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

TL;DR

This study provides a comprehensive multiwavelength analysis of unobscured AGN, revealing correlations between luminosities, spectral indices, and bolometric corrections, and offering new relations to estimate bolometric luminosity from limited data.

Contribution

It presents one of the largest simultaneous optical-to-X-ray SEDs of local unobscured AGN, with detailed modeling and new relations for bolometric corrections based on physical properties.

Findings

Confirmed tight UV-X-ray luminosity correlation.

Found $ ext{α}_{ox}$ decreases with $L_{bol}$ and $ ext{λ}_{Edd}$.

Derived second-order relation between 2-10 keV BC and $ ext{α}_{ox}$.

Abstract

We present one of the largest multiwavelength studies of simultaneous optical-to-X-ray spectral energy distributions (SEDs) of unobscured 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 total bolometric luminosities ($L_{bol}$), optical-to-X-ray spectral indices ($\alpha_{ox}$), and multiple bolometric corrections (BCs) in the optical, UV, and X-rays. We used black hole masses obtained by reverberation mapping and the virial method to estimate Eddington ratios ($\lambda_{Edd}$) for all our AGN. We confirm the tight correlation between UV and X-ray luminosity for our sample. We observe a significant decrease in $\alpha_{ox}$ with $L_{bol}$ and $\lambda_{Edd}$, suggesting that brighter sources emit more UV photons per X-rays. We report a second-order regression relation between the 2-10 keV BC and $\alpha_{ox}$, which is useful to compute $L_{bol}$ in the absence of multiband SEDs. We also investigate the dependence of optical/UV BCs on the physical properties of AGN and obtain a significant increase in the UV BCs with $L_{bol}$ and $\lambda_{Edd}$, unlike those in the optical, which are constant across five orders of $L_{bol}$ and $\lambda_{Edd}$. We obtain significant dispersions (~0.1-1 dex) in all BCs, and hence recommend using appropriate relations with observed quantities while including the reported scatter, instead of their median values.