Optical--to--X-ray emission in low-absorption AGN: Results from the Swift-BAT 9 month catalogue

TL;DR

This study presents simultaneous optical-to-X-ray spectral energy distributions for 26 low-redshift, low-absorption AGN from the Swift-BAT survey, revealing low accretion rates and bolometric corrections, and providing insights into AGN accretion history.

Contribution

First comprehensive SED analysis of low-redshift, low-absorption AGN from Swift-BAT, correcting for host contamination and dust extinction to better understand intrinsic accretion properties.

Findings

Low Eddington ratios (<0.1) for most AGN

Bolometric corrections cluster at 10-20

No correlation between optical-X-ray spectral index and Eddington ratio

Abstract

(Abridged) We present simultaneous optical--to--X-ray spectral energy distributions (SEDs) from Swift's X-ray and UV--optical telescopes (XRT and UVOT) for a well-selected sample of 26 low-redshift (z<0.1) AGN from the Swift/BAT 9-month catalogue, the largest well-studied, hard X-ray selected survey of local AGN to date. Our subsample consists of AGN with low intrinsic X-ray absorption (N_H<10^22 cm^-2) and minimal spectral complexity, to more accurately recover the intrinsic accretion luminosity in these sources. We perform a correction for host galaxy contamination in all available UVOT filter images to recover the intrinsic AGN emission, and estimate intrinsic dust extinction from the resultant nuclear SEDs. Black hole mass estimates are determined from the host-galaxy 2MASS K-band bulge luminosity. Accretion rates determined from our SEDs are on average low (Eddington ratios <~ 0.1) and hard X-ray bolometric corrections cluster at ~10-20, in contrast with the higher values seen for quasars. An average SED for the 22 low accretion rate (Eddington ratio < 0.1) objects is presented, with and without correction for intrinsic extinction. We do not find a correlation of optical--to--X-ray spectral index with Eddington ratio, regardless of the optical reference wavelength chosen for defining the spectral index. The low accretion rates and bolometric corrections found for this representative low-redshift sample are of particular importance for studies of AGN accretion history.