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).
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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