On the Extremely X-ray Variable Active Galactic Nuclei in the XMM-LSS Field

TL;DR

This study systematically investigates extremely X-ray variable AGNs in the XMM-LSS field, revealing that their variability is mainly due to changing accretion states or obscuration, with some confirmed as changing-look AGNs.

Contribution

It provides the first comprehensive analysis of the origins of extreme X-ray variability in AGNs within this region, including spectroscopic confirmation of changing-look behavior.

Findings

Eight variable AGNs identified with flux density changes of 6-12 times.

Variability attributed to changing accretion states or obscuration by clumpy winds.

Confirmed one changing-look AGN through new spectroscopic data.

Abstract

We present a systematic investigation of extremely X-ray variable active galactic nuclei (AGNs) in the $\approx 5.3~{\rm deg}^2$ XMM-SERVS XMM-LSS region. Eight variable AGNs are identified with rest-frame 2 keV flux density variability amplitudes around 6-12. We comprehensively analyze the X-ray and multiwavelength data to probe the origin of their extreme X-ray variability. It is found that their extreme X-ray variability can be ascribed to changing accretion state or changing obscuration from dust-free absorbers. For five AGNs, their X-ray variability is attributed to changing accretion state, supported by contemporaneous multiwavelength variability and the absence of X-ray absorption in the low-state spectra. With new Multiple Mirror Telescope (MMT) spectra for four of these sources, we confirm one changing-look AGN. One MMT AGN lacks multi-epoch spectroscopic observations, while the other two AGNs do not exhibit changing-look behavior, likely because the MMT observations did not capture their high states. The X-ray variability of the other three AGNs is explained by changing obscuration, and they show only mild long-term optical/IR variability. The absorbers of these sources are likely clumpy accretion-disk winds, with variable column densities and covering factors along the lines of sight.