Evidence for Doppler-Shifted Iron Emission Lines in Black Hole Candidate 4U 1630-47
Abstract
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 RXTE observations of the source. At the peak plateau of the outburst, the emission lines are detected, centered mostly at
∼
5.7 keV 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 eV 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
K
α
line of highly ionized iron that is Doppler-shifted either in a Keplerian accretion disk or in a bi-polar 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.