Spectral variability of scorpius X-1, as observed with EXOSAT

Abstract

EXOSAT measurements of Sco X-1 have been carried out with the 500 l/mm and 1000 l/mm grating spectrometers and simultaneously with the gas scintillation proportional counter (GSPC) on two occasions in August 1983. The first observing period, part of the instrument performance verification phase, on August 7 and 8, 1983, lasted for about twenty two hours and showed the source flaring during the first six hours. During the second observing period, August 29 and 30, 1983, the source was again active during part of the time. The lightcurves of the spectrometers and the GSPC show some correlation during the flaring activity. The grating spectra show clear emission features between 12 and 20 Å, which vary with time. When fitting the observed spectra to a thermal bremsstrahlungs model (exponential with gaunt factor) together with absorption by neutral matter, a reduced abundance of oxygen in the absorbing matter with respect to Solar is required.