THE X-RAY-PROPERTIES OF GX-301-2 (4U-1223-62)

The X-ray binary pulsar GX 301-2 (4U 1223 - 62/Wray 977) has been observed with EXOSAT at various orbital phases. As reported in the past, the X-ray luminosity shows a strong maximum near periastron passage of the neutron star (phase 0), but with the peak 0.05 in phase before. The X-ray photoelectric absorption after periastron of around 10(24) H cm-2 decreases after phase 0.25 to a minimum of 1.2 x 10(22) H cm-2 near phase 0.7. A maximum in the absorption with values up to 3 x 10(24) H cm-2 is seen at the same orbital phase as the maximum in X-ray luminosity. During the interval between phase 0.9 and 0.25, when the absorption was high, the X-ray spectra show a strong low-energy excess which can be modeled by X-ray emission scattered around a region of enhanced wind density by the less dense ambient wind. No pulsations in the low-energy excess confirm this. Following Stevens, a model of enhanced mass loss from the primary towards the neutron star forming a dense gas stream is presented, which can reproduce the orbital phase dependence of the X-ray luminosity and absorption. In this model the strong outburst is caused by the neutron star passing through the dense gas stream before periastron. A second pass through the ps stream near phase 0.3 causes another broader increase in X-ray luminosity and leads to two minima in the light curve of GX 301-2 near phase 0.15 and 0.7. On top of the smooth phase dependence, X-ray luminosity and absorption show strong variations on time scales of hours especially during X-ray outburst.