Negative Lags on the Viscous Timescale in Quasar Photometry and Prospects for Detecting More with LSST

Abstract

The variability of quasar light curves can be used to study the structure of quasar accretion disks. For example, continuum reverberation mapping uses delays between variability in short and long wavelength bands (short lags) to measure the radial extent and temperature profile of the disk. Recently, a potential reverse lag, where variations in shorter wavelength bands lag the longer wavelength bands at the much longer viscous timescale, was detected for Fairall 9. Inspired by this detection, we derive a timescale for these long negative lags from fluctuation propagation models and recent simulations. We use this timescale to forecast our ability to detect long lags using the Vera Rubin Legacy Survey of Space and Time (LSST). After exploring several methods, including the interpolated cross-correlation function, a Von-Neumann estimator, javelin, and a maximum-likelihood Fourier method, we find that our two main methods, javelin and the maximum-likelihood method, can together detect long lags of up to several hundred days in mock LSST light curves. Our methods work best on proposed LSST cadences with long season lengths, but can also work for the current baseline LSST cadence, especially if we add observations from other optical telescopes during seasonal gaps. We find that LSST has the potential to detect dozens to hundreds of additional long lags. Detecting these long lags can teach us about the vertical structure of quasar disks and how it scales with different quasar properties.

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

“…variability would be accreted inwards(Hogg & Reynolds 2016), leading to inward-propagating lags (i.e., shorter wavelengths lagging longer wavelengths) on the inflow timescale, otherwise known as long negative lags. Evidence for these long negative lags have been observed for the AGN Fairall 9 (Hernández Santisteban et al 2020;Yao et al 2023;Secunda et al 2023). …”

“…Increasing our understanding of the physical processes that influence reverberation mapping is crucial because there is growing evidence that the standard Shakura & Sunyaev (1973) thin disk model does not accurately model an AGN disk (e.g., Antonucci 1988Antonucci , 2023Antonucci et al 1989). For example, radiation MHD simulations show that pressure support from magnetic fields and radiation can lead to puffed-up disks (e.g., Gaburov et al 2012;Jiang et al 2016Jiang et al , 2019Jiang & Blaes 2020;Hopkins et al 2024), while recent observations provide additional evidence for thicker disks than predicted by the standard thin disk model (Yao et al 2023;Secunda et al 2023). In addition, both microlensing and reverberation mapping campaigns suggest that the radial extent of the disk is larger than predicted by the standard disk model (e.g., Morgan et al 2010;Jiang et al 2017;Guo et al 2022).…”

A Reverberation Mapping Study of a Highly Variable Active Galactic Nucleus 6dFGS gJ022550.0-060145

“…The same conclusion is obtained by Dexter et al (2019) and Marculewicz et al (2023), who obtain interband time lags for an X-ray weak quasar. The recent threedimensional magnetohydrodynamic simulations of an AGN accretion disk also suggest that X-ray is unlikely to be the main driver of UV/optical variations (Secunda et al 2023). The UV reprocessing (Gardner & Done 2017) or magnetic coupling between the corona and disk (Sun et al 2020) may play an important role in driving UV/optical coordinated variations with interband lags.…”

How Long Will the Quasar UV/Optical Flickering Be Damped?

“…Although much progress has been achieved in the studies of AGN variability, the primary physical mechanism causing variability is still an unresolved issue. Many theories have been proposed, such as the global variation of the accretion rate (Lyubarskii 1997;Li & Cao 2008;Liu et al 2016), the local temperature fluctuation (Kelly et al 2009;Dexter & Agol 2011;Cai et al 2016), and the effect of large-scale fluctuations on local temperature fluctuations (Cai et al 2018;Neustadt & Kochanek 2022;Secunda et al 2023). However, these theories failed to fully account for observational results, e.g., AGN power spectral densities (PSDs).…”