A Physical Model for the UV/Optical Power Spectra of AGN

Panagiotou, Christos; Papadakis, I. E.; Kara, Erin; Kammoun, E. S.; Dovčiak, Michal

doi:10.3847/1538-4357/ac7e4d

Cited by 13 publications

(12 citation statements)

References 49 publications

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“…We conclude that the detection of a moderate X-ray/UV CCF does not necessarily contradict the assumption that the UV/ optical variability is driven by the X-ray illumination of the disk. In fact, our analysis renders this scenario as the most likely explanation of the UV/optical variability since it has also been shown to accurately reproduce the observed UV/optical time lags and power spectra (Kammoun et al 2021b;Panagiotou et al 2022). It should be mentioned, though, that the time lag and power spectra analysis assumed a constant response function.…”

Section: Discussionmentioning

confidence: 57%

“…In the following, we simulated an equidistant X-ray light curve of duration D = 150 days and time binning of Δt = 4 • t g , 7 following the method outlined by Timmer & Koenig (1995). We assumed a power spectral density (PSD) given by a bending power-law with the bending frequency and slopes of the NGC 5548 PSD (Panagiotou et al 2022), as a representative X-ray PSD of nearby Seyfert galaxies (see also Markowitz et al 2003). The X-ray light curve is then divided into 10 time intervals, 8 each corresponding to a different height for the X-ray source (Figure 1).…”

Section: Simulations Setup and Resultsmentioning

confidence: 99%

“…Our study was motivated by the limiting assumptions of the definition of CCF, which are very likely not met in the case of AGN light curves. To that end, we decided to probe the expected CCF in the case of a dynamic X-ray source, as it is physically unlikely for the X-ray source to have a steady-state configuration (see, for example, Panagiotou et al 2022, for recent observational evidence of a variable h X ). In particular, we simulated the cases of a variable source height and of a variable photon index.…”

Section: Discussionmentioning

confidence: 99%

“…Moreover, it was suggested that the observed UV/optical light curves cannot be the result of disk X-ray illumination, given the observed X-ray light curve (e.g., Starkey et al 2017;Gardner & Done 2017). However, it should be noted that previous studies assumed a static configuration of the system, while contrarily, recent advanced investigations of high-quality data found evidence of a dynamic X-ray source (Caballero-García et al 2020;Alston et al 2020;Panagiotou et al 2022).…”

Section: Introductionmentioning

confidence: 99%

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Explaining the Moderate UV/X-Ray Correlation in AGN

Panagiotou

Kara

Dovčiak

2022

ApJ

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The UV/optical and X-ray variability of active galactic nuclei (AGN) have long been expected to be well correlated as a result of the X-ray illumination of the accretion disk. Recent monitoring campaigns of nearby AGN, however, found that their X-ray and UV/optical emission are only moderately correlated, challenging the aforementioned paradigm. In this work, we aim to demonstrate that due to the definition of the cross-correlation function, a low UV/X-ray correlation is well expected in the case of an X-ray illuminated accretion disk, when the dynamic variability of the X-ray source is taken into account. In particular, we examine how the variability of the geometric or physical configuration of the X-ray source affects the expected correlation. Variations of the geometric configuration are found to produce a range of UV/X-ray cross correlations, which match well the observed values, while they result in a high correlation between the UV and optical variability, reconciling the observed results with theoretical predictions. We conclude that the detection of a low UV/X-ray correlation does not contradict the assumption of the UV/optical variability being driven by the X-ray illumination of the disk, and we discuss the implications of our results for correlation studies.

