X-Ray Structure between the Innermost Disk and Optical Broad-line Region in NGC 4151

We present an analysis of the narrow Fe K alpha line in Chandra/HETGS observations of the Seyfert active galactic nucleus (AGN) NGC 4151. The sensitivity and resolution afforded by the gratings reveal asymmetry in this line. Models including weak Doppler boosting, gravitational redshifts, and scattering are generally preferred over Gaussians at the 5 sigma level of confidence, and generally measure radii consistent with R similar or equal to 500-1000 GM(BH)/c(2). Separate fits to "high/unobscured" and "low/obscured" phases reveal that the line originates at smaller radii in high-flux states; model-independent tests indicate that this effect is significant at the 4-5 sigma level. Some models and Delta t similar or equal to 2 x 10(4) s variations in line flux suggest that the narrow Fe K alpha line may originate at radii as small as R similar or equal to 50-130 GM(BH)/c(2) in high-flux states. These results indicate that the narrow Fe K alpha line in NGC 4151 is primarily excited in the innermost part of the optical broad line region (BLR), or X-ray BLR. Alternatively, a warp could provide the solid angle needed to enhance Fe K alpha line emission from intermediate radii, and might resolve an apparent discrepancy in the inclination of the innermost and outer disk in NGC 4151. Both warps and the BLR may originate through radiation pressure, so these explanations may be linked. We discuss our results in detail, and consider the potential for future observations with Chandra, XARM, and ATHENA to measure black hole masses and to study the intermediate disk in AGNs using narrow Fe K alpha emission lines.