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}_{{H}}≃ 2× {10}²⁴ {{cm}}^-2) to a Compton-thin ({N}_{{H}}≃ 4× {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 ({{Δ }}{N}_{{H}}∼ {10}²³ {{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× {10}²³ {{cm}}^-2) from that of the varying clouds [{N}_{{H}}∼ (1-150) × {10}²² {{cm}}^-2], and to constrain that of the material responsible for the reprocessed X-ray radiation ({N}_{{H}}∼ 6× {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.