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 μm 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 × 1024 cm−2. The average global column density is high (>3 × 1023 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 μm luminosity (L 2–10 keV,intrinisc ~ 7 × 1042 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.

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Identifiers

ORCID for Peter G. Boorman: orcid.org/0000-0001-9379-4716

ORCID for Poshak Gandhi: orcid.org/0000-0003-3105-2615

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