GX 339-4 : EXOSAT observations in the off and soft states.

Abstract

Exosat observations of the highly variable low-mass X-ray binary GX 339-4, obtained with the low-energy telescope and channel-multiplier array in the off state on Apr. 25, 1985, and with the medium-energy Ar proportional counters and collimator array in the off state in Apr. 29, 1985, and in the soft state on May 7, 1984, are reported. The data are presented in tables and graphs and compared with those obtained by Ariel 6 (Motch et al., 1983 and Ricketts, 1983) in the hard state in March 1981. The off state is characterized by 1.5-microJy emission with a hard power-law spectrum (similar in slope to that of the hard state) and intrinsic 0.1-20-keV luminosity a factor of 140 less than that of the 83-microJy soft state, which has a spectrum with a very soft Comptonized component and a power-law tail. An upper limit of 0.8 percent on the X-ray variability from 1 s to 6 h and a lower limit of 2 kpc on the distance to the object are obtained.