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 ofthe optical emission from Sco X-1. They found that the Sco X-1 B-magnitudehistograms changed from year to year. Subsequent multi-wavelength campaignsconfirmed the variable nature of these optical histograms and also found thatthe X-ray and optical emissions were only correlated when Sco X-1 was brighterthan about B = 12.6. Models had suggested that the optical emission from thissource arose from X-rays reprocessed in an accretion disk surrounding thecentral neutron star. It was therefore difficult to explain why the optical andX-ray fluxes were not more closely correlated. In 1994 and 1995, two newsimultaneous optical and X-ray campaigns on Sco X-1 were conducted with theBurst and Transient Source Experiment on the Compton Gamma Ray Observatory andthe 1 m Yale telescope at Cerro Tololo Inter-American Observatory. Using thesedata and models by Psaltis, Lamb & Miller, it is now possible to provide aqualitative picture of how the X-ray and optical emissions from Sco X-1 arerelated. Differences in the B-magnitude histograms are caused by variations inthe mass accretion rate and the relatively short time period usually covered byoptical investigations. The tilted Gamma pattern seen in plots of thesimultaneous X-ray and optical emission from Sco X-1 arises from (1) the nearlylinear relation between the optical B magnitude and the mass accretion rate inthe range 13.3 > B > 12.3 and an asymptotic behaviour in the B magnitudeoutside this range, and (2) a double-valued relation between the X-ray emissionand mass accretion rate along the normal branch and the lower flaring branch ofthis source.Comment: Accepted to the Astronomical Journal: 7 pages, 3 figure