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NuSTAR observations of four nearby x-ray faint AGN : low luminosity or heavy obscuration?

Zappacosta, L. and Stern, D. and Ricci, C. and Nardini, E. and Masini, A. and Marchesi, S. and Lanz, L. and Koss, M.J. and Harrison, F.A. and Farrah, D. and Moro, A. Del and Chen, C-T J. and Brightman, M. and Brandt, W.N. and Boorman, P.G. and Bauer, F.E. and Ballantyne, D.R. and Baloković, M. and Asmus, D. and Lansbury, G.B. and Gandhi, P. and Alexander, D.M. and Annuar, A. (2020) 'NuSTAR observations of four nearby x-ray faint AGN : low luminosity or heavy obscuration?', Monthly notices of the Royal Astronomical Society., 497 (1). pp. 229-245.


We present NuSTAR observations of four active galactic nuclei (AGN) located within 15 Mpc. These AGN, namely ESO 121-G6, NGC 660, NGC 3486 and NGC 5195, have observed X-ray luminosities of L2-10 keV, obs ≲ 1039 erg s−1, classifying them as low luminosity AGN (LLAGN). We perform broadband X-ray spectral analysis for the AGN by combining our NuSTAR data with Chandra or XMM-Newton observations to directly measure their column densities (NH) and infer their intrinsic power. We complement our X-ray data with archival and new high angular resolution mid-infrared (mid-IR) data for all objects, except NGC 5195. Based on our X-ray spectral analysis, we found that both ESO 121-G6 and NGC 660 are heavily obscured (NH > 1023 cm−2; L2-10 keV, int ∼ 1041 erg s−1), and NGC 660 may be Compton-thick. We also note that the X-ray flux and spectral slope for ESO 121-G6 have significantly changed over the last decade, indicating significant changes in the obscuration and potentially accretion rate. On the other hand, NGC 3486 and NGC 5195 appear to be unobscured and just mildly obscured, respectively, with L2-10 keV, int < 1039 erg s−1; i.e., genuine LLAGN. Both of the heavily obscured AGN have Lbol > 1041 erg s−1 and λEdd ≳ 10−3, and are detected in high angular resolution mid-IR imaging, indicating the presence of obscuring dust on nuclear scale. NGC 3486 however, is undetected in high-resolution mid-IR imaging, and the current data do not provide stringent constraints on the presence or absence of obscuring nuclear dust in the AGN.

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Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:15 June 2020
Date deposited:23 August 2020
Date of first online publication:27 June 2020
Date first made open access:23 August 2020

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