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Published March 20, 2016 | Published + Submitted
Journal Article Open

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_H ≃ 2 x 10^(24)cm^(-2)) to a Compton-thin (N_H ≃ 4 x 10^(23) cm^(-2)) state on timescales of ≾ 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 (ΔN_H ~ 10^(23cm^(-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_H ~ 3.8 x 10^(23)cm^(-2)) from that of the varying clouds [N_H ~ (1-150) x 10^(22 cm^(-2)], and to constrain that of the material responsible for the reprocessed X-ray radiation (N_H ~ 6 x 10^(24) 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.

Additional Information

© 2016 American Astronomical Society. Received 2015 November 28; accepted 2016 January 20; published 2016 March 10. We thank the anonymous referee for comments that helped us to improve the quality of our manuscript. CR acknowledges Marko Stalevski and Sebastien Guillot for fruitful discussions, and Chin-Shin Chang for her comments on the manuscript. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (Caltech, USA). We acknowledge financial support from the CONICYT-Chile grants "EMBIGGEN" Anillo ACT1101 (CR, ET, FEB, PA), FONDECYT 1141218 (CR, FEB), FONDECYT 1140304 (PA), Basal-CATA PFB-06/2007 (CR, ET, FEB), NuSTAR subcontract 44A-1092750 (WNB), the Swiss National Science Foundation and Ambizione fellowship grant PZ00P2 154799/1 (MK), and the Ministry of Economy, Development, and Tourism's Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS (FEB). This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, of data obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC), provided by NASA's Goddard Space Flight Center, and of the SIMBAD Astronomical Database which is operated by the Centre de Données astronomiques de Strasbourg. Facilities: NuSTAR - The NuSTAR (Nuclear Spectroscopic Telescope Array) mission, Swift - Swift Gamma-Ray Burst Mission, XMM-Newton - .

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Published - Ricci,C.Art.5.pdf

Submitted - 1602.00702v1.pdf


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