Mass models of NGC 6624 without an intermediate-mass black hole

TL;DR

This study constructs detailed dynamical models of NGC 6624 without an intermediate-mass black hole, successfully explaining observational data and suggesting that stellar remnants account for the pulsar's timing behavior.

Contribution

The paper presents the first comprehensive dynamical models of NGC 6624 that do not require an intermediate-mass black hole to match observations.

Findings

Models fit surface brightness and proper motion data without IMBH.

Higher-order pulsar period derivatives are caused by passing stars, not an IMBH.

No evidence for an IMBH in NGC 6624 from dynamical modeling.

Abstract

An intermediate-mass black hole (IMBH) was recently reported to reside in the centre of the Galactic globular cluster (GC) NGC 6624, based on timing observations of a millisecond pulsar (MSP) located near the cluster centre in projection. We present dynamical models with multiple mass components of NGC 6624 - without an IMBH - which successfully describe the surface brightness profile and proper motion kinematics from the Hubble Space Telescope (HST) and the stellar mass function at different distances from the cluster centre. The maximum line-of-sight acceleration at the position of the MSP accommodates the inferred acceleration of the MSP, as derived from its first period derivative. With discrete realizations of the models we show that the higher-order period derivatives - which were previously used to derive the IMBH mass - are due to passing stars and stellar remnants, as previously shown analytically in literature. We conclude that there is no need for an IMBH to explain the timing observations of this MSP.