NuSTAR Uncovers an Extremely Local Compton-thick AGN in NGC 4968
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2019-11-13
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LaMassa, Stephanie M.; Yaqoob, Tahir; Boorman, Peter G.; Tzanavaris, Panayiotis; Levenson, N. A.; Gandhi, Poshak; Ptak, Andrew F.; Heckman, Timothy M.; NuSTAR Uncovers an Extremely Local Compton-thick AGN in NGC 4968; High Energy Astrophysical Phenomena (2019); https://arxiv.org/abs/1911.05813
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Abstract
We present the analysis of Chandra and NuSTAR spectra of NGC 4968, a local (D∼44 Mpc) 12μm-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 2×10²⁴ 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 (L₂₋₁₀ₖₑᵥ,intrinsic), where the higher end of L₂₋₁₀ₖₑᵥ,intrinsic is consistent with expectations from the 12μm luminosity (L₂₋₁₀ₖₑᵥ,intrinisc∼7×10⁴² 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.