  We carried out spectroscopic observations of the candidate black-hole binary
GX339-4 during its low-hard and high-soft X-ray states, in 1997 and 1998
respectively. We have found that the spectrum is dominated by emission lines of
neutral elements with asymmetric, round-topped profiles in the low-hard state
(probably due to a dense matter outflow from an inflated outer disk). In the
high-soft state, however, the emission lines from both neutral and ionized
elements have unambiguously resolved double-peaked profiles; this indicates the
presence of an irradiatively-heated thin accretion disk. Higher-ionization
lines have larger peak-to-peak velocity separations. Our data do not show
velocity modulations of the line centres due to the orbital motion of the
compact object; there are no detectable lines from the companion star. These
features are consistent with those of a system with a low-mass companion star
and low orbital inclination. More results (from the 1999 low-hard state) coming
soon in Paper II, stay tuned...
