Thermal iron Kα line emission from the X-ray binary GX 339-4

The accretion rate of the black hole candidate X-ray binary GX 339-4 in the 'off' state is low, and an advection dominated accretion flow (ADAF) is present. Hydrogen-like and helium-like iron Kalpha emission lines at 6.7 and 6.95 keV from hot plasma of ADAF can be produced by recombination-cascade processes with moderately high intensities, which are markedly distinguished from the fluorescent iron Kalpha line at similar to6.4 keV. We show that the observational features of GX 339-4 can be explained by the ADAF model, if the iron abundance is more than 10 times the solar value, though the reason for such a high abundance is still unclear. We suggest that the increase of the accretion rate makes GX 339-4 change from off, low, intermediate, to high and very high states, and the line center of iron Kalpha will therefore shift from similar to6.83 to similar to6.4 keV, i.e. to the fluorescent disc-line, since the disappearance of the ADAF due to its high accretion rate. (C) 2003 Elsevier B.V. All rights reserved.