Explaining the Moderate UV/X-Ray Correlation in AGN

Panagiotou, Christos; Kara, Erin; Dovčiak, Michal

doi:10.3847/1538-4357/aca2a4

Cited by 13 publications

(6 citation statements)

References 29 publications

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“…Similarly, observations of other objects also show that the X-ray to UV correlation is typically weaker than the UV to optical correlation (e.g., Edelson et al 2019;Cackett et al 2020;Hernández Santisteban et al 2020). Dynamic variability in the X-ray source can potentially explain the moderate X-ray to UV correlation (Panagiotou et al 2022). However, in some cases the X-ray and UV are not correlated at all (e.g., Schimoia et al 2015;Buisson et al 2018;Morales et al 2019), presenting a challenge to the simplest reprocessing scenario.…”

Section: Introductionmentioning

confidence: 93%

AGN STORM 2. IV. Swift X-Ray and Ultraviolet/Optical Monitoring of Mrk 817

Cackett,

Gelbord,

Barth

et al. 2023

ApJ

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The AGN STORM 2 campaign is a large, multiwavelength reverberation mapping project designed to trace out the structure of Mrk 817 from the inner accretion disk to the broad emission line region and out to the dusty torus. As part of this campaign, Swift performed daily monitoring of Mrk 817 for approximately 15 months, obtaining observations in X-rays and six UV/optical filters. The X-ray monitoring shows that Mrk 817 was in a significantly fainter state than in previous observations, with only a brief flare where it reached prior flux levels. The X-ray spectrum is heavily obscured. The UV/optical light curves show significant variability throughout the campaign and are well correlated with one another, but uncorrelated with the X-rays. Combining the Swift UV/optical light curves with Hubble Space Telescope UV continuum light curves, we measure interband continuum lags, τ(λ), that increase with increasing wavelength roughly following τ(λ) ∝ λ 4/3, the dependence expected for a geometrically thin, optically thick, centrally illuminated disk. Modeling of the light curves reveals a period at the beginning of the campaign where the response of the continuum is suppressed compared to later in the light curve—the light curves are not simple shifted and scaled versions of each other. The interval of suppressed response corresponds to a period of high UV line and X-ray absorption, and reduced emission line variability amplitudes. We suggest that this indicates a significant contribution to the continuum from the broad-line region gas that sees an absorbed ionizing continuum.

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Section: Introductionmentioning

confidence: 93%

AGN STORM 2. IV. Swift X-Ray and Ultraviolet/Optical Monitoring of Mrk 817

Cackett,

Gelbord,

Barth

et al. 2023

ApJ

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“…If the corona is extended out to ∼100 R g , the X-ray source will preferentially irradiate larger radii in the disk, making the response functions wider, and thus the UV-optical time lags longer (see Equation (1) of Kammoun et al 2021 for more details). Recently, these authors modeled the X-ray-UVOIR lags of several sources, and Panagiotou et al (2022) modeled both the time lags and power spectra in NGC 5548 with general relativistic ray-tracing simulations (Dovčiak et al 2022), and constrained the height of the corona to be somewhere between ∼7-70 R g for previously studied AGN. 31 An important caveat here is that the optical reverberation lags, from which this mass was derived, were measured with respect to the 5100 Å continuum, assuming it all originates in the accretion disk.…”

Section: On the Very Long X-ray-uvoir Lagsmentioning

confidence: 99%

UV–Optical Disk Reverberation Lags despite a Faint X-Ray Corona in the Active Galactic Nucleus Mrk 335

Kara

Barth

Cackett

et al. 2023

ApJ

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We present the first results from a 100-day Swift, NICER, and ground-based X-ray–UV–optical reverberation mapping campaign of the Narrow-line Seyfert 1 Mrk 335, when it was in an unprecedented low X-ray flux state. Despite dramatic suppression of the X-ray variability, we still observe UV–optical lags as expected from disk reverberation. Moreover, the UV–optical lags are consistent with archival observations when the X-ray luminosity was >10 times higher. Interestingly, both low- and high-flux states reveal UV–optical lags that are 6–11 times longer than expected from a thin disk. These long lags are often interpreted as due to contamination from the broad line region; however the u-band excess lag (containing the Balmer jump from the diffuse continuum) is less prevalent than in other active galactic nuclei. The Swift campaign showed a low X-ray-to-optical correlation (similar to previous campaigns), but NICER and ground-based monitoring continued for another 2 weeks, during which the optical rose to the highest level of the campaign, followed ∼10 days later by a sharp rise in X-rays. While the low X-ray countrate and relatively large systematic uncertainties in the NICER background make this measurement challenging, if the optical does lead X-rays in this flare, this indicates a departure from the zeroth-order reprocessing picture. If the optical flare is due to an increase in mass accretion rate, this occurs on much shorter than the viscous timescale. Alternatively, the optical could be responding to an intrinsic rise in X-rays that is initially hidden from our line of sight.

