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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 ($N_{rm,H}simeq 2times 10^{24}rm,cm^{-2}$) to a Compton-thin ($N_{rm,H}simeq 4times 10^{23}rm,cm^{-2}$) 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 $sim2sigma$ 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 ($N_{rm,H}sim 3.8times 10^{23}rm,cm^{-2}$) 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.
We analyse high-quality NuSTAR observations of the local (z = 0.011) Seyfert 2 active galactic nucleus (AGN) IC 3639, in conjunction with archival Suzaku and Chandra data. This provides the first broadband X-ray spectral analysis of the source, spann
We report the discovery of six active galactic nuclei (AGN) caught turning on during the first nine months of the Zwicky Transient Facility (ZTF) survey. The host galaxies were classified as LINERs by weak narrow forbidden line emission in their arch
The uncertain origin of the recently-discovered `changing-looking quasar phenomenon -- in which a luminous quasar dims significantly to a quiescent state in repeat spectroscopy over ~10 year timescales -- may present unexpected challenges to our unde
Hot, Dust-Obscured Galaxies (Hot DOGs), selected from the WISE all sky infrared survey, host some of the most powerful Active Galactic Nuclei (AGN) known, and might represent an important stage in the evolution of galaxies. Most known Hot DOGs are at
We present Nustar 3-40 keV observations of the optically selected Type 2 quasar (QSO2) SDSS J1034+6001 or Mrk 34. The high-quality hard X-ray spectrum and archival XMM-Newton data can be fitted self-consistently with a reflection-dominated continuum