BASS

XLIII. Optical, UV, and X-ray emission properties of unobscured Swift/BAT active galactic nuclei

¹ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
² STAR Institute, Liège Université, Quartier Agora - Allée du six Août, 19c, B-4000 Liège, Belgium
³ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, B-9000 Gent, Belgium
⁴ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
⁵ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, I-00078 Monte Porzio Catone, Italy
⁶ Eureka Scientific, 2452 Delmer Street Suite 100, Oakland, CA 94602-3017, USA
⁷ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA
⁸ Instituto de Astrofísica and Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile, Campus San Joaquin, Av. Vicuña Mackenna 4860, Macul Santiago 7820436, Chile
⁹ Millennium Institute of Astrophysics, Nuncio Monseñor Sótero Sanz 100, Of 104, Providencia, Santiago, Chile
¹⁰ Department of Astronomy and Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA
¹¹ Dipartimento di Matematica e Fisica, Universita Roma Tre, via della Vasca Navale 84, I-00146 Roma, Italy
¹² Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
¹³ Centro de Astronomía (CITEVA), Universidad de Antofagasta, Avenida Angamos 601, Antofagasta, Chile
¹⁴ School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
¹⁵ Joint ALMA Observatory, Avenida Alonso de Cordova 3107, Vitacura, 7630355 Santiago, Chile
¹⁶ Korea Astronomy & Space Science institute, 776, Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea
¹⁷ Department of Earth and Space Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
¹⁸ 6 RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
¹⁹ Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA
²⁰ Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, CA 94305, USA
²¹ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, MS 169-224, Pasadena, CA 91109, USA
²² Yale Center for Astronomy & Astrophysics, Physics Department, PO Box 208120 New Haven CT 06520-8120, USA
²³ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
²⁴ Astrophysics Science Division, NASA Goddard Space Flight Center, Mail Code 661, Greenbelt, MD 20771, USA

^⋆ Corresponding author; kkgupta@uliege.be

Received: 30 April 2024
Accepted: 4 September 2024

Abstract

We present one of the largest multiwavelength studies of simultaneous optical-to-X-ray spectral energy distributions (SEDs) of unobscured (N_H < 10²² cm⁻²) active galactic nuclei (AGN) in the local Universe. Using a representative sample of hard-X-ray-selected AGN from the 70-month Swift/BAT catalog, with optical/UV photometric data from Swift/UVOT and X-ray spectral data from Swift/XRT, we constructed broadband SEDs of 236 nearby AGN (0.001 < z < 0.3). We employed GALFIT to estimate host galaxy contamination in the optical/UV and determine the intrinsic AGN fluxes. We used an absorbed power law with a reflection component to model the X-ray spectra and a dust-reddened multi-temperature blackbody to fit the optical/UV SED. We calculated intrinsic luminosities at multiple wavelengths, total bolometric luminosities (L_bol), optical-to-X-ray spectral indices (α_ox), and multiple bolometric corrections (κ_λ) in the optical, UV, and X-rays. We used black hole masses obtained by reverberation mapping and the virial method to estimate Eddington ratios (λ_Edd) for all our AGN. We confirm the tight correlation (scatter = 0.45 dex) between UV (2500 Å) and X-ray (2 keV) luminosity for our sample. We observe a significant decrease in α_ox with L_bol and λ_Edd, suggesting that brighter sources emit more UV photons per X-rays. We report a second-order regression relation (scatter = 0.15 dex) between the 2–10 keV bolometric correction (κ_2 − 10) and α_ox, which is useful to compute L_bol in the absence of multiband SEDs. We also investigate the dependence of optical/UV bolometric corrections on the physical properties of AGN and obtain a significant increase in the UV bolometric corrections (κ_W2 and κ_M2) with L_bol and λ_Edd, unlike those in the optical (κ_V and κ_B), which are constant across five orders of L_bol and λ_Edd. We obtain significant dispersions (∼0.1–1 dex) in all bolometric corrections, and hence recommend using appropriate relations with observed quantities while including the reported scatter, instead of their median values.