The Likely Orbital Period of the Ultracompact Low-Mass X-Ray Binary 2S 0918-549

TL;DR

This study identifies the orbital period of the ultracompact low-mass X-ray binary 2S 0918-549 as approximately 17.4 minutes using optical photometry, confirming its classification among ultracompact binaries and providing insights into its donor star.

Contribution

First measurement of the orbital period of 2S 0918-549, establishing it as an ultracompact binary with a period around 17.4 minutes.

Findings

Orbital period of 17.4 minutes identified.

2S 0918-549 confirmed as an ultracompact binary.

Donor likely a white dwarf with specific mass and radius estimates.

Abstract

We report the discovery of the likely orbital period of the ultracompact low-mass X-ray binary (LMXB) 2S 0918-549. Using time-resolved optical photometry carried out with the 8-m Gemini South Telescope, we obtained a 2.4-hr long, Sloan r' light curve of 2S 0918-549 and found a periodic, sinusoidal modulation at 17.4+/-0.1 min with a semiamplitude of 0.015+/-0.002 mag, which we identify as the binary period. In addition to 4U 0513-40 in the globular cluster NGC 1851 and the Galactic disk source 4U 1543-624, 2S 0918-549 is the third member of the ultracompact LMXBs that have orbital periods around 18 min. Our result verifies the suggestion of 2S 0918-549 as an ultracompact binary based on its X-ray and optical spectroscopic properties. Given that the donor in 2S 0918-549 has been suggested to be either a C-O or He white dwarf, its likely mass and radius are around 0.024--0.029 M_sun and 0.03--0.032 R_sun, respectively, for the former case and 0.034--0.039 M_sun and 0.033--0.035 R_sun for the latter case. If the optical modulation arises from X-ray heating of the mass donor, its sinusoidal shape suggests that the binary has a low inclination angle, probably around 10 deg.