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

Scaringi, S., Maccarone, T.J., Hynes, R.I., Körding, E., Ponti, G., Knigge, C., Britt, C.T. and van Winckel, H. (2015) Sco X-1 revisited with Kepler, MAXI and HERMES: outflows, time-lags and echoes unveiled. Monthly Notices of the Royal Astronomical Society, 451 (4), 3857-3867. (doi:10.1093/mnras/stv1216).

Abstract

Sco X-1 has been the subject of many multiwavelength studies in the past, being the brightest persistent extrasolar X-ray source ever observed. Here, we revisit Sco X-1 with simultaneous short cadence Kepler optical photometry and Monitor of All-sky X-ray Image X-ray photometry over a 78 d period, as well as optical spectroscopy obtained with High Efficiency and Resolution Mercator Echelle Spectrograph (HERMES). We find Sco X-1 to be highly variable in all our data sets. 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 h optical lag. However, during the normal branch phase, the X-ray and optical fluxes are anticorrelated, 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 disc, some is absorbed and re-processed on the thermal time-scale, giving rise to both the observed optical lags and optical echoes.

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