Pulsars in Globular Clusters With the SKAO

TL;DR

Globular clusters host dense, dynamically active environments that produce large MSP populations and exotic pulsar systems, offering unique tests of gravity, dense-matter physics, and cluster dynamics. The paper assesses how SKAO instruments (SKA-MID/LOW) can perform targeted, high-sensitivity GC pulsar surveys, detailing strategies for binary searches, propagation considerations, new analysis techniques, and archival data usage. It predicts substantial gains in discoveries (potentially hundreds to thousands of GC pulsars) and outlines the scientific payoff, from neutron-star mass measurements and IMBH probes to GC evolution and gravitational-wave progenitors, while stressing early SKAO science and cross-band synergies. A key practical point is a robust archival-data program to facilitate long-term timing and orbital analyses, maximizing the return of SKAO observations across GC populations.

Abstract

Because of their extreme stellar densities, globular clusters are highly efficient factories of X-ray binaries and radio pulsars: per unit of stellar mass, they contain about 1000 times more of these exotic objects. Thus far, 345 radio pulsars have been found in globular clusters. These can be used as precision probes of the structure, gas content, magnetic field, and dynamic history of their host clusters; some of them are also highly interesting in their own right because they probe exotic stellar evolution scenarios as well as the physics of dense matter, accretion, and gravity; one of them (PSR~J0514$-$4002E) might even be the first pulsar - black hole system known. Deep searches with SKA-MID and SKA-LOW will only require one to a few tied-array beams, and can be done during early commissioning of the telescope, before an all-sky pulsar survey using hundreds to thousands of tied-array beams is feasible. Even a conservative approach predicts new discoveries only with the core of SKA-MID AA*, and the full AA* and eventually AA4 is expected to increase the number of discoveries even more, leading to more than doubling the current known population. This offers a great opportunity for early SKAO pulsar science, even before all the collecting area is in place. On the other hand, a more optimistic prediction calls for a 4-5 times growth of the population, leading to a total of about 1700 pulsars to be detectable with SKA-MID AA4 configuration in all Galactic GCs visible by SKA telescopes. Thus, a dedicated search for pulsars in globular clusters will fully exploit the best possible natural laboratories to study many branches of physics and astrophysics, including properties of dense matter, stellar evolution, and the dynamical history of the Galactic globular cluster systems.

Figures (7)

• Figure 1: Pulsar population in globular clusters with time.
• Figure 2: Sky distributions of Galactic globular clusters, demonstrating that there are many GCs in the sky observable by the SKA telescopes observable sky (pink hexagons) and many of those GCs still do not have any pulsars discovered in them (pink hexagons without blue stars). GCs in the declination range $+15^{\circ}$ to $-90^{\circ}$ have been considered as visible by the SKA telescopes.
• Figure 3: Cumulative luminosity distribution of all 176 pulsars with published values of flux densities. The blue line is a power-law with an exponent of $\alpha = -1$.
• Figure 4: The upper panel shows the correlation between estimated pulsar content, $N$, and the two-body encounter rate, $\Gamma$. The solid line compares these estimates to the best fit found by hct2010 in which $N = \Gamma^{0.7}$. The lower panel shows the significance of the correlation expressed as the probability density of Pearson's correlation coefficient for the data in the upper panel.
• Figure 5: Simulated GC pulsars detectable in a 2 hr baseline SKA-MID (AA4 configuration) survey shown alongside the underlying model population and the current sample of GC pulsars with measured flux densities. Although a number of faint pulsars will still remain undetectable with such a survey, the enhancement in the size of the population of high flux density pulsars is significant.