Evolution of Iron Kα Line Emission in the Black Hole Candidate GX 339−4

GX 339-4 was regularly monitored with RXTE during a period (in 1999) when itsX-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 revealedthe presence of a prominent iron K$_{\alpha}$ line in the observed spectrum ofthe source for all observations. The line shows an interesting evolution: it iscentered 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. Theequivalent width of the line appears to increase significantly toward lowerfluxes, although it is likely to be sensitive to calibration uncertainties.While the fluorescent emission of neutral or mildly ionized iron atoms in theaccretion disk can perhaps account for the 6.4 keV line, as is often invokedfor black hole candidates, it seems difficult to understand the 6.7 keV linewith 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 linemight be due to recombination cascade of hydrogen or helium like iron ions inan optically thin, highly ionized plasma. We discuss the results in the contextof proposed accretion models.Comment: 18 pages, 2 figures, accepted for publication in the ApJ in v552n2p May 10, 2001 issu