IC 751: a new changing-look AGN discovered by NuSTAR

Abstract

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 ($N_{\mathrm{H}}\simeq 2\times 10^{24}\mathrm{c}{\mathrm{m}}^{-2}$) to a Compton-thin ($N_{\mathrm{H}}\simeq 4\times 10^{23}\mathrm{c}{\mathrm{m}}^{-2}$) 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 $\sim 2\ddot{I}ƒ$ level are also found on a time-scale of $\sim 48$ hours ($\hat{I}”N_{\mathrm{H}}\sim 10^{23}\mathrm{c}{\mathrm{m}}^{-2}$). 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 ($N_{\mathrm{H}}\sim 3.8\times 10^{23}\mathrm{c}{\mathrm{m}}^{-2}$) from that of the varying clouds [$N_{\mathrm{H}}\sim (1-150)\times 10^{22}\mathrm{c}{\mathrm{m}}^{-2}$], and to constrain that of the material responsible for the reprocessed X-ray radiation ($N_{\mathrm{H}}\sim 6\times 10^{24}\mathrm{c}{\mathrm{m}}^{-2}$). 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.

Accepted for publication in ApJ, 11 pages, 6 figures