Kavli Affiliate: Steven W. Allen
| First 5 Authors: Zhefu Yu, Zhefu Yu, , ,
| Summary:
X-ray reverberation, which exploits the time delays between variability in
different energy bands as a function of Fourier frequency, probes the structure
of the inner accretion disks and X-ray coronae of active galactic nuclei. We
present a systematic X-ray spectroscopic and reverberation study of the
high-Eddington-ratio narrow-line Seyfert 1 galaxy Ark 564, using over 900 ks of
textitXMM-Newton and textitNuSTAR observations spanning 13 years. The
time-averaged spectra can be well described by the a model consisting of a
coronal continuum, relativistic disk reflection, warm Comptonization, and three
warm absorbers. Leveraging the high X-ray brightness of Ark 564, we are able to
resolve the time evolution of the spectra and contemporaneous reverberation
lags. The soft-band lag relative to the continuum increases with the X-ray
flux, while Fe K$alpha$ lags are detected in only a subset of epochs and do
not correlate with soft lags. Models based on a lamppost corona and reflection
from a standard thin disk can broadly reproduce the observed lag-energy spectra
of low-flux epochs; however, additional reverberation from the warm Comptonized
atmosphere is required to explain the soft lags observed in high-flux epochs. A
vertically puffed-up inner disk and a variable, vertically extended corona can
better explain the observed evolution of the lags and covariance spectra. Our
study underscores the importance of multi-epoch, multi-band analyses for a
comprehensive understanding the inner accretion disk and corona.
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