Evidence for an intermediate-mass black hole in the globular cluster NGC 6624

Abstract

PSR B1820$-$30A is located in the globular cluster NGC 6624 and is the closest known pulsar to the centre of any globular cluster. We present more than 25 years of high-precision timing observations of this millisecond pulsar and obtain four rotational frequency time derivative measurements. Modelling these higher-order derivatives as being due to orbital motion, we find solutions which indicate that the pulsar is in either a low-eccentricity ($0.33\lesssim e\lesssim 0.4$) smaller orbit with a low mass companion (such as a main sequence star, white dwarf, neutron star, or stellar mass black hole) or a high-eccentricity ($e\gtrsim 0.9$) larger orbit with a massive companion. The cluster mass properties and the observed properties of 4U 1820$-$30 and the other pulsars in the cluster argue against the low-eccentricity possibility. The high-eccentricity solution reveals that the pulsar is most likely orbiting around an intermediate-mass black hole (IMBH) of mass $>7,500$~M${}_{\odot }$ located at the cluster centre. A gravitational model for the globular cluster, which includes such a central black hole (BH), predicts an acceleration that is commensurate with that measured for the pulsar. It further predicts that the model-dependent minimum mass of the IMBH is $\sim 60,000$~M${}_{\odot }$. Accounting for the associated contribution to the observed period derivative indicates that the $\gamma$-ray efficiency of the pulsar should be between 0.08 and 0.2. Our results suggest that other globular clusters may also contain central black holes and they may be revealed by the study of new pulsars found sufficiently close to their centres. Note that we found an erratum in Section 5 and thus, the $\sim$60,000~M${}_{\odot }$ mass mentioned above has to be replaced by the correct model-dependent mass limit of $\sim$20,000~M${}_{\odot }$. See the erratum appended.

15 pages, 10 figures, Accepted by MNRAS on 23 February 2017. Erratum was accepted by MNRAS on 17 May 2017

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Cited by in corpus (6)

• Mass models of NGC 6624 without an intermediate-mass black hole
• Pulsar Timing Perturbations from Galactic Gravitational Wave Bursts with Memory
• Spin dynamics of a millisecond pulsar orbiting closely around a massive black hole
• Intermediate mass black holes in globular clusters: effects on jerks and jounces of millisecond pulsars
• Mergers of globular clusters in the Galactic disc: intermediate mass black hole coalescence and implications for gravitational waves detection
• The dynamics of Galactic centre pulsars: constraining pulsar distances and intrinsic spin-down