A Comparison of Two Methods for Estimating Black Hole Spin in Active Galactic Nuclei

TL;DR

This paper compares two methods for estimating black hole spin in active galactic nuclei, applying them to two AGN and finding generally consistent results, which helps validate these techniques for broader astrophysical studies.

Contribution

It provides a direct comparison of the X-ray reflection and continuum-fitting methods for black hole spin estimation in AGN, highlighting their agreement and potential improvements.

Findings

Both methods yield consistent spin estimates for the two AGN studied.

Adding a disk wind model improves the spin estimate for NGC 3783.

Refinement of techniques and better data will enable larger sample analyses.

Abstract

Angular momentum, or spin, is a fundamental property of black holes (BHs), yet it is much more difficult to estimate than mass or accretion rate (for actively accreting systems). In recent years, high-quality X-ray observations have allowed for detailed measurements of the Fe K$\alpha$ emission line, where relativistic line broadening allows constraints on the spin parameter (the X-ray reflection method). Another technique uses accretion disk models to fit the AGN continuum emission (the continuum-fitting, or CF, method). Although each technique has model-dependent uncertainties, these are the best empirical tools currently available and should be vetted in systems where both techniques can be applied. A detailed comparison of the two methods is also useful because neither method can be applied to all AGN. The X-ray reflection technique targets mostly local (z $\lesssim$ 0.1) systems, while the CF method can be applied at higher redshift, up to and beyond the peak of AGN activity and growth. Here, we apply the CF method to two AGN with X-ray reflection measurements. For both the high-mass AGN, H1821+643, and the Seyfert 1, NGC 3783, we find a range in spin parameter consistent with the X-ray reflection measurements. However, the near-maximal spin favored by the reflection method for NGC 3783 is more probable if we add a disk wind to the model. Refinement of these techniques, together with improved X-ray measurements and tighter BH mass constraints, will permit this comparison in a larger sample of AGN and increase our confidence in these spin estimation techniques.