EXOSAT observations of broad iron K line emission from Scorpius X-1

Using the Exosat Gas Scintillation Proportional Counter a broad iron K emission line is found to be present in the X-ray spectrum of Sco X-1. The line full width half-maximum (FWHM) is 0.7 keV, independent of the source intensity state. During flaring intervals the line intensity remains constant such that the equivalent width, decreases from 50 eV to 25 eV across a flare. During quiescent intervals lasting several hours or more the line flux can decrease by up to a factor of 2. The line centroid is centered on 6.75 keV. The observed line width suggests Compton broadening by a hot scattering cloud with an optical depth of a few. The invariance of the line FWHM during flares rules out the previously suggested self-Comptonized thermal bremsstrahlung model. The line more likely originates in an optically thick accretion disk corona that lies at 100 to 1000 neutron star radii. The 2-25 keV continuum can be modeled as a blackbody plus unsaturated Comptonized spectrum. The flaring is caused by increases in the luminosity of the blackbody component from 10 percent to 40 percent of the total. The two-component continuum model is interpreted in terms of disk accretion onto a neutron star with a weak magnetic field and the inner accretion disk terminated at the neutron star surface by a boundary layer. 45 references.