Pasi Hakala, Gavin Ramsay, Thomas Barclay, Phil Charles

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

Figure 1. The K2 optical (top panel) and MAXI X-ray (lower panel) light curves of Sco X-1. The data have been normalized to unity and the mean has been subtracted. The error on the optical photometry is negligible and the error on the X-ray points are typically 1 percent.

Abstract

Sco X-1 is the archetypal low mass X-ray binary (LMXB) and the brightest per- sistent 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 observa- tions of other accreting compact binaries. We reconfirm that Sco X-1 shows a bimodal distribution of optical òÀÝhighòÀÝ and òÀÝlowòÀÝ 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 òÀÝhighòÀÝ and òÀÝlowòÀÝ optical states respectively.

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Last Revised: 2015 July 7th