Discovery of a 693.5 S Period in the X-ray Binary 4u 1820−30: a Superhump Interpretation

The X-ray source 4U 1820-30 in the globular cluster NGC 6624 is known as the most compact binary among the identified X-ray binaries (XRBs). Having an orbital period of 685.0 s, the source consists of a neutron star (NS) primary and likely a 0.06-0.08 M [subscript ☉] white dwarf (WD) secondary. Here, we report on far-ultraviolet (FUV) observations of this XRB, 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 WD/NS mass ratio around 0.06. Considering 4U 1820-30 as a superhump source, we discuss the implications