Evidence for an intermediate-mass black hole in NGC 6624

Abstract

PSR B1820-30A is located in the globular cluster NGC 6624 and has the smallest projected distance to the centre of any globular cluster in the sky plane. We observe this millisecond pulsar over more than 25 years and obtain higher-order rotational frequency time derivative measurements through high-precision timing. Modelling these higher-order derivatives as being due to orbital motion, we find that the pulsar is in either a low-eccentricity smaller orbit with a low mass companion or a high-eccentricity larger orbit with a massive companion. The cluster mass properties and the observed properties of other nearby sources indicate that the high-eccentricity solution is more probably. This reveals that the pulsar is orbiting around an intermediate-mass black hole (IMBH) of mass >7500 M⊙ located at the cluster centre. This contribution is based on previous work published in MNRAS 471, 1258 (2017).