Abstract
ABSTRACT Spectral-timing techniques have proven valuable in studying the interplay between the X-ray corona and the accretion disc in variable active galactic nuclei (AGNs). Under certain conditions, photoionized outflows emerging from central AGN regions also play a role in the observable spectral-timing properties of the nuclear components. The variable ionizing flux causes the intervening gas to ionize or recombine, resulting in a time-dependent absorption spectrum. Understanding the spectral-timing properties of these outflows is critical not only for the determination of their role in the AGN environment but also for the correct interpretation of timing signatures of other AGN components. In this paper, we test the capabilities of the Athena X-IFU instrument in studying the spectral and spectral-timing properties of a black hole system displaying a variable outflow. We take the narrow-line Seyfert 1 IRAS 13224−3809 as a test case. Our findings show that while the non-linear response of the absorbing medium can result in complex behaviour of time lags, the resulting decrease in the coherence can be used to constrain gas density and distance to the central source. Ultimately, modelling the coherence spectra of AGN outflows may constitute a valuable tool in studying the physical properties of the outflowing gas.
Highlights
The variability of active galactic nuclei (AGN) X-ray emission has been extensively used to study the properties of the innermost regions surrounding the central supermassive black hole
We aim to demonstrate the capabilities of Fourier spectral-timing analysis of AGN outflows, applied to future Athena XIFU observations
The light curves were extracted in bins of width ΔE = 2.5 eV to match the X-ray Integral Field Unit (X-IFU) resolution, in the energy range 0.35 – 10 keV and were all compared to a reference, virtually unabsorbed light curve, extracted from 0.2 to 0.35 keV
Summary
The variability of AGN X-ray emission has been extensively used to study the properties of the innermost regions surrounding the central supermassive black hole. Fourier spectral-timing techniques (see Uttley et al 2014, for a review) have proven invaluable for overcoming this limitation, as they rely on statistical properties of the analysed light curves, rather than a high signal-to-noise ratio in a given time and energy bin. They enable isolation and examination of processes occurring on different time-scales, provided they can be distinguished by the energy at which they are manifested. The use of Fourier techniques allows measurement of delays associated with light-travel time between individual sources of X-rays within the central region, facilitating studies of the geometry and dynamics of the innermost accretion flow (Fabian et al 2009; Zoghbi et al 2012; De Marco et al 2013; Kara et al 2019; Alston et al 2020)
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