The Orbital Period of Scorpius X-1

TL;DR

This study confirms the historically established orbital period of Sco X-1 using nearly a century of optical data, refuting recent claims of a different period based on RXTE/ASM data.

Contribution

The paper validates the original orbital period of Sco X-1 with modern optical data, clarifying previous ambiguities and providing an updated time of minimum light.

Findings

Confirmed the orbital period of Sco X-1 as per Gottlieb et al. (1975).

Refuted the one-year alias suggested by RXTE/ASM data.

Provided a new modern time of minimum light.

Abstract

The orbital period of Sco X-1 was first identified by Gottlieb et al. (1975). While this has been confirmed on multiple occasions, this work, based on nearly a century of photographic data, has remained the reference in defining the system ephemeris ever since. It was, however, called into question when Vanderlinde et al. (2003) claimed to find the one-year alias of the historical period in RXTE/ASM data and suggested that this was the true period rather than that of Gottlieb et al. (1975). We examine data from the All Sky Automated Survey (ASAS) spanning 2001-2009. We confirm that the period of Gottlieb et al. (1975) is in fact the correct one, at least in the optical, with the one-year alias strongly rejected by these data. We also provide a modern time of minimum light based on the ASAS data.