  We present an analysis of the narrow Fe K-alpha line in Chandra/HETGS
observations of the Seyfert AGN, NGC 4151. The sensitivity and resolution
afforded by the gratings reveal asymmetry in this line. Models including weak
Doppler boosting, gravitational red-shifts, and scattering are generally
preferred over Gaussians at the 5 sigma level of confidence, and generally
measure radii consistent with R ~ 500-1000 GM/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 ~ 2 E+4 s
variations in line flux suggest that the narrow Fe K-alpha line may originate
at radii as small as R ~ 50-130 GM/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 AGN using narrow Fe
K-alpha emission lines.
