A Monte Carlo Markov Chain based investigation of black hole spin in the active galaxy NGC3783

TL;DR

This study uses Monte Carlo Markov Chain methods to analyze Suzaku X-ray data of NGC3783, confirming a rapidly spinning black hole and high iron abundance, while discussing the impact of modeling degeneracies and physical processes like radiative levitation.

Contribution

It demonstrates the application of MCMC techniques to assess black hole spin and iron abundance, revealing degeneracies and proposing radiative levitation as a factor affecting abundance measurements.

Findings

NGC3783 has a rapidly spinning black hole (a>0.88).

The disk shows supersolar iron abundance (Z_Fe=2-4Zsun).

Radiative levitation may enhance iron abundance in AGN disks.

Abstract

The analysis of relativistically broadened X-ray spectral features from the inner accretion disk provides a powerful tool for measuring the spin of supermassive black holes (SMBH) in active galactic nuclei (AGN). However, AGN spectra are often complex and careful analysis employing appropriate and self-consistent models are required if one is to obtain robust results. In this paper, we revisit the deep July-2009 Suzaku observation of the Seyfert galaxy NGC3783 in order to study in a rigorous manner the robustness of the inferred black hole spin parameter. Using Monte Carlo Markov Chain (MCMC) techniques, we identify a (partial) modeling degeneracy between the iron abundance of the disk and the black hole spin parameter. We show that the data for NGC3783 strongly require both supersolar iron abundance (Z_Fe=2-4Zsun) and a rapidly spinning black hole (a>0.88). We discuss various astrophysical considerations that can affect the measured abundance. We note that, while the abundance enhancement inferred in NGC3783 is modest, the X-ray analysis of some other objects has found extreme iron abundances. We introduce the hypothesis that the radiative levitation of iron ions in the innermost regions of radiation-dominated AGN disks can enhance the photospheric abundance of iron. We show that radiative levitation is a plausible mechanism in the very inner regions of high accretion rate AGN disks.