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Published November 20, 2016 | Accepted Version
Journal Article Open

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

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 Kα 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 Kα 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 blue features are actually highly variable on timescales of ~10–15 hr, 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.

Additional Information

© 2016 The American Astronomical Society. Received 2016 June 20; revised 2016 August 25; accepted 2016 September 8; published 2016 November 15. The authors would like to thank the anonymous referee for useful comments that helped improve the clarity of this paper. EN is supported by the UK Science and Technology Facilities Council under grant ST/M001040/1. DP acknowledges financial support from the French GdR PCHE and the EU 7th Framework Programme FP7 (2013–2017) under grant agreement number 312789. JNR acknowledges support from Chandra grant number GO4-15092X and NASA grant NNX15AF12G. AL acknowledges support from the UK STFC. The results presented in this paper are based on data obtained with the Chandra X-ray Observatory; XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and NASA; and Suzaku, a collaborative mission between the space agency of Japan (JAXA) and NASA. We have made use of software provided by the Chandra X-ray Center (CXC) in the application package ciao. The figures were generated using matplotlib (Hunter 2007), a python library for publication of quality graphics.

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August 19, 2023
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