Work

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

Year: 2024

Type: article

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 ρp(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.

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202451763

Authors: Andrés Bañares-Hernández, Francesca Calore, Jorge Martin Camalich, Justin I. Read