Weighing Super-Massive Black Holes with Narrow Fe K$\alpha$ Line

TL;DR

This study explores using narrow Fe Kα line widths to estimate supermassive black hole masses in AGNs, comparing results with other methods and discussing potential origins and uncertainties.

Contribution

It introduces a new technique for black hole mass estimation using Fe Kα line widths and compares it with existing methods, highlighting current limitations.

Findings

Narrow Fe Kα line widths are on average 2.6 times broader than expected from isotropic torus models.

The correlation between black hole masses from this method and others is statistically insignificant.

Large uncertainties and small sample size hinder definitive conclusions about the technique's reliability.

Abstract

It has been suggested that the narrow cores of the Fe K$\alpha$ emission lines in Active Galactic Nuclei (AGNs) are likely produced in the torus, the inner radius of which can be measured by observing the lag time between the $V$ and $K$ band flux variations. In this paper we compare the virial products of the infrared time lags and the narrow Fe K$\alpha$ widths for 10 type 1 AGNs with the black hole masses from other techniques. We find the narrow Fe K$\alpha$ line width is in average 2.6$^{+0.9}_{-0.4}$ times broader than expected assuming an isotropic velocity distribution of the torus at the distance measured by the infrared lags. We propose the thick disk model of the torus could explain the observed larger line width. Another possibility is the contamination by emission from the broad line region or the outer accretion disk. Alternatively, the narrow iron line might originate from the inner most part of the obscuring torus within the sublimation radius, while the infrared emission from outer cooler part. We note the correlation between the black hole masses based on this new technique and those based on other known techniques is statistically insignificant. We argue that this could be attributed to the small sample size and the very large uncertainties in the measurements of iron K line widths. The next generation of X-ray observatories could help verify the origin of the narrow iron K$\alpha$ line and the reliability of this new technique.