The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick active galactic nucleus in NGC7674

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; F. E. Christensen; A. Comastri; W. W. Craig; A. Del Moro; M. Elvis; M. Guainazzi; C. J. Hailey; F. A. Harrison; M. Koss; I. Lamperti; G. Malaguti; A. Masini; G. Matt; S. Puccetti; C. Ricci; E. Rivers; D. J. Walton; W. W. Zhang

doi:10.1093/mnras/stx357

The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick active galactic nucleus in NGC7674

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, F. E. 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 NGC7674. 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-offAGN 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 longterm X-ray light curve of NGC7674 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-offAGN 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 (N_H) 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.

Original language	English (US)
Pages (from-to)	4606-4621
Number of pages	16
Journal	Monthly Notices of the Royal Astronomical Society
Volume	467
Issue number	4
DOIs	https://doi.org/10.1093/mnras/stx357
State	Published - Jun 1 2017

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Astronomy and Astrophysics
Space and Planetary Science

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10.1093/mnras/stx357

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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., ... Zhang, W. W. (2017). The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick active galactic nucleus in NGC7674. Monthly Notices of the Royal Astronomical Society, 467(4), 4606-4621. https://doi.org/10.1093/mnras/stx357

@article{bef2ffbcc1ef46f8855ca1e94d33a18f,

title = "The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick active galactic nucleus in NGC7674",

abstract = "We present NuSTAR X-ray observations of the active galactic nucleus (AGN) in NGC7674. 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-offAGN 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 longterm X-ray light curve of NGC7674 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-offAGN 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.",

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

note = "Publisher Copyright: {\textcopyright} 2017 The Authors.",

year = "2017",

month = jun,

day = "1",

doi = "10.1093/mnras/stx357",

language = "English (US)",

volume = "467",

pages = "4606--4621",

journal = "Monthly Notices of the Royal Astronomical Society",

issn = "0035-8711",

publisher = "Oxford University Press",

number = "4",

}

Gandhi, P, Annuar, A, Lansbury, GB, Stern, D, Alexander, DM, Bauer, FE, Bianchi, S, Boggs, SE, Boorman, PG, Brandt, WN, Brightman, M, Christensen, FE, Comastri, A, Craig, WW, Del Moro, A, Elvis, M, Guainazzi, M, Hailey, CJ, Harrison, FA, Koss, M, Lamperti, I, Malaguti, G, Masini, A, Matt, G, Puccetti, S, Ricci, C, Rivers, E, Walton, DJ & Zhang, WW 2017, 'The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick active galactic nucleus in NGC7674', Monthly Notices of the Royal Astronomical Society, vol. 467, no. 4, pp. 4606-4621. https://doi.org/10.1093/mnras/stx357

TY - JOUR

T1 - The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick active galactic nucleus in NGC7674

AU - Gandhi, P.

AU - Annuar, A.

AU - Lansbury, G. B.

AU - Stern, D.

AU - Alexander, D. M.

AU - Bauer, F. E.

AU - Bianchi, S.

AU - Boggs, S. E.

AU - Boorman, P. G.

AU - Brandt, W. N.

AU - Brightman, M.

AU - Christensen, F. E.

AU - Comastri, A.

AU - Craig, W. W.

AU - Del Moro, A.

AU - Elvis, M.

AU - Guainazzi, M.

AU - Hailey, C. J.

AU - Harrison, F. A.

AU - Koss, M.

AU - Lamperti, I.

AU - Malaguti, G.

AU - Masini, A.

AU - Matt, G.

AU - Puccetti, S.

AU - Ricci, C.

AU - Rivers, E.

AU - Walton, D. J.

AU - Zhang, W. W.

PY - 2017/6/1

Y1 - 2017/6/1

N2 - We present NuSTAR X-ray observations of the active galactic nucleus (AGN) in NGC7674. 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-offAGN 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 longterm X-ray light curve of NGC7674 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-offAGN 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.

AB - We present NuSTAR X-ray observations of the active galactic nucleus (AGN) in NGC7674. 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-offAGN 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 longterm X-ray light curve of NGC7674 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-offAGN 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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UR - http://www.scopus.com/inward/citedby.url?scp=85029110577&partnerID=8YFLogxK

U2 - 10.1093/mnras/stx357

DO - 10.1093/mnras/stx357

M3 - Article

AN - SCOPUS:85029110577

SN - 0035-8711

VL - 467

SP - 4606

EP - 4621

JO - Monthly Notices of the Royal Astronomical Society

JF - Monthly Notices of the Royal Astronomical Society

IS - 4

ER -

The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick active galactic nucleus in NGC7674

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