Optical spectroscopy of GX 339-4 during the high-soft and low-hard states - II. Line ionization and emission region

We have carried out observations of the X-ray transient GX 339-4 during its high-soft and low-hard X-ray spectral states. Our high-resolution spectroscopic observation in 1999 April suggests that the H alpha line has a single-peaked profile in the low-hard state as speculated in our previous paper. The He II lambda 4686 line, however, has a double-peaked profile in both the high-soft and low-hard states. This suggests that the line-emission mechanism is different in the two states. Our interpretation is that double-peaked lines are emitted from a temperature-inversion layer on the accretion disc surface when it is irradiatively heated by soft X-rays. Single-peaked lines may be emitted from outflow/wind matter driven by hard X-ray heating. We have constructed a simple plane-parallel model and we use it to illustrate that a temperature-inversion layer can be formed at the disc surface under X-ray illumination. We also discuss the conditions required for the formation of temperature inversion and line emission. Based on the velocity separations measured for the double-peaked lines in the high-soft state, we propose that GX 339-4 is a low-inclination binary system. The orbital inclination is about 15 degrees if the orbital period is 14.8 h.