Evidence for Doppler-shifted Iron Emission Lines in Black Hole Candidate 4U 1630–47

We report the first detection of a pair of correlated emission lines in the X-ray spectrum of black hole candidate 4U 1630-47 during its 1996 outburst, based on Rossi X-Ray Timing Explorer (RXTE) observations of the source. At the peak plateau of the outburst, the emission lines are detected, centered mostly at ~5.7 and ~7.7 keV, respectively, while the line energies exhibit random variability ~5%. Interestingly, the lines move in a concerted manner to keep their separation roughly constant. The lines also vary greatly in strength, but with the lower energy line always much stronger than the higher energy one. The measured equivalent width ranges from ~50 to ~270 eV for the former, and from insignificant detection to ~140 eV for the latter; the two are reasonably correlated.

The correlation between the lines implies a causal connection; perhaps they share a common origin. Both lines may arise from a single Kα line of highly ionized iron that is Doppler shifted either in a Keplerian accretion disk or in a bipolar outflow or even both. In both scenarios, a change in the line energy might simply reflect a change in the ionization state of line-emitting matter. We discuss the implication of the results and also raise some questions about such interpretations.