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 durin g a period (in 1999) when its X-ray flux decreased significantly (from 4.2 x 10(-10) to 7.6 x 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 pre sence of a prominent iron Kcl line in the observed spectrum of the source f or all observations. The line shows an interesting evolution: it is centere d at similar to6.4 keV when the measured flux is above 5 x 10(-11) ergs cm( -2) s(-1) but is shifted to similar to6.7 keV at lower fluxes. The equivale nt 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 acc retion disk can perhaps account for the 6.4 keV Line, as is often invoked f or 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 fluxe s (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 ir on ions in an optically thin, highly ionized plasma. We discuss the results in the context of proposed accretion models.