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NuSTAR observations of heavily obscured Swift/BAT AGN: constraints on the Compton-thick AGN fraction

Georgantopoulos, I. and Akylas, A. (2019) NuSTAR observations of heavily obscured Swift/BAT AGN: constraints on the Compton-thick AGN fraction. Astronomy and Astrophysics, 621 . Art. No. A28. ISSN 0004-6361. doi:10.1051/0004-6361/201833038.

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The evolution of the accretion history of the Universe has been studied in unprecedented detail owing to recent X-ray surveys performed by Chandra and XMM-Newton. A focus on the most heavily obscured or Compton-thick active galactic nuclei (AGNs) is missing in these studies. These AGNs evade detection even in X-ray surveys owing to their extreme hydrogen column densities, which exceed 10^(24) cm^(−2). Recently, the all-sky hard X-ray survey performed by Swift/BAT brought a breakthrough, allowing the detection of many of these AGNs. This is because of the very high energy bandpass (14–195 keV) of this instrument, which helps to minimise attenuation effects. In our previous work, we identified more than 50 candidate Compton-thick AGNs in the local Universe, corresponding to an observed fraction of about 7% of the total AGNs population. This number can only be converted to the intrinsic Compton-thick AGNs number density if we know their exact selection function. This function sensitively depends on the form of the Compton-thick AGN spectrum, that is the energy of their absorption turnover, photon-index and its cut-off energy at high energies, and the strength of the reflection component on the matter surrounding the nucleus. For example, the reflection component at hard energies 20–40 keV antagonises the number density of missing Compton-thick AGNs in the sense that the stronger the reflection the easier these sources are detected in the BAT band. In order to constrain their number density, we analysed the spectra of 19 Compton-thick AGNs that have been detected with Swift/BAT and have been subsequently observed with NuSTAR in the 3–80 keV band. We analysed their X-ray spectra using the MYTORUS models which properly take into account the Compton scattering effects. These were combined with physically motivated Comptonisation models, which accurately describe the primary coronal X-ray emission. We derived absorbing column densities that are consistent with those derived by the previous Swift/BAT analyses. We estimate the coronal temperatures to be roughly between 25 and 80 keV corresponding to high energy cut-offs roughly between 75 and 250 keV. Furthermore, we find that the majority of our AGNs lack a strong reflection component in the 20–40 keV band placing tighter constraints on the intrinsic fraction of Compton-thick AGNs. Combining these results with our X-ray background synthesis models, we estimate a percentage of Compton-thick AGNs in the local Universe of ≈20 ± 3 % relative to the type-II AGNs population.

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Additional Information:© 2018 ESO. Article published by EDP Sciences. Received 16 March 2018; Accepted 31 August 2018; Published online 03 January 2019. We would like to thank the anonymous referee for comments and suggestions that helped to improve the paper. We acknowledge the use of MYTORUS spectral models by Yaqoob & Murphy (2011). We would also like to thank Tahir Yaqoob for his help and suggestions in the application of the above models. The research leading to these results has received funding from the European Union’s Horizon 2020 Programme under the AHEAD project (grant agreement n. 654215).
Subject Keywords:X-rays: general – galaxies: active – catalogs – quasars: supermassive black holes
Record Number:CaltechAUTHORS:20180912-092926899
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Official Citation:NuSTAR observations of heavily obscured Swift/BAT AGNs: Constraints on the Compton-thick AGNs fraction. I. Georgantopoulos and A. Akylas. A&A, 621 (2019) A28. DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:89558
Deposited By: Joy Painter
Deposited On:12 Sep 2018 18:55
Last Modified:16 Nov 2021 00:36

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