Evolution of Iron K Alpha Line Emission in the Black Hole Candidate GX 339-4

GX 339-4 was regularly monitored with the Rossi X-Ray Timing Explorer during a period (in 1999) when its X-ray flux decreased significantly (from 4.2 × 10-10 to 7.6 × 10-12 ergs 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 Kα line in the observed spectrum of the source for all observations. The line shows an interesting evolution: it is centered at ~6.4 keV when the measured flux is above 5 × 10-11 ergs cm-2 s-1 but is shifted to ~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 a 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.