Modelling the stellar soft-photon energy density profile of globular clusters

P L Prinsloo; C Venter; I Büsching; A Kopp

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arXiv · 1311.3791

Modelling the stellar soft-photon energy density profile of globular clusters

P L Prinsloo·C Venter·I Büsching·A Kopp

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Abstract

Recent observations by the Fermi Large Area Telescope (LAT) and the High Energy Stereoscopic System (H.E.S.S.) have revealed globular clusters (GC) to be sources of high-energy (HE) and very-high-energy (VHE) gamma-rays. It has been suggested that the presence of large numbers of millisecond pulsars (MSPs) within these clusters may be either directly responsible for these gamma-ray fluxes through emission of pulsed curvature radiation, or indirectly through the injection of relativistic leptons into the cluster. These relativistic particles are plausibly re-accelerated in shocks, created by the collision of stellar winds, before interacting with the soft-photon radiation field set up by the stellar population of the host cluster. Inverse Compton (IC) scattering then produces gamma-radiation in the TeV band. In order to calculate the IC spectrum, an accurate profile for the energy density of the soft-photon field is required. We construct such a profile by deriving a radially-dependent expression for the stellar energy density, and then solve it numerically. As a next step, the average energy density values for three different regions of the cluster (demarcated by its core, half-mass, and tidal radii) are determined, which we consequently import into an existing radiation code to predict the TeV gamma-ray spectrum. As an application, we consider the case of Terzan 5, boasting a population of 34 radio MSPs, and compare our predicted spectrum with that measured by H.E.S.S.

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P L Prinsloo, C Venter, I Büsching, A Kopp. 2014-12-12. Modelling the stellar soft-photon energy density profile of globular clusters. https://arxiv.org/abs/1311.3791

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