  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 multi-wavelength 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 usually covered by
optical investigations. The tilted Gamma 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 behaviour 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 the lower flaring branch of
this source.
