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The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick active galactic nucleus in NGC 7674

The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick active galactic nucleus in NGC 7674
The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick active galactic nucleus in NGC 7674
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 ionized line (EW ≈ 0.2 keV). We construct an updated long-term X-ray light curve of NGC 7674 and find that the observed 2-10 keV flux has remained constant for the past ≈ 20 yr, following a high-flux state probed by Ginga. Light travel time arguments constrain the minimum radius of the reflector to be ∼ 3.2 pc under the switched-off AGN scenario, ≈ 30 times larger than the expected dust sublimation radius, rendering this possibility unlikely. A patchy Compton-thick AGN (CTAGN) solution is plausible, requiring a minimum line-of-sight column density (NH) of 3 × 1024 cm-2 at present, 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 is potentially a local analogue of bolometrically luminous systems showing complex neutral and ionized Fe emission. It exemplifies the difficulty of identification and proper characterization of distant CTAGN based on the strength of the neutral Fe Kα line.
galaxies: Seyfert, X-rays: individual (NGC 7674)
0035-8711
4606-4621
Gandhi, P.
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Annuar, A.
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Lansbury, G.B.
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Stern, D.
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Alexander, D.M.
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Bauer, F.E.
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Bianchi, S.
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Boggs, S.E.
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Boorman, P.G.
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Brightman, M.
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Christensen, F.E.
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Comastri, A.
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Craig, W.W.
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Del Moro, A.
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Hailey, C.J.
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Koss, M.
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Lamperti, I.
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Malaguti, G.
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Masini, A.
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Matt, G.
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Puccetti, S.
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Ricci, C.
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Rivers, E.
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Walton, D.J.
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Gandhi, P.
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Annuar, A.
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Lansbury, G.B.
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Stern, D.
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Alexander, D.M.
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Bauer, F.E.
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Bianchi, S.
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Boggs, S.E.
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Boorman, P.G.
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Brandt, W.N.
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Brightman, M.
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Christensen, F.E.
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Comastri, A.
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Craig, W.W.
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Del Moro, A.
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Elvis, M.
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Guainazzi, M.
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Hailey, C.J.
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Harrison, F.A.
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Koss, M.
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Lamperti, I.
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Malaguti, G.
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Masini, A.
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Matt, G.
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Puccetti, S.
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Ricci, C.
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Rivers, E.
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Walton, D.J.
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Gandhi, P., Annuar, A., Lansbury, G.B., Stern, D., Alexander, D.M., Bauer, F.E., Bianchi, S., Boggs, S.E., Boorman, P.G., Brandt, W.N., Brightman, M., Christensen, F.E., Comastri, A., Craig, W.W., Del Moro, A., Elvis, M., Guainazzi, M., Hailey, C.J., Harrison, F.A., Koss, M., Lamperti, I., Malaguti, G., Masini, A., Matt, G., Puccetti, S., Ricci, C., Rivers, E. and Walton, D.J. (2017) The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick active galactic nucleus in NGC 7674. Monthly Notices of the Royal Astronomical Society, 467 (4), 4606-4621. (doi:10.1093/mnras/stx357).

Record type: Article

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 ionized line (EW ≈ 0.2 keV). We construct an updated long-term X-ray light curve of NGC 7674 and find that the observed 2-10 keV flux has remained constant for the past ≈ 20 yr, following a high-flux state probed by Ginga. Light travel time arguments constrain the minimum radius of the reflector to be ∼ 3.2 pc under the switched-off AGN scenario, ≈ 30 times larger than the expected dust sublimation radius, rendering this possibility unlikely. A patchy Compton-thick AGN (CTAGN) solution is plausible, requiring a minimum line-of-sight column density (NH) of 3 × 1024 cm-2 at present, 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 is potentially a local analogue of bolometrically luminous systems showing complex neutral and ionized Fe emission. It exemplifies the difficulty of identification and proper characterization of distant CTAGN based on the strength of the neutral Fe Kα line.

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Accepted/In Press date: 8 February 2017
e-pub ahead of print date: 13 February 2017
Published date: 1 June 2017
Keywords: galaxies: Seyfert, X-rays: individual (NGC 7674)

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Local EPrints ID: 413360
URI: http://eprints.soton.ac.uk/id/eprint/413360
ISSN: 0035-8711
PURE UUID: 2bac45f1-2c16-4ffa-9974-a978eb4115f4
ORCID for P. Gandhi: ORCID iD orcid.org/0000-0003-3105-2615
ORCID for P.G. Boorman: ORCID iD orcid.org/0000-0001-9379-4716

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Date deposited: 22 Aug 2017 16:31
Last modified: 09 Oct 2020 16:36

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Contributors

Author: P. Gandhi ORCID iD
Author: A. Annuar
Author: G.B. Lansbury
Author: D. Stern
Author: D.M. Alexander
Author: F.E. Bauer
Author: S. Bianchi
Author: S.E. Boggs
Author: P.G. Boorman ORCID iD
Author: W.N. Brandt
Author: M. Brightman
Author: F.E. Christensen
Author: A. Comastri
Author: W.W. Craig
Author: A. Del Moro
Author: M. Elvis
Author: M. Guainazzi
Author: C.J. Hailey
Author: F.A. Harrison
Author: M. Koss
Author: I. Lamperti
Author: G. Malaguti
Author: A. Masini
Author: G. Matt
Author: S. Puccetti
Author: C. Ricci
Author: E. Rivers
Author: D.J. Walton

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