The contribution to Galactic Centre γ-ray excess from cluster-born millisecond pulsars

Constraints from direct N-body simulations

¹ Heriot-Watt University Aktobe Campus, K. Zhubanov Aktobe Regional University, A. Moldagulova ave. 34, 030000 Aktobe, Kazakhstan
² Fesenkov Astrophysical Institute, Observatory 23, 050020 Almaty, Kazakhstan
³ Faculty of Physics and Technology, Al-Farabi Kazakh National University, al-Farabi ave. 71, 050040 Almaty, Kazakhstan
⁴ Main Astronomical Observatory, National Academy of Sciences of Ukraine, 27 Akademika Zabolotnoho St, 03143 Kyiv, Ukraine
⁵ Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
⁶ Rudolf Peierls Centre for Theoretical Physics, Parks Road, Oxford OX1 3PU, UK
⁷ Szechenyi Istvan University, Space Technology and Space Law Research Center, 9026 Gyor, Egyetem ter 1, Hungary

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 November 2025
Accepted: 20 February 2026

Abstract

Context. The Galactic-Centre γ-ray excess (GCE) is a spatially extended surplus of γ-rays around Sgr A* observed by Fermi-LAT that exceeds predictions from standard cosmic-ray interactions in the inner Galaxy. The GCE has been mainly attributed either to dark-matter annihilation or to an unresolved population of millisecond pulsars (MSPs) dynamically delivered to the inner Galaxy by globular clusters (GCs).

Aims. We reassess the MSP interpretation by following a fully dynamical framework to explore how neutron stars (NSs) produced in GCs are stripped and deposited into the central kiloparsec. We also quantify the resulting γ-ray emission to compare it with Fermi-LAT measurements.

Methods. We ran high-resolution direct N-body simulations of GCs in a time-varying Milky Way (MW) potential, capturing the internal cluster dynamics and external tides. We modelled two channels: (i) present-day observed MW GCs on their orbits and (ii) a population of early, now-destroyed clusters whose debris was accreted during their evolution into the inner Galaxy. The simulations provide the full phase-space distribution of deposited NSs to central 1 kpc from these both sources. We converted the NS count to an MSP count using an empirically calibrated efficiency, adopting a sample-averaged ratio between observed and predicted MSP counts and NSs that are bound to clusters. As a result, we were able to generate mock sky maps and cumulative flux profiles assuming representative per-pulsar luminosities.

Results. We find that MSPs inferred from the observed GCs already supply a substantial γ-ray signal, including disrupted clusters increases both amplitude and central concentration. By taking the observational ratio of MSPs to the total number of NSs in observed Galactic stellar systems as a basis and adopting an average luminosity of ⟨L⟩∼8 × 10³³ erg s⁻¹, we were able to reproduce the GCE quite well. As a free parameter to achieve better agreement with the observed flux, we had to increase the number of NSs originating from previously disrupted stellar systems by approximately a factor of 2. The deposited NSs from destroyed clusters exhibit an axisymmetric morphology with pronounced over-densities in the Galactic plane and perpendicular to it.

Conclusions. The results of our modelling favour an MSP origin of the GalC γ-ray excess over dark matter annihilation, primarily because the combined contribution of MSPs delivered by surviving and disrupted GCs naturally reproduces both the amplitude and concentration of the observed signal under reasonable assumptions.