Sco X-1 revisited with Kepler, MAXI and HERMES: outflows, time-lags and echoes unveiled

TL;DR

This study revisits Sco X-1 using simultaneous multi-wavelength observations, revealing distinct optical states, variable correlations with X-ray flux, and optical echoes indicative of complex reprocessing mechanisms.

Contribution

It provides new insights into the optical/X-ray variability, lag, and echo phenomena in Sco X-1 through combined Kepler, MAXI, and HERMES data analysis.

Findings

Optical fluxes are bimodal, indicating two distinct states.

A broad ~12.5 hour optical lag is observed between optical and X-ray emissions.

Significant optical echoes are detected, consistent with optical lags.

Abstract

Sco X-1 has been the subject of many multi-wavelength studies in the past, being the brightest persistent extra-solar X-ray source ever observed. Here we revisit Sco X-1 with simultaneous short cadence Kepler optical photometry and MAXI X-ray photometry over a 78 day period, as well as optical spectroscopy obtained with HERMES. We find Sco X-1 to be highly variable in all our datasets. The optical fluxes are clearly bimodal, implying the system can be found in two distinct optical states. These states are generally associated with the known flaring/normal branch X-ray states, although the flux distributions associated with these states overlap. Furthermore, we find that the optical power spectrum of Sco X-1 differs substantially between optical luminosity states. Additionally we find rms-flux relations in both optical states, but only find a linear relation during periods of low optical luminosity. The full optical/X-ray discrete correlation function displays a broad ~12.5 hour optical lag. However during the normal branch phase the X-ray and optical fluxes are anti-correlated, whilst being correlated during the flaring branch. We also performed a Cepstrum analysis on the full Kepler light curve to determine the presence of any echoes within the optical light curve alone. We find significant echo signals, consistent with the optical lags found using the discrete cross-correlation. We speculate that whilst some of the driving X-ray emission is reflected by the disk, some is absorbed and re-processed on the thermal timescale, giving rise to both the observed optical lags and optical echoes.