Accretion-disk reflection in the Seyfert 1 galaxy NGC 3783 as viewed by Chandra grating spectroscopy

TL;DR

This study analyzes Chandra X-ray data of NGC 3783 to understand accretion disk reflection, SMBH spin, and disk structure, revealing a high spin, spectral state evolution, and possible disk warping.

Contribution

It provides a comprehensive multi-epoch spectral analysis of NGC 3783, constraining SMBH spin and disk geometry using high-resolution X-ray spectroscopy.

Findings

Relativistically broadened iron emission indicates a near-maximal SMBH spin (a=0.98).

Spectral states evolved from softer to harder over time.

Distant Fe Kα line shows excess velocity, suggesting twisted disk layers.

Abstract

In active galactic nuclei, X-ray illumination of the accretion disk around a supermassive black hole (SMBH) results in the production of the K$\alpha$ fluorescent line of iron, which provides insights into accretion physics and SMBH spins. In this work, we studied X-ray reflection from the accretion disk in the Seyfert 1 galaxy NGC 3783 using all the data collected by the Chandra High Energy Transmission Grating Spectrometer. We used hardness-ratio diagrams to distinguish between different spectral states and conducted spectral analysis of all the multi-epoch datasets, as well as the source in the observed spectral states. Our hardness analysis indicates that the source gradually evolved into a harder state (2013-2016) compared to the previous epochs (2000-2001). Our spectral modeling implies that the relativistically broadened iron emission from the innermost accretion disk is associated with a near-maximal SMBH spin ($a=0.98^{+0.02}_{-0.12}$) in all the datasets, even though the hard state was present in 17% of them, and a consistent spin is also found in different spectral states. In addition, the narrow, bright Fe K$\alpha$ line from distant regions has an excess velocity of $620^{+80}_{-70}$ km s$^{-1}$ relative to the rest frame, implying that some distant layers of the disk could be twisted. Our results suggest that, despite long-term changes in the X-ray brightness of NGC 3783, likely caused by eclipsing material, the relativistic reflection can be constrained thanks to the substantial counts provided by multi-epoch observations, while a warped disk structure may be present.