2016ApJ...820....5R

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≃2×10²⁴ cm^–2) to a Compton-thin (N_H≃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 (ΔN_H∼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 (N_H∼3.8×10²³ cm^–2) from that of the varying clouds [N_H∼ (1-150) × 10²² cm^–2], and to constrain that of the material responsible for the reprocessed X-ray radiation (N_H∼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.