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NuSTAR Observations of Four Nearby X-ray Faint AGN: Low Luminosity or Heavy Obscuration?

Annuar, A. and Alexander, D. M. and Gandhi, P. and Lansbury, G. B. and Asmus, D. and Baloković, M. and Ballantyne, D. R. and Bauer, F. E. and Boorman, P. G. and Brandt, W. N. and Brightman, M. and Chen, C.-T. J. and Del Moro, A. and Farrah, D. and Harrison, F. A. and Koss, M. J. and Lanz, L. and Marchesi, S. and Masini, A. and Nardini, E. and Ricci, C. and Stern, D. and Zappacosta, L. (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. ISSN 0035-8711.

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We present NuSTAR (Nuclear Spectroscopic Telescope Array) observations of four active galactic nuclei (AGNs) located within 15 Mpc. These AGNs, namely ESO 121-G6, NGC 660, NGC 3486, and NGC 5195, have observed X-ray luminosities of L_(2–10 keV,obs) ≲ 10³⁹ erg s⁻¹, classifying them as low-luminosity AGN (LLAGN). We perform broad-band X-ray spectral analysis for the AGN by combining our NuSTAR data with Chandra or XMM–Newton observations to directly measure their column densities (N_H) 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 (N_H > 10²³ cm⁻²; L_(2–10 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 L_(2–10 keV,int) < 10³⁹ erg s⁻¹, i.e. genuine LLAGN. Both of the heavily obscured AGNs have L_(bol) > 10⁴¹ erg s⁻¹ and λ_(Edd) ≳ 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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Annuar, A.0000-0003-0387-1429
Alexander, D. M.0000-0002-5896-6313
Gandhi, P.0000-0003-3105-2615
Lansbury, G. B.0000-0002-5328-9827
Asmus, D.0000-0003-0220-2063
Baloković, M.0000-0003-0476-6647
Ballantyne, D. R.0000-0001-8128-6976
Bauer, F. E.0000-0002-8686-8737
Boorman, P. G.0000-0001-9379-4716
Brandt, W. N.0000-0002-0167-2453
Brightman, M.0000-0002-8147-2602
Chen, C.-T. J.0000-0002-4945-5079
Farrah, D.0000-0003-1748-2010
Harrison, F. A.0000-0003-2992-8024
Koss, M. J.0000-0002-7998-9581
Lanz, L.0000-0002-3249-8224
Marchesi, S.0000-0001-5544-0749
Masini, A.0000-0002-7100-9366
Nardini, E.0000-0001-9226-8992
Ricci, C.0000-0001-5231-2645
Stern, D.0000-0003-2686-9241
Zappacosta, L.0000-0002-4205-6884
Additional Information:© 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Accepted 2020 June 15. Received 2020 June 8; in original form 2020 March 23. Published: 27 June 2020. We acknowledge financial support from the Ministry of Higher Education Malaysia Fundamental Research Grant Scheme grant code FRGS/1/2019/STG02/UKM/02/7 (AA), the Science and Technology Facilities Council grant codes ST/P000541/1 and ST/T000244/1 (DMA), and ST/R000506/1 (PG). FEB acknowledges support from the CONICYT grant CATA-Basal AFB-170002, FONDECYT Regular 1190818 and 1200495, and Chile’s Ministry of Economy, Development, and Tourism’s Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS. DA acknowledges funding through the European Union’s Horizon 2020 and Innovation programme under the Marie Sklodowska-Curie grant agreement no. 793499 (DUSTDEVILS). MB acknowledges support from the Black Hole Initiative at Harvard University, which is funded in part by the Gordon and Betty Moore Foundation (grant GBMF8273) and in part by the John Templeton Foundation. CR acknowledges support from the CONICYT+PAI Convocatoria Nacional subvencion a instalacion en la academia convocatoria año 2017 PAI77170080, and Fondecyt Iniciacion grant 11190831. PB acknowledges financial support from the STFC and the Czech Science Foundation project no. 19-05599Y. EN acknowledges financial contribution from the agreement ASI-INAF n.2017-14-H.0 and partial support from the EU Horizon 2020 Marie Skłodowska-Curie grant agreement no. 664931. NuSTAR is a project led by the California