K2 and MAXI observations of Sco X-1 - Evidence for disc precession?

TL;DR

This study uses K2 and MAXI observations to analyze Sco X-1, revealing rapid state transitions, a potential disc precession period of 4.8 days, and complex optical-X-ray correlations, demonstrating K2's utility for accreting binary systems.

Contribution

First continuous 71-day optical monitoring of Sco X-1 with K2, identifying a possible disc precession period and detailed optical-X-ray correlation behavior.

Findings

Bimodal optical high/low states with rapid transitions under 3 hours

Evidence for a 4.8-day systemic timescale possibly indicating disc precession

Complex optical and X-ray correlation and anticorrelation patterns

Abstract

Sco X-1 is the archetypal low mass X-ray binary (LMXB) and the brightest persistent extra-solar X-ray source in the sky. It was included in the K2 Campaign 2 field and was observed continuously for 71 days with 1 minute time resolution. In this paper we report these results and underline the potential of K2 for similar observations of other accreting compact binaries. We reconfirm that Sco X-1 shows a bimodal distribution of optical "high" and "low" states and rapid transitions between them on timescales less than 3 hours (or 0.15 orbits). We also find evidence that this behaviour has a typical systemic timescale of 4.8 days, which we interpret as a possible disc precession period in the system. Finally, we confirm the complex optical vs. X-ray correlation/anticorrelation behaviour for "high" and "low" optical states respectively.