Modeling high-energy and very-high-energy γ-rays from the Terzan 5 cluster
De Jager, O.C.
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The Fermi Large Area Telescope (LAT) has recently detected a population of globular clusters (GCs) in highenergy (HE) γ-rays. Their spectral properties and energetics are consistent with cumulative emission from a population of millisecond pulsars (MSPs) hosted by these clusters. For example, the HE spectra exhibit fairly hard power-law indices and cutoffs around a few GeV, typical of pulsed spectra measured for the γ-ray pulsar population. The energetics may be used to constrain the number of visible MSPs in the cluster (Nvis), assuming canonical values for the average γ-ray efficiency and spin-down power. This interpretation is indeed strengthened by the fact that the first γ-ray MSP has now been identified in the GC NGC 6624, and this MSP is responsible for almost all of the HE emission from this cluster . On the other hand, it has been argued that the MSPs are also sources of relativistic leptons which may be reaccelerated in shocks originating in collisions of stellar winds in the cluster core, and may upscatter bright starlight and cosmic microwave background photons to very high energies. Therefore, this unpulsed component may give an independent constraint on the total number of MSPs (Ntot) hosted in the GC, for a given cluster magnetic field B and diffusion coefficient k0. Lastly, the transport properties of the energetic leptons may be further constrained using multiwavelength data, e.g., to infer the radial dependence of k0 and B. We present results on our modeling of the pulsed and unpulsed γ-ray fluxes from the GC Terzan 5