Continuum reverberation in bright quasars using NASA/ATLAS

ABSTRACT Variable continuum emission from active galactic nuclei (AGNs) can be used to probe the structure of their accretion discs via reverberation mapping. Assuming a variable, hot inner light source irradiates the surrounding accretion disc, 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 AGNs are ubiquitously $\sim 3$ times longer than expected from standard disc theory, with evidence that this size discrepancy may decrease in more luminous AGNs. We have analysed high-cadence light curves of 9498 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 anticorrelation 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, larger optical Fe ii EWs, higher iron ratios (both UV Fe ii/Mg ii and optical Fe ii/H$\, \beta$), C iv broad absorption troughs, and lower C iv blueshift.