Revisiting the Evidence for an Intermediate-mass Black Hole in the Center of NGC 6624 with Simulations

TL;DR

This study reevaluates evidence for an intermediate-mass black hole in NGC 6624, finding that gravitational wave emission explains observed pulsar acceleration, and simulations suggest a possible IMBH of around 950 solar masses.

Contribution

The paper demonstrates that the previously attributed acceleration is due to gravitational waves, and uses simulations to estimate the IMBH mass in NGC 6624.

Findings

Gravitational wave emission accounts for the pulsar acceleration.

Simulations suggest an IMBH with mass around 950 solar masses.

Nearby star perturbations likely cause the second period derivative.

Abstract

The acceleration of LMXB 4U 1820-30 that derived from its orbital-period derivative $\dot P_{\rm b}$ was supposed to be the evidence for an Intermediate-mass Black Hole (IMBH) in the Galactic globular cluster (GC) NGC 6624. However, we find that the anomalous $\dot P_{\rm b}$ is mainly due to the gravitational wave emission, rather than the acceleration in cluster potential. Using the standard structure models of GCs, we simulate acceleration distributions for pulsars in the central region of the cluster. By fitting the acceleration of J1823-3021A with the simulated distribution profiles (maximum values), it is suggested that an IMBH with mass $M\gtrsim 950^{+550}_{-350}~M_{\odot}$ may reside in the cluster center. We further show that the second period derivative $\ddot P$ of J1823-3021A is probably due to the gravitational perturbation of a nearby star.