Echoes from the companion star in Sco X-1

TL;DR

This study uses simultaneous X-ray and optical observations of Sco X-1 to detect echoes from the companion star and accretion disc, revealing reprocessing timescales and orbital parameters.

Contribution

First simultaneous high-resolution X-ray and optical observations of Sco X-1 to detect and analyze reprocessing echoes from the companion star and accretion disc.

Findings

Echo from the companion star detected with 11-16s lag

Echoes at the bottom of the Flaring Branch show 5-10s lag

Results inform orbital parameters of Sco X-1

Abstract

We present simultaneous X-ray (RXTE) and optical (ULTRACAM) narrow band (Bowen blend/HeII and nearby continuum) observations of Sco X-1 at 2-10 Hz time resolution. We find that the Bowen/HeII emission lags the X-ray light-curves with a light travel time of ~11-16s which is consistent with reprocessing in the companion star. The echo from the donor is detected at orbital phase ~0.5 when Sco X-1 is at the top of the Flaring Branch. Evidence of echoes is also seen at the bottom of the Flaring Branch but with time-lags of 5-10s which are consistent with reprocessing in an accretion disc with a radial temperature profile. We discuss the implication of our results for the orbital parameters of Sco X-1.