Continuum Reverberation in Active Galactic Nuclei Disks Only With Sufficient X-ray Luminosity and Low Albedo (2501.06304v1)

Abstract: Disk continuum reverberation mapping is one of the primary ways we learn about active galactic nuclei (AGN) accretion disks. Reverberation mapping assumes that time-varying X-rays incident on the accretion disk drive variability in UV-optical light curves emitted by AGN disks, and uses lags between X-ray and UV-optical variability on the light-crossing timescale to measure the radial temperature profile and extent of AGN disks. However, recent reverberation mapping campaigns have revealed oddities in some sources such as weakly correlated X-ray and UV light curves, longer than anticipated lags, and evidence of intrinsic variability from disk fluctuations. To understand how X-ray reverberation works with realistic accretion disk structures, we perform 3D multi-frequency radiation magnetohydrodynamic simulations of X-ray reprocessing by the UV-emitting region of an AGN disk using sophisticated opacity models that include line opacities for both the X-ray and UV radiation. We find there are two important factors that determine whether X-ray irradiation and UV emission will be well-correlated, the ratio of X-ray to UV luminosity and significant absorption. When these factors are met, the reprocessing of X-rays into UV is nearly instantaneous, as is often assumed, although linear reprocessing models are insufficient to fully capture X-ray reprocessing in our simulations. Nevertheless, we can still easily recover mock lags in our light curves using software that assumes linear reprocessing. Finally, the X-rays in our simulation heat the disk, increasing temperatures by a factor of 2--5 in the optically thin region, which could help explain the discrepancy between measured and anticipated lags.

• The paper demonstrates that sufficient X-ray flux combined with refined opacity models enhances UV continuum reprocessing in AGN disks.
• It employs 3D radiation magnetohydrodynamic simulations to reveal how X-ray variability and power spectral density affect light curve correlations.
• The findings imply rapid reprocessing (<1 hour) and a 2–5 fold temperature boost in disk surface layers, challenging traditional linear models.

Analyzing X-ray Continuum Reverberation in Active Galactic Nuclei Disks

This paper investigates the intricacies of disk continuum reverberation in active galactic nuclei (AGN) through three-dimensional radiation magnetohydrodynamic simulations. The research primarily focuses on understanding how time-varying X-ray irradiation impacts ultraviolet (UV) emission from AGN accretion disks, with an emphasis on addressing previously noted discrepancies in reverberation mapping campaigns. The paper provides necessary insight into the dependence of light curve correlation on X-ray flux levels and variability patterns, thus offering a novel approach for evaluating conventional linear reprocessing models.

The authors employ advanced opacity models that include both X-ray and UV radiation line opacities, diverging from prior studies that often relied on simplified opacity estimates. A significant finding is the substantial correlation between X-ray and UV light curves when sufficient X-ray flux and appropriate variability models are considered. In particular, the paper demonstrates that a refined opacity treatment amplifies the absorption and correlation of X-ray and UV emissions, suggesting this is a crucial component often overlooked in earlier models.

The implications of this paper extend theoretical and practical understanding in AGN accretion disk research. The simulations reveal that X-ray irradiation predominantly influences the disk's surface layers, enhancing the temperature by a factor of 2–5 in the optically thin region, which could account for larger than expected lags in empirical observations. Such findings align with previous observational discrepancies, like the oversized radial extent of disks detected through micro-lensing and reverberation mapping compared to standard models.

The authors also explore various models for the power spectral density (PSD) of the X-ray light curves. It is evident that the PSD structure significantly impacts the resulting UV variability, with distinct variability patterns observed when modifying the X-ray flux level or its power-law slope. In cases where the X-ray flux is reduced, the correlation between X-ray and UV emissions diminishes—highlighting the pivotal role of flux levels in reprocessing efficiency. This nuanced examination of PSD aligns with empirical data noting moderate correlations in some AGNs and calls for updated reverberation models that can better integrate these findings.

Through fitting response functions (RF) to the simulation results, the paper scrutinizes the standard models often applied in observational data analysis. The extracted RFs suggest that the reprocessing timeframes are considerably brief, often under an hour, endorsing assumptions of instantaneous reprocessing prevalent in traditional reverberation mapping studies. However, the findings also underscore that a linear reprocessing model may not fully encapsulate the complexities in interplay between different frequency variabilities.

This research facilitates the path forward for future studies by providing a framework to test lag detection mechanisms such as {\sc javelin}, commonly used to decode AGN variability. The adaptability of such models requires validation on more intricate, realistic light curves as presented in this paper, where multi-frequency impacts and intrinsic variabilities are dynamically simulated.

Ultimately, this research calls for a revisiting of AGN accretion disk models to accommodate the observed complexities in reverberation mapping data. Considering the imminent survey influx from facilities like the Vera Rubin Observatory, where data on numerous quasars will be procured, bridging the gap between empirical observations and reference models becomes not only relevant but necessary. This paper contributes significantly towards that objective by refining the understanding of how AGN disks process incoming radiation and emit detectable light, potentially reshaping standard theoretical models and methodologies in astrophysical studies.