New constraints on the central mass contents of Omega Centauri from combined stellar kinematics and pulsar timing

¹ Instituto de Astrofísica de Canarias, La Laguna, Tenerife 38200, Spain
² Departamento de Astrofísica, Universidad de La Laguna, Santa Cruz de Tenerife, Spain
³ CNRS, Laboratoire d’Annecy-le-Vieux de Physique Théorique, 74940 Annecy, France
⁴ CERN, Theoretical Physics Department, 1211 Geneva 23, Switzerland
⁵ Department of Physics, University of Surrey, Guildford, GU2 7XH, UK

^★ Corresponding author; a.banareshernandez@gmail.com

Received: 2 August 2024
Accepted: 26 November 2024

Abstract

Aims. We performed a combined analysis of stellar kinematics with line-of-sight accelerations of millisecond pulsars (MSPs) to probe the mass content of Omega Centauri (ω Cen). Our mass model includes the stellar mass distribution, a more concentrated mass component linked to the observed MSP population, a generic cluster of stellar remnants (assumed to be more concentrated than the stars and MSPs), and an intermediate-mass black hole (IMBH), allowing us to determine which of these is statistically preferred to account for these observations.

Methods. We mass-modeled ω Cen using the package GravSphere to solve the Jeans equations, including constraints in the form of proper motions, line-of-sight velocities, the surface density profile of the stars, the spatial distribution of MSPs, and the recently measured line-of-sight accelerations of a subset of these MSPs, self-consistently modeling their intrinsic spin-down. We explore the impact of different assumed centers of ω Cen on our results and we infer the posterior distributions of the model parameters from the combined likelihood using the nested sampling package dynesty.

Results. Our analysis favors an extended central mass of ~2−3 × 10⁵ M_⊙ over an IMBH, setting a 3σ upper limit on the IMBH mass of 6 × 10³ M_⊙. We find that pulsar timing observations are an important additional constraint, favoring a central mass distribution that is ~20% more massive and extended than implied by models that are constrained by the stellar kinematics alone. Finally, we find a 3σ confidence level (CL) upper bound of 6 × 10⁴ M_⊙ on the total mass traced by the MSPs, with the density profile following ρ_p(r) ∝ ρ_⋆(r)^γ/σ(r), with γ = 1.9 ± 0.3, where ρ_⋆(r) is the stellar mass density and σ(r) is the stellar velocity dispersion profile. This favors models in which MSPs form via stellar encounters, as in the leading paradigm whereby MSPs are the progeny of low-mass X-ray binaries.

Conclusions. Our analysis demonstrates how combining stellar kinematics with MSP accelerations produces new constraints on mass models, shedding light on the presence or absence of IMBHs at the centers of globular clusters. Further, we provide the first validation of its kind where MSP positions are linked to their place of formation in globular clusters, which is in excellent agreement with the expectations of stellar encounter models of MSP formation. This sets a promising precedent amid the rapid growth in the number of observations and discoveries currently taking place in this field.