Simultaneous X-ray and optical observations of rapid variability in Scorpius X-1<xref ref-type="corresp" rid="cor1">⋆</xref>

Abstract

<it>Copernicus</it> and <it>SAS-3</it> observations of Sco X-l during very active states show strong and rapid X-ray flickering with time scales as short as 20 s. Simultaneous optical photometry reveals similar and remarkably well correlated flickering of lower amplitude. The level of correlation decreases when the X-ray source becomes fainter, but optical and X-ray fluxes, averaged over 1 min, are usually well correlated for <it>B</it> < 12.9. Cross-correlation analysis of the data during periods of high activity shows no really significant time delay between X-ray and optical features, although the cross-correlation function peaks systematically for positive lag values (optical following X-rays). The amplitude of the X-ray flickering exhibits a strong energy-dependence consistent with the well-known intensity versus spectral-hardness relation derived from previous observations. We interpret the highly correlated optical activity as due to reprocessing of X-ray photons in the binary system.