Gamma-rays from millisecond pulsars in Globular Clusters

Globular clusters (GCs) with their ages of the order of several billion years contain many final products of evolution of stars such as: neutron stars, white dwarfs and probably also black holes. These compact objects can be at present responsible for the acceleration of particles to relativistic energies. Therefore, gamma-ray emission is expected from GCs as a result of radiation processes occurring either in the inner magnetosperes of millisecond pulsars or in the vicinity of accreting neutron stars and white dwarfs or as a result of interaction of particles leaving the compact objects with the strong radiation field within the GC. Recently, GeV gamma-ray emission has been detected from several GCs by the new satellite observatory Fermi. Also Cherenkov telescopes reported interesting upper limits at the TeV energies which start to constrain the content of GCs. We review the results of these gamma-ray observations in the context of recent scenarios for their origin.

💡 Research Summary

Globular clusters (GCs) are dense, old stellar systems containing 10⁵–10⁶ stars within a few parsecs. Their high stellar density creates an intense internal radiation field with an energy density of roughly 300 eV cm⁻³, far exceeding the cosmic microwave background. This photon bath provides an efficient target for relativistic particles to produce high‑energy γ‑rays via inverse Compton scattering (ICS) and hadronic interactions.

GCs host a rich population of compact objects: hundreds of millisecond pulsars (MSPs), low‑mass X‑ray binaries (LMXBs), cataclysmic variables (CVs), and possibly intermediate‑mass black holes (IMBHs). Radio surveys have identified about 140 MSPs in 26 clusters; dynamical estimates suggest many more, especially in core‑collapsed clusters such as Terzan 5, where up to ~200 MSPs may reside. MSPs are old neutron stars spun up by accretion from a companion; although their individual spin‑down power is lower than that of young pulsars, the collective output of dozens to hundreds of MSPs can reach γ‑ray luminosities detectable by modern instruments.

The Fermi Large Area Telescope (LAT) has detected GeV emission from eight GCs (including 47 Tuc and Terzan 5). Their spectra are remarkably uniform: power‑law indices between 0.7 and 1.4, exponential cut‑offs at 1–2 GeV, and total γ‑ray powers of 10³⁴–10³⁶ erg s⁻¹. These characteristics closely match the average spectrum of field MSPs, supporting the hypothesis that the observed GeV flux is the cumulative magnetospheric emission of the MSP population. The γ‑ray luminosity correlates better with the inferred number of MSPs than with the total mass of the cluster.

At TeV energies, ground‑based Cherenkov telescopes (Whipple, MAGIC, H.E.S.S., VERITAS) have so far reported only upper limits. MAGIC’s deep observation of M 13 (E > 140 GeV) and H.E.S.S.’s limit on 47 Tuc (E > 800 GeV) already constrain models that assume a modest conversion efficiency (~1 %) of MSP spin‑down power into relativistic electrons. The non‑detections imply either a relatively low number of MSPs (≤ 50) or that accelerated electrons lose energy primarily via synchrotron radiation or escape before up‑scattering the dense stellar photon field. Future facilities such as the Cherenkov Telescope Array (CTA) will reach the sensitivity needed to test these scenarios.

Theoretical models for GC γ‑ray production fall into two main categories. (1) Direct magnetospheric emission from MSPs, based on polar‑cap, slot‑gap, or outer‑gap geometries, predicts curvature‑radiation spectra consistent with the observed GeV cut‑offs. (2) Leptonic models where MSP winds interact with companion stellar winds or intra‑cluster material, forming shocks that accelerate electrons to tens or hundreds of GeV. These electrons then up‑scatter the intense stellar photon field, producing both the observed GeV component and a potential TeV tail. Model parameters—electron injection spectrum, diffusion coefficient, magnetic field strength, and radiation field density—are tuned to reproduce the Fermi data while respecting TeV upper limits.

Additional contributions could arise from accreting neutron stars or white dwarfs, where turbulent magnetospheric regions accelerate particles, and from hypothetical IMBHs that might generate jets or outflows. However, current observations provide no definitive evidence for these mechanisms.

In summary, the paper synthesizes observational results and theoretical frameworks to argue that the GeV γ‑ray emission from globular clusters is dominated by the collective magnetospheric output of MSPs, modulated by the cluster’s intense stellar radiation field. TeV upper limits already place meaningful constraints on the efficiency of particle acceleration and diffusion within the clusters. Continued multi‑wavelength observations, especially with next‑generation TeV instruments, will be crucial to disentangle the relative roles of MSP magnetospheres, wind‑wind shocks, and any exotic sources such as IMBHs in shaping the high‑energy phenomenology of globular clusters.