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$\sim$44 Mpc) 12$\mu$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 years), and 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 AGN X-ray emission, is well within the Compton-thick regime, with a minimum column density of $2\times 10^{24}$ cm${}^{-2}$. The average global column density is high ($>3\times 10^{23}$ 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 ($L_{2-10keV,\mathrm{intrinsic}}$), where the higher end of $L_{2-10keV,\mathrm{intrinsic}}$ is consistent with expectations from the 12$\mu$m luminosity ($L_{2-10keV,\mathrm{intrinisc}}\sim 7\times 10^{42}$ erg s${}^{-1}$). Compared with Compton-thick AGN previously observed by {\it 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-BAT survey, underscoring the importance of multi-wavelength selection for obtaining the most complete census of the most hidden black holes.

accepted for publication in ApJ; 20 pages, 8 figures

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