Millisecond Pulsars in Globular Clusters and Implications for the Galactic Center Gamma-Ray Excess

We study the gamma-ray emission from millisecond pulsars within the Milky Way’s globular cluster system in order to measure the luminosity function of this source population. We find that these pulsars have a mean luminosity of $\langle L_γ\rangle \sim (1-8)\times 10^{33}, {\rm erg/s}$ (integrated between 0.1 and 100 GeV) and a log-normal width of $σ_L \sim 1.4-2.8$. If the Galactic Center Gamma-Ray Excess were produced by pulsars with similar characteristics, Fermi would have already detected $N \sim 17-37$ of these sources, whereas only three such pulsar candidates have been identified. We conclude that the excess gamma-ray emission can originate from pulsars only if they are significantly less bright, on average, than those observed within globular clusters or in the Galactic Plane. This poses a serious challenge for pulsar interpretations of the Galactic Center Gamma-Ray Excess.

💡 Research Summary

In this paper the authors perform a comprehensive study of the gamma‑ray emission from millisecond pulsars (MSPs) residing in the Milky Way’s globular cluster system, with the goal of constraining the MSP gamma‑ray luminosity function and testing whether such a population can account for the Galactic Center Excess (GCE). Using 15.8 years of Fermi‑LAT data (100 MeV–100 GeV) and the latest source catalogs, they analyze each of the 157 known globular clusters. Their pipeline follows the standard LAT analysis: a 14° × 14° region of interest is modeled with all cataloged sources, the Galactic diffuse and isotropic backgrounds, and a new point source placed at the cluster’s nominal position. The spectrum of each candidate is fit with a power‑law × exponential‑cutoff model. They detect statistically significant emission (TS > 25) from 56 clusters, eight of which are new detections not present in previous LAT catalogs.

To translate the observed gamma‑ray fluxes into an MSP luminosity function, the authors exploit two well‑known cluster properties: the visible‑band luminosity (L_V, a proxy for stellar mass) and the stellar encounter rate (Γ_e), which correlates with the formation of low‑mass X‑ray binaries and recycled pulsars. They consider five scaling hypotheses for the mean number of MSPs per cluster (linear or non‑linear dependence on Γ_e, proportionality to L_V, and mixed models). By fitting the flux distribution across the 56 detected clusters within a Bayesian framework, they find that a log‑normal luminosity function provides the best description. The inferred parameters are a mean gamma‑ray luminosity ⟨L_γ⟩≈(1–8) × 10³³ erg s⁻¹ and a width σ_L≈1.4–2.8, values that are fully consistent with those measured for MSPs in the Galactic plane.

Armed with this luminosity function, the authors ask how many MSPs would have to be present in the inner Galaxy to generate the observed GCE. Assuming the same ⟨L_γ⟩ and σ_L, the expected number of individually detectable sources in the current Fermi catalogs is 17–37. In reality, only three plausible MSP candidates have been identified near the Galactic Center. This discrepancy implies that, if MSPs are responsible for the GCE, they must be systematically fainter—by at least a factor of five—than the MSPs observed in globular clusters or the Galactic disk. Consequently, the required total MSP population would be of order 10⁵, far larger than any reasonable extrapolation from known systems.

The paper discusses possible ways to reconcile this tension, such as invoking an older, intrinsically dimmer MSP population in the bulge, or proposing exotic formation channels (e.g., accretion‑induced collapse of O‑Ne white dwarfs) that could produce a distinct, low‑luminosity MSP class. However, the authors note that current observations do not support these scenarios convincingly, and that the required dimness would also make the MSPs difficult to detect in radio or X‑ray bands. They conclude that the GCE cannot be readily explained by a population of MSPs with the same luminosity characteristics as those found in globular clusters or the Galactic plane, posing a serious challenge to pulsar‑based interpretations and keeping dark‑matter annihilation among the viable explanations.