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 spectros copic 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 th e high-soft and low-hard states. This suggests that the line-emission mecha nism is different in the two states. Our interpretation is that double-peak ed lines are emitted from a temperature-inversion layer on the accretion di sc surface when it is irradiatively heated by soft X-rays. Single-peaked li nes may be emitted from outflow/wind matter driven by hard X-ray heating. W e have constructed a simple plane-parallel model and we use it to illustrat e that a temperature-inversion layer can be formed at the disc surface unde r X-ray illumination. We also discuss the conditions required for the forma tion of temperature inversion and line emission. Based on the velocity sepa rations measured for the double-peaked lines in the high-soft state, we pro pose that GX 339-4 is a low-inclination binary system. The orbital inclinat ion is about 15 degrees if the orbital period is 14.8 h.