A GINGA observation of the supergiant X-ray binary system 4U 1700-37/HD 153919.

A Ginga observation of the massive X-ray binary system 4U 1700-37/HD 153919 in April 1990 revealed a strong soft component in the X-ray spectra consistent with thermal bremsstrahlung of temperature 0.5 keV, with an average luminosity of 8 x 10 exp 34 erg/s. The soft component might be explained by emission from a region of X-ray-induced shocks trailing the X-ray source. The soft component is also visible in eclipse spectra from an Exosat observation in 1985, but with a factor of about five lower luminosity compared with the Ginga spectra around eclipse ingress. The increase in luminosity of the soft component might be correlated with the increase in overall absorption and luminosity of the direct spectral component by a factor of about two observed since 1985, indicating an increase in the stellar wind density caused by a factor of about two higher mass loss rate of HD 153919.