IC 751: a new changing-look AGN discovered by NuSTAR
  We present the results of five NuSTAR observations of the type 2 active
galactic nucleus (AGN) in IC 751, three of which were performed simultaneously
with XMM-Newton or Swift/XRT. We find that the nuclear X-ray source underwent a
clear transition from a Compton-thick ($N_{\rm\,H}\simeq 2\times
10^{24}\rm\,cm^{-2}$) to a Compton-thin ($N_{\rm\,H}\simeq 4\times
10^{23}\rm\,cm^{-2}$) 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 a $\sim2\sigma$ level are also found on a
time-scale of $\sim 48$ hours ($\Delta N_{\rm\,H}\sim 10^{23}\rm\,cm^{-2}$).
From the lack of spectral variability on timescales of $\sim 100$ ks we infer
that the varying absorber is located beyond the emission-weighted average
radius of the broad-line region, and could therefore be related either to the
external part of the broad-line region 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 ($N_{\rm\,H}\sim 3.8\times
10^{23}\rm\,cm^{-2}$) from that of the varying clouds
[$N_{\rm\,H}\sim(1-150)\times10^{22}\rm\,cm^{-2}$], and to constrain that of
the material responsible for the reprocessed X-ray radiation ($N_{\rm\,H} \sim
6 \times 10^{24}\rm\,cm^{-2}$). 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.
