# Deep observations of the globular cluster M15 with the MAGIC telescopes

## TL;DR

Deep observations of globular cluster M15 with MAGIC telescopes set stringent limits on TeV gamma-ray emission, constraining lepton acceleration efficiency in millisecond pulsars and challenging existing high-energy emission models.

## Contribution

This study provides the first deep TeV gamma-ray observational limits for M15, constraining lepton acceleration efficiency in MSPs and their wind regions, and refining models of high-energy processes in globular clusters.

## Key findings

- No significant TeV gamma-ray emission detected from M15.
- Upper limit on gamma-ray flux established at <3.2×10⁻¹³ cm⁻² s⁻¹ above 300 GeV.
- Constraints placed on lepton acceleration efficiency in MSP magnetospheres and wind regions.

## Abstract

A population of globular clusters (GCs) has been recently established by the Fermi-LAT telescope as a new class of GeV $\gamma$-ray sources. Leptons accelerated to TeV energies, in the inner magnetospheres of MSPs or in their wind regions, should produce $\gamma$-rays through the inverse Compton scattering in the dense radiation field from the huge population of stars. We have conducted deep observations of the globular cluster M15 with the MAGIC telescopes and used 165 hrs in order to search for $\gamma$-ray emission. A strong upper limit on the TeV $\gamma$-ray flux $<3.2\times 10^{-13}\mathrm{cm^{-2}s^{-1}}$ above 300 GeV ($<0.26\%$ of the Crab nebula flux) has been obtained. We interpret this limit as a constraint on the efficiency of the acceleration of leptons in the magnetospheres of the MSPs. We constrain the injection rate of relativistic leptons, $\eta_{\rm e}$, from the MSPs magnetospheres and their surrounding. We conclude that $\eta_{\rm e}$ must be lower than expected from the modelling of high energy processes in MSP inner magnetospheres. For leptons accelerated with the power law spectrum in the MSP wind regions, $\eta_{\rm e}$ is constrained to be much lower than derived for the wind regions around classical pulsars. These constraints are valid for the expected range of magnetic field strengths within the GC and for the range of likely energies of leptons injected from the inner magnetospheres, provided that the leptons are not removed from the globular cluster very efficiently due to advection process. We discuss consequences of these constraints for the models of radiation processes around millisecond pulsars.