GX 339-4 was regularly monitored with RXTE during a period ( in 1999 ) when its X-ray flux decreased significantly ( from 4.2 \times 10 ^ { -10 } erg cm ^ { -2 } s ^ { -1 } to 7.6 \times 10 ^ { -12 } erg cm ^ { -2 } s ^ { -1 } in the 3–20 keV band ) , as the source settled into the “ off state ” .
Our spectral analysis revealed the presence of a prominent iron K _ { \alpha } line in the observed spectrum of the source for all observations .
The line shows an interesting evolution : it is centered at \sim 6.4 keV when the measured flux is above 5 \times 10 ^ { -11 } erg cm ^ { -2 } s ^ { -1 } , but is shifted to \sim 6.7 keV at lower fluxes .
The equivalent width of the line appears to increase significantly toward lower fluxes , although it is likely to be sensitive to calibration uncertainties .
While the fluorescent emission of neutral or mildly ionized iron atoms in the accretion disk can perhaps account for the 6.4 keV line , as is often invoked for black hole candidates , it seems difficult to understand the 6.7 keV line with this mechanism , because the disk should be less ionized at lower fluxes ( unless its density changes drastically ) .
On the other hand , the 6.7 keV line might be due to recombination cascade of hydrogen or helium like iron ions in an optically thin , highly ionized plasma .
We discuss the results in the context of proposed accretion models .