IC 751: A New Changing Look AGN Discovered By NuSTAR

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

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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 (NH ≃ 2 × 1024 cm−2) to a Compton-thin (NH ≃ 4 × 1023 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 (ΔNH ∼ 1023 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 (NH ∼ 3.8 × 1023 cm−2) from that of the varying clouds [NH ∼ (1−150) × 1022 cm−2], and to constrain that of the material responsible for the reprocessed X-ray radiation (NH ∼ 6 × 1024 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.
Original languageEnglish
Article number5
JournalAstrophysical Journal
Issue number1
Number of pages11
Publication statusPublished - 2016


  • Galaxies: active
  • Galaxies: individual: (IC 751)
  • Galaxies: Seyfert
  • X-rays: galaxies


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