A 2 hour periodic variation in the low mass X-ray binary Ser X-1

TL;DR

This study reports a ~2 hour periodic variability in the low mass X-ray binary Ser X-1, likely due to orbital motion, providing new insights into its orbital period and system configuration.

Contribution

First indication of Ser X-1's orbital period based on spectroscopic variability, with a proposed system model involving a main sequence K-dwarf and a neutron star.

Findings

Detected ~2 hr periodic variability in emission lines.

Narrow emission lines suggest low inclination (<10°).

System likely consists of a K-dwarf and a neutron star.

Abstract

Spectroscopy of the low mass X-ray binary Ser X-1 using the Gran Telescopio Canarias have revealed a ~2 hr periodic variability that is present in the three strongest emission lines. We tentatively interpret this variability as due to orbital motion, making it the first indication of the orbital period of Ser X-1. Together with the fact that the emission lines are remarkably narrow, but still resolved, we show that a main sequence K-dwarf together with a canonical 1.4Msun neutron star gives a good description of the system. In this scenario the most likely place for the emission lines to arise is the accretion disk, instead of a localized region in the binary (such as the irradiated surface or the stream-impact point), and their narrowness is due instead to the low inclination (<10 degrees) of Ser X-1.