The Puzzling Negative Orbit-Period Derivative of the Low-Mass X-Ray Binary 4U 1820-30 in NGC 6624

TL;DR

This study precisely measures the orbital period of the low-mass X-ray binary 4U 1820-30, finding an unexpected negative period derivative that suggests the presence of dark matter or an intermediate-mass black hole in its globular cluster.

Contribution

First precise measurement of 4U 1820-30's orbital period derivative revealing a negative value, challenging existing theoretical models for such systems.

Findings

Orbital period P = 685.01197 s with high precision.

Period derivative dP/dt /P = -5.3 x 10^-8 yr^-1, significantly negative.

Possible presence of dark remnants or an IMBH in NGC 6624.

Abstract

4U 1820-30 is a low-mass X-ray binary near the center of the globular cluster NGC 6624 consisting of, at least, one neutron star and one helium white dwarf. Analyzing 16 years of data from the Rossi X-ray Timing Explorer (RXTE) allows us to measure its orbital period and its time derivative with unprecedented accuracy to be P = 685.01197 +- 0.00003 s and dP/dt /P = -5.3 +- 0.3x10^-8 yr^-1. Hence, we confirm that the period derivative is significantly negative at the >17 sigma level, contrary to theoretical expectations for an isolated X-ray binary. We discuss possible scenarios that could explain this discrepancy, and conclude that the center of NGC 6624 most likely contains large amounts of non-luminous matter such as dark remnants. We also discuss the possibility of an IMBH inside NGC 6624, or that a dark remnant close to 4U 1820-30 causes the observed shift.