The LSST era of supermassive black holes accretion-disk reverberation mapping

TL;DR

This paper evaluates how LSST survey strategies affect the measurement of supermassive black hole accretion disk sizes via reverberation mapping, forecasting the number of detectable quasars and optimal observing cadences.

Contribution

It introduces a new metric, AGNTimeLagMetric, to assess LSST cadence strategies for reverberation mapping and provides performance estimates for AD size measurements across redshifts.

Findings

Over 1000 sources in each Deep Drilling field will have measurable AD time lags.

Optimal cadence (~5 days) and long observing seasons (~9 years) improve measurement success.

The number of measurable quasars increases as the survey's mean sampling rate decreases.

Abstract

The Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will detect an unprecedentedly large sample of actively accreting supermassive black holes with typical accretion disk (AD) sizes of a few light days. This brings us to face challenges in the reverberation mapping (RM) measurement of AD sizes in active galactic nuclei (AGNs) using interband continuum delays. We examine the effect of LSST cadence strategies on AD RM using our metric AGNTimeLagMetric. It accounts for redshift, cadence, the magnitude limit, and magnitude corrections for dust extinction. Running our metric on different LSST cadence strategies, we produce an atlas of the performance estimations for LSST photometric RM measurements. We provide an upper limit on the estimated number of quasars for which the AD time lag can be computed within 0<z<7 using the features of our metric. We forecast that the total counts of such objects will increase as the mean sampling rate of the survey decreases. The AD time lag measurements are expected for >1000 sources in each Deep Drilling field (DDF, 10 sq. deg) in any filter, with the redshift distribution of these sources peaking at z~1. We find the LSST observation strategies with a good cadence (~ 5 days) and a long cumulative season (~9 yr), as proposed for LSST DDF, are favored for the AD size measurement. We create synthetic LSST light curves for the most suitable DDF cadences and determine RM time lags to demonstrate the impact of the best cadences based on the proposed metric.