Institute of Technology (Caltech), managed by the Jet Propulsion Laboratory (JPL), and funded by the National Aeronautics and Space Administration (NASA). We thank the NuSTAR Operations, Software and Calibrations teams for support with these observations. This research has made use of the NuSTAR Data Analysis Software (NUSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). The scientific results reported in this article are based on observations made by the Chandra X-ray Observatory and data obtained from the Chandra Data Archive. This research has made use of software provided by the Chandra X-ray Center (CXC) in the application packages CIAO. This work was also based on observations obtained with XMM–Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. Besides these, we also used observations collected at the European Organisation for Astronomical Research in the Southern hemisphere under ESO programme 0101.B-0386(A). This research made use of Astropy,12 a community-developed core Python package for Astronomy (Astropy Collaboration 2013; Astropy Collaboration 2018). We also used data obtained through the High Energy Astrophysics Science Archive Research Center (HEASARC) Online Service, provided by the NASA/Goddard Space Flight Center, and the NASA/IPAC extragalactic Database (NED) operated by JPL, Caltech under contract with NASA. Facilities: Chandra, Gemini, NuSTAR, Swift, VLT, WISE, and XMM–Newton. Data availability: The data used in this paper are publicly available to access and download as follows: i. X-ray data a. From NASA’s High Energy Astrophysics Science Archive Research Center ( Details of the observations, including the observation identification numbers, are listed in Table 2. ii. Mid-IR data a. ESO121-G6: From the Gemini Observatory Archive (; Program ID: GS-2010B-Q-3; PI: F. Bauer. b. NGC 660: From the European Southern Observatory Science Archive Facility (; Program ID: 0101.B-0386(A); PI: A. Annuar.
Group:Astronomy Department, NuSTAR, Space Radiation Laboratory
Funding AgencyGrant Number
Ministry of Education (Malaysia)FRGS/1/2019/STG02/UKM/02/7
Science and Technology Facilities Council (STFC)ST/P000541/1
Science and Technology Facilities Council (STFC)ST/T000244/1
Science and Technology Facilities Council (STFC)ST/R000506/1
Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT)1190818
Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT)1200495
Iniciativa Científica Milenio del Ministerio de Economía, Fomento y TurismoIC120009
Marie Curie Fellowship793499
Harvard UniversityUNSPECIFIED
Gordon and Betty Moore FoundationGBMF8273
John Templeton FoundationUNSPECIFIED
Comisión Nacional de Investigación Científica y Tecnológica (CONICYT)2017 PAI77170080
Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT)11190831
Czech Science Foundation19-05599Y
Agenzia Spaziale Italiana (ASI)2017-14-H.0
Marie Curie Fellowship664931
Subject Keywords:galaxies: active – X-rays: individual: ESO 121-G6 –X-rays: individual:NGC660 –X-rays: individual:NGC3486 – X-rays: individual: NGC 5195
Issue or Number:1
Record Number:CaltechAUTHORS:20200714-115518491
Persistent URL:
Official Citation:A Annuar, D M Alexander, P Gandhi, G B Lansbury, D Asmus, M Baloković, D R Ballantyne, F E Bauer, P G Boorman, W N Brandt, M Brightman, C-T J Chen, A Del Moro, D Farrah, F A Harrison, M J Koss, L Lanz, S Marchesi, A Masini, E Nardini, C Ricci, D Stern, L Zappacosta, NuSTAR Observations of Four Nearby X-ray Faint AGN: Low Luminosity or Heavy Obscuration?, Monthly Notices of the Royal Astronomical Society, 497(1); 2020; 229-245,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:104379
Deposited By: Tony Diaz
Deposited On:14 Jul 2020 19:49
Last Modified:31 Jul 2020 16:09

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