Broadband reverberation mapping of quasars at cosmic noon

Abstract

Broadband reverberation mapping provides a powerful tool to investigate the structure of the accretion flow around supermassive black holes. The ultraviolet region of the quasar spectrum is of particular interest, as it enables the disentanglement of the relative contributions from the accretion disc and the broad-line region to the observed variability of quasars in the local Universe. Observations of quasars at cosmic noon ($z\sim 1-2$) offer the opportunity to probe the UV emission across a diverse population, spanning a broad range of black hole masses and luminosities. Here we re-analysed the 36 multi-year quasar lightcurves from the Dark Energy Survey (DES) between $z\sim0.7-2$ to retrieve interband delays and analyse their spectral energy distribution. Using PyROA, we were able to fit all multiple years of observations and the four optical bands simultaneously, significantly improving the lag detection as this method is able to interpolate over the seasonal gaps. We find robust detections of interband delays in 31/36 quasars, suggesting high-Eddington ratios for a large fraction of objects. Their spectral energy distribution is consistent with a geometrically thin disk $f_ν\proptoλ^{-1/3}$, with an ensemble slope of $-0.28\pm0.02$. The full sample of quasars do not show excess around the Balmer jump region, suggesting the impact of the broad-line region to the reverberation signals in high-Eddington sources could be small as documented in some AGN in the local Universe. The prospect of LSST to observe thousands of quasars in this redshift range should motivate the detailed study of the UV-rest frame of high-Eddington quasars to account for the BLR contribution and properly infer disc sizes.