The Behavior of the Optical and X-Ray Emission from Scorpius X-1

In 1970, Hiltner & Mook reported the results of the first multiyear study of the optical emission from Sco X-1. They found that the Sco X-1 B-magnitude histograms changed from year to year. Subsequent multiwavelength campaigns confirmed the variable nature of these optical histograms and also found that the X-ray and optical emissions were only correlated when Sco X-1 was brighter than about B=12.6. Models had suggested that the optical emission from this source arose from X-rays reprocessed in an accretion disk surrounding the central neutron star. It was therefore difficult to explain why the optical and X-ray fluxes were not more closely correlated. In 1994 and 1995, two new simultaneous optical and X-ray campaigns on Sco X-1 were conducted with the Burst and Transient Source Experiment on the Compton Gamma Ray Observatory and the 1 m Yale telescope at Cerro Tololo Inter-American Observatory. Using these data and models by Psaltis, Lamb, & Miller, it is now possible to provide a qualitative picture of how the X-ray and optical emissions from Sco X-1 are related. Differences in the B-magnitude histograms are caused by variations in the mass accretion rate and the relatively short time period typically covered by optical investigations. The tilted-Γ pattern seen in plots of the simultaneous X-ray and optical emission from Sco X-1 arises from (1) the nearly linear relation between the optical B magnitude and the mass accretion rate in the range 13.3>=B>=12.3 and an asymptotic behavior in the B magnitude outside this range, and (2) a double-valued relation between the X-ray emission and mass accretion rate along the normal branch and lower flaring branch of this source.