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.