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Section: Discussionmentioning

confidence: 57%

Section: Simulations Setup and Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Explaining the Moderate UV/X-Ray Correlation in AGN

Panagiotou

Kara

Dovčiak

2022

ApJ

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“…However, the variability seen in the drop is similar to standard AGN behavior, whereby the strongest variability is in X-rays and decreases with increasing wavelength. This sort of behavior can be explained by the standard reprocessing scenario, whereby X-rays illuminate the disk and imprint their variability on short timescales, which gets dampened by reprocessing at longer wavelengths (e.g., Edelson et al 1996;Cackett et al 2007;Panagiotou et al 2022).…”

Section: Optical/uv Variability During Nicer Coveragementioning

confidence: 99%

Unusual Hard X-Ray Flares Caught in NICER Monitoring of the Binary Supermassive Black Hole Candidate AT2019cuk/Tick Tock/SDSS J1430+2303

Masterson

Kara

Pasham

et al. 2023

ApJL

Self Cite

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The nuclear transient AT2019cuk/Tick Tock/SDSS J1430+2303 has been suggested to harbor a supermassive black hole (SMBH) binary near coalescence. We report results from high-cadence NICER X-ray monitoring with multiple visits per day from 2022 January to August, as well as continued optical monitoring during the same time period. We find no evidence of periodic/quasiperiodic modulation in the X-ray, UV, or optical bands; however, we do observe exotic hard X-ray variability that is unusual for typical active galactic nuclei (AGN). The most striking feature of the NICER light curve is repetitive hard (2–4 keV) X-ray flares that result in distinctly harder X-ray spectra compared to the nonflaring data. In its nonflaring state, AT2019cuk looks like a relatively standard AGN, but it presents the first case of day-long, hard X-ray flares in a changing-look AGN. We consider a few different models for the driving mechanism of these hard X-ray flares, including (1) corona/jet variability driven by increased magnetic activity, (2) variable obscuration, and (3) self-lensing from the potential secondary SMBH. We prefer the variable corona model, as the obscuration model requires rather contrived timescales and the self-lensing model is difficult to reconcile with a lack of clear periodicity in the flares. These findings illustrate how important high-cadence X-ray monitoring is to our understanding of the rapid variability of the X-ray corona and necessitate further high-cadence, multiwavelength monitoring of changing-look AGN like AT2019cuk to probe the corona-jet connection.

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X-ray reverberation as an explanation for UV/optical variability in nearby Seyferts

Papoutsis,

Papadakis,

Panagiotou

et al. 2024

A&A

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Context. Active galactic nuclei (AGNs) are known to be variable across all wavelengths. Significant observational efforts have been invested in the last decade in studying their ultraviolet (UV) and optical variability. Long and densely sampled, multi-wavelength monitoring campaigns of numerous Seyfert galaxies have been conducted with the aim of determining the X-ray/UV/optical continuum time lags. Time-lag studies can be used to constrain theoretical models. The observed time lags can be explained by thermal reprocessing of the X-rays illuminating the accretion disc (known as the X-ray reverberation model). However, the observed light curves contain more information that can be used to further constrain physical models. Aims. Our primary objective is to investigate whether, in addition to time lags, the X-ray reverberation model can also explain the UV/optical variability amplitude of nearby Seyferts. Methods. We measured the excess variance of four sources (namely Mrk 509, NGC 4151, NGC 2617, and Mrk 142) as a function of wavelength using data from archival long, multi-wavelength campaigns with Swift, and ground-based telescopes. We also computed the model excess variance in the case of the X-ray reverberation model by determining the disc’s transfer function and assuming a bending power law for the X-ray power spectrum. We tested the validity of the model by comparing the measured and model variances for a range of accretion rates and X-ray source heights. Results. Our main result is that the X-ray thermal reverberation model can fit both the continuum, UV/optical time lags, as well as the variance (i.e. the variability amplitude) in these AGNs, for the same physical parameters. Our results suggest that the accretion disc is constant and that all the observed UV/optical variations, on timescales of days and up to a few weeks, can be fully explained by the variable X-rays as they illuminate the accretion disc.

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A Physical Model for the UV/Optical Power Spectra of AGN

Cited by 13 publications

References 49 publications

Explaining the Moderate UV/X-Ray Correlation in AGN

Explaining the Moderate UV/X-Ray Correlation in AGN

Unusual Hard X-Ray Flares Caught in NICER Monitoring of the Binary Supermassive Black Hole Candidate AT2019cuk/Tick Tock/SDSS J1430+2303

X-ray reverberation as an explanation for UV/optical variability in nearby Seyferts

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