Probing Black Holes in Distant Quasars with PRM: 150x faster and more efficient

Reverberation mapping accurately determines virial black hole masses for redshifts z < 0.2 using the relationship between the Hß broad-line region (BLR) size and the 5100 Angstrom continuum luminosity established with ~200 AGNs. For quasars at z ~ 2-3, determining the BLR size is time-consuming and limited by seasonal gaps, requiring ~10-20 years of monitoring CIV emission lines. In this work, we demonstrate that an efficient alternative is to use a continuum size-luminosity relation, which can be obtained up to 150 times faster using photometric reverberation mapping (PRM). We outline the method and its feasibility based on simulations and showcase the spectacular first results carried out with ground-based meter-class telescopes equipped with narrow and medium-band filters. We focus on the ESO La Silla 2.2-meter telescope observations with a well-defined sampling rate, which recovers our predictions - a testament to the validity of our scaling relation. These observations provide the scaling factor between the accretion disk and the CIV-based BLR sizes, which is (1) crucial for estimating the masses of black holes at higher redshifts extending beyond the cosmic noon, (2) evaluating the contribution of the diffused continuum emission and assessing the standard accretion disk theory, and, (3) validating quasars as cosmological distance indicators and bridge the gap between the local and early Universe.