Probing the Structure and Evolution of BASS Active Galactic Nuclei through Eddington Ratios

Creators: Ananna, Tonima Tasnim¹; Urry, C. Megan²; Ricci, Claudio^{3, 4}; Natarajan, Priyamvada^{2, 5}; Hickox, Ryan C.¹; Trakhtenbrot, Benny⁶; Treister, Ezequiel⁷; Weigel, Anna K.⁸; Ueda, Yoshihiro⁹; Koss, Michael J.^{10, 11}; Bauer, F. E.^{7, 12}; Temple, Matthew J.³; Baloković, Mislav²; Mushotzky, Richard¹³; Auge, Connor¹⁴; Sanders, David B.¹⁴; Kakkad, Darshan¹⁵; Sartori, Lia F.¹⁶; Marchesi, Stefano^{17, 18}; Harrison, Fiona A.¹⁹; Stern, Daniel²⁰; Oh, Kyuseok^{9, 21}; Caglar, Turgay²²; Powell, Meredith C.²³; Podjed, Stephanie A.¹; Mejía-Restrepo, Julian E.²⁴

1. Dartmouth College
2. Yale University
3. Diego Portales University
4. Peking University
5. Harvard University
6. Tel Aviv University
7. Pontificia Universidad Católica de Chile
8. Modulos AG, Technoparkstrasse 1, CH-8005 Zurich, Switzerland
9. Kyoto University
10. Eureka Scientific
11. Space Science Institute
12. Millennium Institute of Astrophysics
13. University of Maryland, College Park
14. University of Hawaii at Manoa
15. Space Telescope Science Institute
16. ETH Zurich
17. INAF—Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti, 93/3, 40129, Bologna, Italy
18. Clemson University
19. California Institute of Technology
20. Jet Propulsion Lab
21. Korea Astronomy and Space Science Institute
22. Leiden University
23. Stanford University
24. European Southern Observatory

Abstract

We constrain the intrinsic Eddington ratio (λ Edd) distribution function for local active galactic nuclei (AGN) in bins of low and high obscuration [log (N_H / cm⁻²) ⩽ 22 and 22 < log (N_H / cm⁻²) < 25], using the Swift Burst Alert Telescope 70 month/BASS DR2 survey. We interpret the fraction of obscured AGN in terms of circumnuclear geometry and temporal evolution. Specifically, at low Eddington ratios (log λ_(Edd) < −2), obscured AGN outnumber unobscured ones by a factor of ∼4, reflecting the covering factor of the circumnuclear material (0.8, or a torus opening angle of ∼34°). At high Eddington ratios (log λ_(Edd) > −1), the trend is reversed, with <30% of AGN having log (N_H / cm⁻²) > 22 , which we suggest is mainly due to the small fraction of time spent in a highly obscured state. Considering the Eddington ratio distribution function of narrow-line and broad-line AGN from our prior work, we see a qualitatively similar picture. To disentangle temporal and geometric effects at high λ Edd, we explore plausible clearing scenarios such that the time-weighted covering factors agree with the observed population ratio. We find that the low fraction of obscured AGN at high λ_(Edd) is primarily due to the fact that the covering factor drops very rapidly, with more than half the time spent with <10% covering factor. We also find that nearly all obscured AGN at high-λ_(Edd) exhibit some broad lines. We suggest that this is because the height of the depleted torus falls below the height of the broad-line region, making the latter visible from all lines of sight.

Copyright and License

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

We thank the referee for the thoughtful comments that helped us greatly improve this Letter. This Letter is part of a series presented by the BASS Collaboration. Specifically, this is BASS XXXVIII.

Funding

T.T.A. acknowledges support from NASA through ADAP award NNH22ZDA001N. T.T.A. and R.C.H. acknowledge support from NASA through ADAP award 80NSSC19K0580, and the National Science Foundation through CAREER award 1554584. C.M.U. acknowledges support from the National Science Foundation under grant No. AST-1715512, and from NASA through ADAP award 80NSSC18K0418. P.N. acknowledges the black hole Initiative (BHI) at Harvard University, which is supported by grants from the Gordon and Betty Moore Foundation and the John Templeton Foundation. B.T. acknowledges support from the Israel Science Foundation (grant No. 1849/19) and from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No. 950533). M.K. acknowledges support from NASA through ADAP award NNH16CT03C. M.B. acknowledges support from the YCAA Prize Postdoctoral Fellowship. K.O. acknowledges the support of the Korea Astronomy and Space Science Institute under the R&D program (Project No. 2022-1-830-06) supervised by the Ministry of Science and ICT and from the National Research Foundation of Korea (NRF-2020R1C1C1005462). We acknowledge funding support through ANID programs: Millennium Science Initiative NCN19\_058 (E.T.), ICN12\_009 (FEB); CATA-BASAL - ACE210002 (E.T., F.E.B.), and FB210003 (E.T., F.E.B., C.R.); FONDECYT Regular - 1190818 (E.T., F.E.B.) and 1200495 (F.E.B., E.T.); and FONDECYT Iniciacion 11190831 (C.R.). This work was performed in part at the Aspen Center for Physics, which is supported by National Science Foundation grant PHY-1607611.

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