A deep X-ray view of the bare AGN Ark 120. II. Evidence for Fe K emission transients

TL;DR

This study provides a detailed X-ray analysis of the bare AGN Ark 120, revealing transient Fe K emission features that suggest dynamic, short-lived hotspots on the accretion disk surface near the supermassive black hole.

Contribution

It presents the first detailed temporal and spectral analysis of Fe K emission transients in Ark 120, highlighting rapid variability and potential magnetic reconnection effects.

Findings

Fe Kα line from neutral iron is resolved and consistent with the broad-line region origin.

Red excess component varies on ~1 year timescale, indicating disk dynamics.

Both red and blue Fe K features vary on 10-15 hour timescales, suggesting short-lived hotspots.

Abstract

We report on the results from a large observational campaign on the bare Seyfert galaxy Ark 120, jointly carried out in 2014 with XMM-Newton, Chandra, and NuSTAR. The fortunate line of sight to this source, devoid of any significant absorbing material, provides an incomparably clean view to the nuclear regions of an active galaxy. Here we focus on the analysis of the iron fluorescence features, which form a composite emission pattern in the 6$-$7 keV band. The prominent K$\alpha$ line from neutral iron at 6.4 keV is resolved in the Chandra High-Energy Transmission Grating spectrum to a full-width at half maximum of 4700$^{+2700}_{-1500}$ km s$^{-1}$, consistent with an origin from the optical broad-line region. Excess components are detected on both sides of the narrow K$\alpha$ line: the red one (6.0$-$6.3 keV) clearly varies in strength in about one year, and hints at the presence of a broad, mildly asymmetric line from the accretion disk; the blue one (6.5$-$7.0 keV), instead, is likely a blend of different contributions, and appears to be constant when integrated over long enough exposures. However, the Fe K excess emission map computed over the 7.5 days of the XMM-Newton monitoring shows that both the red and the blue features are actually highly variable on timescales of $\sim$10$-$15 hours, suggesting that they might arise from short-lived hotspots on the disk surface, located within a few tens of gravitational radii from the central supermassive black hole and possibly illuminated by magnetic reconnection events. Any alternative explanation would still require a highly dynamic, inhomogeneous disk/coronal system, involving clumpiness and/or instability.