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“…In addition, AGN disks with scattering atmospheres may help account for the longer-than-anticipated lag timescales of recent AGN reverberation mapping campaigns (Hall et al 2018). However, other models, such as those in Panagiotou et al (2022a), suggest that the canonical reprocessing of X-rays through absorption can also produce longer lag timescales and poorly correlated X-ray and UV light curves, like those observed for NGC 5548. In addition, observations using X-ray spectroscopy suggest that AGN disks may not be fully ionized or scattering dominated (e.g., García et al 2019).…”

Section: Hard X-rays Have Little Impact On Uv Emissionmentioning

confidence: 97%

“…This correlation agrees with observational results that suggest that X-ray and UV light curves are not as well correlated as we might expect if X-rays are the main driver of UV and optical variability as in traditional reprocessing models (e.g., Schimoia et al 2015;Edelson et al 2019;Cackett et al 2023). However, canonical reprocessing models could produce lower X-ray to UV correlations if the X-ray light curve is nonstationary, for example due to changes in the height or temperature of the corona on timescales shorter than the observing campaign (Panagiotou et al 2022a).…”

Section: Summary and Future Workmentioning

confidence: 99%

“…However, there is significant evidence for additional variability in UV-optical light curves that does not appear to come from X-ray light curves (e.g., Uttley et al 2003;Arévalo et al 2009;Edelson et al 2019;Yu et al 2022;Cackett et al 2023;Hagen & Done 2023;Kara et al 2023). The lack of correlation between X-ray and UV-optical light curves could be due to absorption by intervening material (e.g., Kara et al 2021) or changes in the height and temperature of the corona on timescales shorter than the observing timescale (Panagiotou et al 2022a). Another alternative is that this variability could come from fluctuations in the UV-optical regions of the accretion disk due to magnetorotational instability (MRI)driven turbulence (Balbus & Hawley 1991) or convection resulting from the enhanced opacity in the UV-optical region of the disk (Jiang & Blaes 2020).…”

Section: Introductionmentioning

confidence: 99%

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Simulating X-Ray Reverberation in the Ultraviolet-emitting Regions of Active Galactic Nuclei Accretion Disks with Three-dimensional Multifrequency Radiation Magnetohydrodynamic Simulations

Secunda,

Jiang 姜,

Greene

2024

ApJL

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Active galactic nuclei (AGN) light curves observed with different wave bands show that the variability in longer wavelength bands lags the variability in shorter wavelength bands. Measuring these lags, or reverberation mapping, is used to measure the radial temperature profile and extent of AGN disks, typically with a reprocessing model that assumes X-rays are the main driver of the variability in other wavelength bands. To demonstrate how this reprocessing works with realistic accretion disk structures, we use 3D local shearing box multifrequency radiation magnetohydrodynamic simulations to model the UV-emitting region of an AGN disk, which is unstable to the magnetorotational instability and convection. At the same time, we inject hard X-rays (>1 keV) into the simulation box to study the effects of X-ray irradiation on the local properties of the turbulence and the resulting variability of the emitted UV light curve. We find that disk turbulence is sufficient to drive intrinsic variability in emitted UV light curves and that a damped random walk model is a good fit to this UV light curve for timescales >5 days. Meanwhile, X-ray irradiation has negligible impact on the power spectrum of the emitted UV light curve. Furthermore, the injected X-ray and emitted UV light curves are only correlated if there is X-ray variability on timescales >1 day, in which case we find a correlation coefficient r = 0.34. These results suggest that if the opacity for hard X-rays is scattering dominated as in the standard disk model, hard X-rays are not the main driver of reverberation signals.

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Explaining the Moderate UV/X-Ray Correlation in AGN

Cited by 13 publications

References 29 publications

AGN STORM 2. IV. Swift X-Ray and Ultraviolet/Optical Monitoring of Mrk 817

AGN STORM 2. IV. Swift X-Ray and Ultraviolet/Optical Monitoring of Mrk 817

UV–Optical Disk Reverberation Lags despite a Faint X-Ray Corona in the Active Galactic Nucleus Mrk 335

Simulating X-Ray Reverberation in the Ultraviolet-emitting Regions of Active Galactic Nuclei Accretion Disks with Three-dimensional Multifrequency Radiation Magnetohydrodynamic Simulations

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