Neutron stars and millisecond pulsars in star clusters: implications for the diffuse $\gamma$-radiation from the Galactic Centre

TL;DR

This study models neutron star and millisecond pulsar evolution in globular clusters, linking their gamma-ray emissions to the gamma-ray excess observed in the Galactic Centre, and highlights the impact of black holes on neutron star distribution.

Contribution

It provides new insights into the dynamical evolution of neutron stars in globular clusters and their role in gamma-ray emissions, considering various initial cluster conditions.

Findings

Neutron star radial distribution is influenced by black hole content.

Average neutron star position is about 60-80% of the cluster's half-mass radius.

Gamma-ray emission from globular clusters matches observations and supports MSP origin of Galactic Centre excess.

Abstract

Globular clusters (GCs) are the ideal environment for the formation of neutron stars (NSs) and millisecond pulsars (MSPs). NSs origin and evolution provide a useful information on stellar dynamics and evolution in star clusters, and are among the most interesting astrophysical objects, being precursors of several high-energy phenomena such as gravitational waves and gamma-ray bursts. Due to a large velocity kick that they receive at birth, most of the NSs escape the local field, affecting the evolution and dynamics of their parent cluster. In this paper, we study the origin and dynamical evolution of NSs within GCs with different initial masses, metallicities and primordial binary fractions. We find that the radial profile of NSs is shaped by the BH content of the cluster, which partially quenches the NS segregation until most of the BHs are ejected from the system. Independently on the cluster mass and initial configuration, the NSs map the average stellar population, as their average radial distance is $\approx 60-80\%$ of the cluster half-mass radius. Finally, by assuming a recycling fraction of $f_\mathrm{rec}=0.1$ and an average MSP gamma-ray emission of $L_\gamma=2\times 10^{33}$ erg s$^{-1}$, we show that the typical gamma-ray emission from our GCs agrees with observations and supports the MSP origin of the gamma-ray excess signal observed by the Fermi-LAT telescope in the Galactic Centre.