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IC 751: a new changing look AGN discovered by NuSTAR

IC 751: a new changing look AGN discovered by NuSTAR
IC 751: a new changing look AGN discovered by NuSTAR
We present results of five Nuclear Spectroscopic Telescope Array (NuSTAR) observations of the type 2 active galactic nucleus (AGN) in IC 751, three of which were performed simultaneously with XMM-Newton or Swift/X-Ray Telescope. We find that the nuclear X-ray source underwent a clear transition from a Compton-thick (${N}_{{\rm{H}}}\simeq 2\times {10}^{24}\;{\mathrm{cm}}^{-2}$) to a Compton-thin (${N}_{{\rm{H}}}\simeq 4\times {10}^{23}\;{\mathrm{cm}}^{-2}$) state on timescales of $\lesssim 3$ months, which makes IC 751 the first changing look AGN discovered by NuSTAR. Changes of the line of sight column density at the ~2σ level are also found on a timescale of ~48 hr (${\rm{\Delta }}{N}_{{\rm{H}}}\sim {10}^{23}\;{\mathrm{cm}}^{-2}$). From the lack of spectral variability on timescales of ~100 ks, we infer that the varying absorber is located beyond the emission-weighted average radius of the broad-line region (BLR), and could therefore be related either to the external part of the BLR or a clumpy molecular torus. By adopting a physical torus X-ray spectral model, we are able to disentangle the column density of the non-varying absorber (${N}_{{\rm{H}}}\sim 3.8\times {10}^{23}\;{\mathrm{cm}}^{-2}$) from that of the varying clouds $[{N}_{{\rm{H}}}\sim \;(1-150)\;\times \;{10}^{22}\;{\mathrm{cm}}^{-2}]$, and to constrain that of the material responsible for the reprocessed X-ray radiation (${N}_{{\rm{H}}}\sim 6\times {10}^{24}\;{\mathrm{cm}}^{-2}$). We find evidence of significant intrinsic X-ray variability, with the flux varying by a factor of five on timescales of a few months in the 2–10 and 10–50 keV band.
1538-4357
1-11
Ricci, C.
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Bauer, F.E.
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Arevalo, P.
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Boggs, S.
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Brandt, W.N.
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Christensen, F.E.
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Craig, W.W.
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Gandhi, P.
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Hailey, C.J.
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Harrison, F.A.
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Koss, M.
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Markwardt, C.B.
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Stern, D.
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Treister, E.
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Zhang, W.W.
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Ricci, C.
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Bauer, F.E.
4090334d-93b3-4fd4-83bf-3c65385dce47
Arevalo, P.
eb2c36e7-2984-4048-b2fd-22e1cd8ca325
Boggs, S.
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Brandt, W.N.
2ccd8265-d819-4b7a-8ec8-ced4bfb7baeb
Christensen, F.E.
ca546034-0593-4892-a1db-7987f2113467
Craig, W.W.
a15d85e8-3d7c-4bf5-895a-6b754550be47
Gandhi, P.
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Hailey, C.J.
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Harrison, F.A.
b71f286a-75db-46ef-8953-bd41865e74bc
Koss, M.
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Markwardt, C.B.
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Stern, D.
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Treister, E.
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Zhang, W.W.
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Ricci, C., Bauer, F.E., Arevalo, P., Boggs, S., Brandt, W.N., Christensen, F.E., Craig, W.W., Gandhi, P., Hailey, C.J., Harrison, F.A., Koss, M., Markwardt, C.B., Stern, D., Treister, E. and Zhang, W.W. (2016) IC 751: a new changing look AGN discovered by NuSTAR. The Astrophysical Journal, 820 (1), 1-11. (doi:10.3847/0004-637X/820/1/5).

Record type: Article

Abstract

We present results of five Nuclear Spectroscopic Telescope Array (NuSTAR) observations of the type 2 active galactic nucleus (AGN) in IC 751, three of which were performed simultaneously with XMM-Newton or Swift/X-Ray Telescope. We find that the nuclear X-ray source underwent a clear transition from a Compton-thick (${N}_{{\rm{H}}}\simeq 2\times {10}^{24}\;{\mathrm{cm}}^{-2}$) to a Compton-thin (${N}_{{\rm{H}}}\simeq 4\times {10}^{23}\;{\mathrm{cm}}^{-2}$) state on timescales of $\lesssim 3$ months, which makes IC 751 the first changing look AGN discovered by NuSTAR. Changes of the line of sight column density at the ~2σ level are also found on a timescale of ~48 hr (${\rm{\Delta }}{N}_{{\rm{H}}}\sim {10}^{23}\;{\mathrm{cm}}^{-2}$). From the lack of spectral variability on timescales of ~100 ks, we infer that the varying absorber is located beyond the emission-weighted average radius of the broad-line region (BLR), and could therefore be related either to the external part of the BLR or a clumpy molecular torus. By adopting a physical torus X-ray spectral model, we are able to disentangle the column density of the non-varying absorber (${N}_{{\rm{H}}}\sim 3.8\times {10}^{23}\;{\mathrm{cm}}^{-2}$) from that of the varying clouds $[{N}_{{\rm{H}}}\sim \;(1-150)\;\times \;{10}^{22}\;{\mathrm{cm}}^{-2}]$, and to constrain that of the material responsible for the reprocessed X-ray radiation (${N}_{{\rm{H}}}\sim 6\times {10}^{24}\;{\mathrm{cm}}^{-2}$). We find evidence of significant intrinsic X-ray variability, with the flux varying by a factor of five on timescales of a few months in the 2–10 and 10–50 keV band.

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Accepted/In Press date: 20 January 2016
Published date: 10 March 2016
Organisations: Astronomy Group

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Local EPrints ID: 399907
URI: http://eprints.soton.ac.uk/id/eprint/399907
ISSN: 1538-4357
PURE UUID: 7b955f0f-6ae0-496f-805e-f3cba8a924ea
ORCID for P. Gandhi: ORCID iD orcid.org/0000-0003-3105-2615

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Date deposited: 02 Sep 2016 11:12
Last modified: 17 Dec 2019 01:34

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Contributors

Author: C. Ricci
Author: F.E. Bauer
Author: P. Arevalo
Author: S. Boggs
Author: W.N. Brandt
Author: F.E. Christensen
Author: W.W. Craig
Author: P. Gandhi ORCID iD
Author: C.J. Hailey
Author: F.A. Harrison
Author: M. Koss
Author: C.B. Markwardt
Author: D. Stern
Author: E. Treister
Author: W.W. Zhang

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