Kavli Affiliate: Linhua Jiang
| First 5 Authors: Zijian Zhang, Bin Luo, Linhua Jiang, W. N. Brandt, Jian Huang
| Summary:
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.
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