Discovery of A 693.5 s Period in the X-ray Binary 4U 1820-30: A Superhump Interpretation

TL;DR

This paper reports the discovery of a 693.5 s period in the X-ray binary 4U 1820-30, interpreted as a superhump indicating an eccentric accretion disk, which suggests a specific mass ratio and system dynamics.

Contribution

The study presents the first detection of a superhump period in 4U 1820-30, proposing a new interpretation of the system's accretion disk behavior and mass ratio.

Findings

Detected a 693.5 s FUV period differing from the orbital period.

Interpreted the period as a superhump caused by an eccentric accretion disk.

Estimated a superhump excess of about 1% and a mass ratio around 0.06.

Abstract

The X-ray source 4U1820-30 in the globular cluster NGC 6624 is known as the most compact binary among the identified X-ray binaries. Having an orbital period of 685.0 s, the source consists of a neutron star primary and likely 0.06--0.08 Msun white dwarf secondary. Here we report on far-ultraviolet (FUV) observations of this X-ray binary, made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. From our Fourier spectral analysis of the FUV timing data, we obtain a period of 693.5+/-1.3 s, which is significantly different from the orbital period. The light curve folded at this period can be described by a sinusoid, with a fractional semiamplitude of 6.3% and the phase zero (maximum of the sinusoid) at MJD 50886.015384+/-0.000043 (TDB). While the discovered FUV period may be consistent with a hierarchical triple system model that was previously considered for 4U 1820-30, we suggest that it could instead be the indication of superhump modulation, which arises from an eccentric accretion disk in the binary. The X-ray and FUV periods would be the orbital and superhump periods, respectively, indicating a 1% superhump excess and a white-dwarf/neutron-star mass ratio around 0.06. Considering 4U 1820-30 as a superhump source, we discuss the implications.