Probing the accretion disk and central engine structure of NGC4258 with Suzaku and XMM-Newton observations

TL;DR

This study uses Suzaku and XMM-Newton data to analyze the structure of the accretion disk and central engine of NGC4258, revealing a clean environment with a warped disk and rapid X-ray variability indicating activity close to the black hole.

Contribution

It provides the first detailed analysis of the accretion disk structure and iron line origin in NGC4258, highlighting a warped disk and a lack of typical obscuring torus in this low-luminosity AGN.

Findings

Weak X-ray reprocessing signatures and a faint iron line suggest a clean circumnuclear environment.

The iron line likely originates from the surface of a warped accretion disk.

Rapid X-ray variability indicates activity in the innermost disk regions.

Abstract

[abridged] We present an X-ray study of the low-luminosity active galactic nucleus (AGN) in NGC4258 using data from Suzaku, XMM-Newton, and the Swift/BAT survey. We find that signatures of X-ray reprocessing by cold gas are very weak in the spectrum of this Seyfert-2 galaxy; a weak, narrow fluorescent-Kalpha emission line of cold iron is robustly detected in both the Suzaku and XMM-Newton spectra but at a level much below that of most other Seyfert-2 galaxies. We conclude that the circumnuclear environment of this AGN is very "clean" and lacks the Compton-thick obscuring torus of unified Seyfert schemes. From the narrowness of the iron line, together with evidence for line flux variability between the Suzaku and XMM-Newton observations, we constrain the line emitting region to be between $3\times 10^3r_g$ and $4\times 10^4r_g$ from the black hole. We show that the observed properties of the iron line can be explained if the line originates from the surface layers of a warped accretion disk. In particular, we present explicit calculations of the expected iron line from a disk warped by Lens-Thirring precession from a misaligned central black hole. Finally, the Suzaku data reveal clear evidence for large amplitude 2-10keV variability on timescales of 50ksec as well as smaller amplitude flares on timescales as short as 5-10ksec. If associated with accretion disk processes, such rapid variability requires an origin in the innermost regions of the disk ($r\approx 10r_g$ or less).