EXOSAT observations of X 1822 – 371: modelling of the accretion disc rim

We present X-ray data from three observations of X 1822 – 371 made using EXOSAT. The quasi-sinusoidal modulation dominating the X-ray light curve is not accompanied by spectral changes. We do, however, find a softening of the spectrum during eclipse and a hardening of the spectrum just prior to eclipse. The spectral change during eclipse indicates that the effective temperature of the ADC decreases with height. A similar energy dependence in the sinusoidal modulation may be masked by photo-electric absorption in the disc. We model the binary system with an X-ray emitting ADC partially obscured by an optically emitting disc whose height varies with azimuthal angle. Using disc structure parameterized by as few as two straight lines we are able to reproduce simultaneously both the X-ray and optical light curves of X 1822 – 371 using a system inclination of |$85^\circ\pm3^\circ$|⁠. Our results determine the absolute sizes of the X-ray and optical emitting regions as well as fixing the position of the X-ray obscuring material within the disc. We are able to exclude the possibility that this material lies at a similar radius to that of the X-ray emitting ADC and place it at the outer edge of the accretion disc. An analysis of X-ray and optical eclipse timings suggests that the 5.57-hr period of X 1822 – 371 is increasing on a time-scale of 1.8×106 yr.