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The contribution to Galactic Centre γ-ray excess from cluster-born millisecond pulsars. Constraints from direct N-body simulations

M. Kalambay, M. Ishchenko, D. Kuvatova, T. Panamarev, P. Berczik

Abstract

The Galactic Centre γ-ray excess (GCE), observed by Fermi-LAT around Sgr A*, exceeds expectations from standard cosmic-ray models and is commonly attributed either to dark matter annihilation or to unresolved millisecond pulsars (MSPs). We revisit the MSP scenario within a fully dynamical framework by tracking neutron stars (NSs) formed in globular clusters (GCs) and deposited into the central kiloparsec. Using high-resolution direct N-body simulations of GCs evolving in a time-dependent Milky Way potential, we model both present-day clusters and an early population of disrupted systems. From the simulated NS distributions, we infer the MSP population via an empirically calibrated MSP-to-NS ratio and construct mock γ-ray flux profiles assuming representative pulsar luminosities. MSPs associated with surviving clusters already produce a substantial γ-ray contribution, while disrupted clusters enhance both the amplitude and central concentration of the signal. Under reasonable assumptions, the combined MSP population reproduces the observed GCE properties, favouring an astrophysical origin over dark matter interpretations.

The contribution to Galactic Centre γ-ray excess from cluster-born millisecond pulsars. Constraints from direct N-body simulations

Abstract

The Galactic Centre γ-ray excess (GCE), observed by Fermi-LAT around Sgr A*, exceeds expectations from standard cosmic-ray models and is commonly attributed either to dark matter annihilation or to unresolved millisecond pulsars (MSPs). We revisit the MSP scenario within a fully dynamical framework by tracking neutron stars (NSs) formed in globular clusters (GCs) and deposited into the central kiloparsec. Using high-resolution direct N-body simulations of GCs evolving in a time-dependent Milky Way potential, we model both present-day clusters and an early population of disrupted systems. From the simulated NS distributions, we infer the MSP population via an empirically calibrated MSP-to-NS ratio and construct mock γ-ray flux profiles assuming representative pulsar luminosities. MSPs associated with surviving clusters already produce a substantial γ-ray contribution, while disrupted clusters enhance both the amplitude and central concentration of the signal. Under reasonable assumptions, the combined MSP population reproduces the observed GCE properties, favouring an astrophysical origin over dark matter interpretations.
Paper Structure (7 sections, 4 equations, 8 figures, 3 tables)

This paper contains 7 sections, 4 equations, 8 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Integration procedure, initial mass function, and mass loss
  3. NS distribution in Galaxy at the present day based on GC simulations
  4. $\gamma$-ray fluxes from the distribution of MSPs
  5. Fitting the MSP-to-NS ratio
  6. Mock observations: NSs from real and destroyed GCs
  7. Conclusions and discussion

Figures (8)

  • Figure 1: Evolution of the mass loss in the GCs in percent due to stellar evolution and orbital tidal stripping for our 12 MW GCs.
  • Figure 2: Position and velocity distribution of the NSs in the Galaxy from the GC's $N$-body simulations. Top: Real MW GCs. Bottom: Early GCs that were destroyed over a 5 Gyr evolution time.
  • Figure 3: Dependence of the velocity distribution on distance. Top: Real MW GCs. Bottom: Destroyed GCs. The colour coding corresponds to the legend in Fig. \ref{['fig:mass-loss']}.
  • Figure 4: Distribution of NSs in the central region of the Galaxy is less than 1 kpc. This distribution is shown in a three-coordinate projection. From left to right: $X$ vs $Y$, $Y$ vs $Z$ and $R$ vs $Z$, where $R$ is the galactic plate radius. Top: NSs from MW GC simulations (different colours correspond to different models, as in Fig. \ref{['fig:mass-loss']}). Bottom: NSs from GCs that were destroyed early on.
  • Figure 5: Relationship between GC mass and NS populations. Colours denote individual clusters as listed in legend of Fig. \ref{['fig:dr-gc']}. The full dataset is given in Table \ref{['tab:ns-stat']}. Top: Initial mass of GCs vs the total number of NSs formed up to the present day. The dashed line shows the best-fit linear relation (slope = 9428.42 NSs per $10^6 M_\odot$, $R^2 = 0.87$), with the shaded region representing the $\pm$1436 NSs scatter around the fit. Bottom: Current mass of GCs vs bound number of NSs. The dashed line shows the best-fit linear relation (slope = 95.29 NSs per $10^5 M_\odot$, $R^2 = 0.11$), with the shaded region representing the $\pm$842 NSs scatter around the fit.
  • ...and 3 more figures