NuSTAR Uncovers an Extremely Local Compton-thick AGN in NGC 4968

Abstract

We present the analysis of Chandra and NuSTAR spectra of NGC 4968, a local ( D  ∼ 44 Mpc) 12 μ m selected Seyfert 2 galaxy, enshrouded within Compton-thick layers of obscuring gas. We find no evidence of variability between the Chandra and NuSTAR observations (separated by 2 yr), nor between the two NuSTAR observations (separated by 10 months). Using self-consistent X-ray models, we rule out the scenario where the obscuring medium is nearly spherical and uniform, contradicting the results implied by the <10 keV Chandra spectrum. The line-of-sight column density, from intervening matter between the source and observer that intercepts the intrinsic active galactic nucleus (AGN) X-ray emission, is well within the Compton-thick regime, with a minimum column density of 2 × 10 24 cm −2 . The average global column density is high (>3 × 10 23 cm −2 ), with both Compton-thick and Compton-thin solutions permitted, depending on the X-ray spectral model. The spectral models provide a range of intrinsic AGN continuum parameters and implied 2–10 keV luminosities ( L 2–10 keV,intrinsic ), where the higher end of L 2–10 keV,intrinsic is consistent with expectations from the 12 μ m luminosity ( L 2–10 keV,intrinisc  ∼ 7 × 10 42 erg s −1 ). Compared with Compton-thick AGN previously observed by NuSTAR , NGC 4968 is among the most intrinsically X-ray luminous. However, despite its close proximity and relatively high intrinsic X-ray luminosity, it is undetected by the 105 month Swift -Burst Alert Telescope survey, underscoring the importance of multiwavelength selection for obtaining the most complete census of the most hidden black holes.