Iron-line emission as a probe of Bardeen-Petterson accretion disks

In this work we show that Bardeen-Petterson accretion disks can exhibit unique, detectable features in relativistically broadened emission line profiles. Some of the unique characteristics include inverted line profiles with sharper red horns and softer blue horns and even profiles with more than two horns from a single rest-frame line. We demonstrate these points by constructing a series of synthetic line profiles using simple two-component disk models. We find that the resulting profiles are very sensitive to the two key parameters one would like to constrain, namely, the Bardeen-Petterson transition radius r(BP) and the relative tilt beta between the two disk components over a range of likely values [10 <= r(BP)/(GM/c(2)) <= 40, 15 degrees <= beta <= 45 degrees]. We use our findings to show that some of the "extra'' line features observed in the spectrum of the Seyfert 1 galaxy MCG-6-30-15 may be attributable to a Bardeen-Petterson disk structure. Similarly, we apply our findings to two likely Bardeen-Petterson candidate Galactic black holes, GRO J1655-40 and XTE J1550-564. We provide synthetic line profiles of these systems using observationally constrained sets of parameters. Although we do not formally fit the data for any of these systems, we confirm that our synthetic spectra are consistent with current observations.