X-ray Structure between the Innermost Disk and Optical Broad Line Region in NGC 4151

TL;DR

This study analyzes the narrow Fe K-alpha line in NGC 4151, revealing its origin close to the black hole within the optical broad line region, with implications for understanding AGN inner disk structures.

Contribution

It provides the first detailed analysis linking the narrow Fe K-alpha line to the innermost regions of the AGN's accretion disk and broad line region, using high-resolution X-ray spectroscopy.

Findings

Fe K-alpha line originates at radii ~50-130 GM/c^2 in high flux states

Line asymmetry and shifts suggest emission from the innermost disk or BLR

Potential for future observations to measure black hole masses

Abstract

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.