescopes
• Galactic Observatory Science with the ASTRI Mini-Array at the Observatorio del Teide
• Gamma-ray emission from high Galactic latitude globular clusters
• Identifying the brightest Galactic globular clusters for future observations by H.E.S.S. and CTA
• Probing the pulsar population of Terzan 5 via spectral modeling
• A history of the gamma-ray burst flux at the Earth from Galactic globular clusters
• Assessing Uncertainties in the Predicted Very-High-Energy Flux of Globular Clusters in the Cherenkov Telescope Array Era
• Characterizing the Gamma-ray Emission Properties of the Globular Cluster M5 with the Fermi-LAT
• H.E.S.S.: The High Energy Stereoscopic System