The weak neutral Fe fluorescence line and long-term X-ray evolution of the Compton-thick AGN in NGC 7674

P. Gandhi, A. Annuar, G. B. Lansbury, D. Stern, D. M. Alexander, F. E. Bauer, S. Bianchi, S. E. Boggs, P. G. Boorman, W. N. Brandt, M. Brightman, Finn Erland Christensen, A. Comastri, W. W. Craig, A. Del Moro, M. Elvis, M. Guainazzi, C. J. Hailey, F. A. Harrison, M. KossI. Lamperti, G. Malaguti, A. Masini, G. Matt, S. Puccetti, C. Ricci, E. Rivers, D. J. Walton, W. W. Zhang

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Abstract

We present NuSTAR X-ray observations of the active galactic nucleus (AGN) in NGC 7674. The source shows a flat X-ray spectrum, suggesting that it is obscured by Compton-thick gas columns. Based upon long-term flux dimming, previous work suggested the alternate possibility that the source is a recently switched-off AGN with the observed X-rays being the lagged echo from the torus. Our high-quality data show the source to be reflection-dominated in hard X-rays, but with a relatively weak neutral Fe Kα emission line (equivalent width [EW] of ≈ 0.4 keV) and a strong Fe XXVI ionised line (EW ≈ 0.2 keV). We construct an X-ray light curve of NGC 7674 spanning 37 years and find that the observed 2-10 keV X-ray flux has remained constant for the past ≈ 20 years. Light travel time arguments constrain the minimum radius of the reflector to be ∼3.2pc under the switched-off AGN scenario, ≈30 times larger than the expected dust sublimation radius, rendering this possibility unlikely. A combination of intrinsic fading and patchy obscuration cannot be ruled out. A Compton-thick AGN (CTAGN) solution requires a minimum line-of-sight column density (NH) of 3 × 1024 cm−2, and yields an intrinsic 2-10 keV luminosity of (3-5) × 1043 erg s−1. Realistic uncertainties span the range of (1-13) × 1043 erg s−1. The source has one of the weakest fluorescence lines amongst bona fide CTAGN, and potentially a local analogue of bolometrically luminous systems showing complex neutral and ionised Fe emission. It exemplifies the difficulty of identification and proper characterisation of distant CTAGN based on the strength of the neutral Fe Kα line.
Original languageEnglish
JournalMonthly Notices of the Royal Astronomical Society
Volume467
Issue number4
Pages (from-to) 4606-4621
ISSN0035-8711
DOIs
Publication statusPublished - 2017

Bibliographical note

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2017 The authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

Keywords

  • Seyfert
  • X-rays: individual (NGC7674)

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