BASS XXXIX: Swift-BAT AGN with changing-look optical spectra

TL;DR

This study investigates changing-look active galactic nuclei (AGN) using optical and X-ray data, revealing that most changes are due to intrinsic accretion variability rather than obscuration, with a rate of 0.7-6.2%.

Contribution

First comprehensive analysis of CL AGN in the Swift-BAT sample combining optical spectroscopy and hard X-ray data, revealing intrinsic accretion variability as the primary cause.

Findings

8 new CL events identified in optical spectra.

21 AGN in BASS exhibit CL behavior, with some linked to X-ray flux changes.

Most CL events are not caused by obscuration, indicating intrinsic accretion changes.

Abstract

Changing-look (CL) AGN are unique probes of accretion onto supermassive black holes (SMBHs), especially when simultaneous observations in complementary wavebands allow investigations into the properties of their accretion flows. We present the results of a search for CL behaviour in 412 Swift-BAT detected AGN with multiple epochs of optical spectroscopy from the BAT AGN Spectroscopic Survey (BASS). 125 of these AGN also have 14-195 keV ultra-hard X-ray light-curves from Swift-BAT which are contemporaneous with the epochs of optical spectroscopy. Eight CL events are presented for the first time, where the appearance or disappearance of broad Balmer line emission leads to a change in the observed Seyfert type classification. Combining with known events from the literature, 21 AGN from BASS are now known to display CL behaviour. Nine CL events have 14-195 keV data available, and five of these CL events can be associated with significant changes in their 14-195 keV flux from BAT. The ultra-hard X-ray flux is less affected by obscuration and so these changes in the 14-195 keV band suggest that the majority of our CL events are not due to changes in line-of-sight obscuration. We derive a CL rate of 0.7-6.2 per cent on 10-25 year time-scales, and show that many transitions happen within at most a few years. Our results motivate further multi-wavelength observations with higher cadence to better understand the variability physics of accretion onto SMBHs.