An ultracompact X-ray binary in the globular cluster NGC 1851

TL;DR

This paper reports the discovery of an ultracompact X-ray binary in a globular cluster, characterized by a 17-minute orbital period, confirmed through ultraviolet photometry, and highlights its significance as a gravitational wave source.

Contribution

First detection of an ultracompact X-ray binary in NGC 1851 with a confirmed 17-minute orbital period using Hubble ultraviolet data.

Findings

Detected a 17-minute periodic signal in UV data.

Confirmed the system as an ultracompact X-ray binary.

Identified potential as a gravitational wave source.

Abstract

We present far-ultraviolet photometry obtained with the Hubble Space Telescope of the low-mass X-ray binary 4U 0513-40 in the globular cluster NGC 1851. Our observations reveal a clear, roughly sinusoidal periodic signal with $P \simeq 17$ min and amplitude 3%-10%. The signal appears fully coherent and can be modelled as a simple reprocessing effect associated with the changing projected area presented by the irradiated face of a white dwarf donor star in the system. All of these properties suggest that the signal we have detected is orbital in nature, thus confirming 4U 0513-40 as an ultracompact X-ray binary (UCXB). All four confirmed UCXBs in globular clusters have orbital periods below 30 minutes, whereas almost all UCXBs in the Galactic field have orbital periods longer than this. This suggests that the dynamical formation processes dominate UCXB production in clusters, producing a different orbital period distribution than observed among field UCXBs. Based on the likely system parameters, we show that 4U 0513-40 should be a strong gravitational wave source and may be detectable by LISA over the course of a multi-year mission.