IC 751: a new changing look AGN discovered by NuSTAR

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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ORCID for P. Gandhi: orcid.org/0000-0003-3105-2615

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