Continuum Reverberation in Bright Quasars Using NASA/ATLAS

TL;DR

This study analyzes continuum reverberation in a large sample of bright quasars, revealing persistent size discrepancies with standard theory and complex wavelength-dependent delays influenced by various quasar properties.

Contribution

It provides the largest high-cadence reverberation analysis of quasars, demonstrating persistent size discrepancies and complex delay behaviors across a broad luminosity range.

Findings

Size discrepancy persists in high-luminosity quasars.

Anti-correlation between delays and luminosity is wavelength-driven.

Variable diffuse emission contaminates inter-band delay measurements.

Abstract

Variable continuum emission from AGN can be used to probe the structure of their accretion disks via reverberation mapping. Assuming a variable, hot inner light source irradiates the surrounding accretion disk, time delays between different continuum band light curves reveal light-travel times between their respective emission regions. Inter-band delays measured in several low-luminosity AGN are ubiquitously $\sim 3$ times longer than expected from standard disk theory, with evidence this size discrepancy may decrease in more luminous AGN. We have analysed high-cadence light curves of 9,498 of the brightest quasars between redshift 0.3-2.5 in the largest continuum reverberation study to date. Given the large sample size, we construct bins and fit delays jointly to combine inference across the parameter space and improve lag detections. We find that the size discrepancy persists in our high-luminosity sample, and that the previously seen anti-correlation with luminosity is likely driven by wavelength effects. The complex, non-monotonic wavelength dependence of delay amplitudes strongly suggests that contamination of inter-band delays by variable diffuse emission is widespread in the AGN population. We test delay behaviour against a variety of quasar properties finding longer lags in quasars with: higher Eddington ratios, redder colours, stronger optical FeII equivalent widths, higher iron ratios (both UV FeII/MgII and optical FeII/H$\beta$), CIV broad absorption troughs, and lower CIV blueshift.