A New Approach to Constrain Black Hole Spins in Active Galaxies Using Optical Reverberation Mapping

TL;DR

This paper proposes a novel method to estimate black hole spins in active galaxies by analyzing the reverberation lags of Hbeta emission, revealing that spin significantly influences the BLR size-luminosity relation.

Contribution

It introduces a relativistic accretion disk model showing how black hole spin affects ionizing luminosity and reverberation lags, enabling spin estimation from optical reverberation mapping data.

Findings

Retrograde spins cause shorter Hbeta lags at high luminosity.

Most observed active galaxies likely host rapidly spinning black holes.

The model explains deviations from the standard BLR size-luminosity relation.

Abstract

A tight relation between the size of the broad-line region (BLR) and optical luminosity has been established in about 50 active galactic nuclei studied through reverberation mapping of the broad Hbeta emission line. The R_blr-L relation arises from simple photoionization considerations. Using a general relativistic model of an optically thick, geometrically thin accretion disk, we show that the ionizing luminosity jointly depends on black hole mass, accretion rate, and spin. The non-monotonic relation between the ionizing and optical luminosity gives rise to a complicated relation between the BLR size and the optical luminosity. We show that the reverberation lag of Hbeta to the varying continuum depends very sensitively to black hole spin. For retrograde spins, the disk is so cold that there is a deficit of ionizing photons in the BLR, resulting in shrinkage of the hydrogen ionization front with increasing optical luminosity, and hence shortened Hbeta lags. This effect is specially striking for luminous quasars undergoing retrograde accretion, manifesting in strong deviations from the canonical R_blr-L relation. This could lead to a method to estimate black hole spins of quasars and to study their cosmic evolution. At the same time, the small scatter of the observed R_blr-L relation for the current sample of reverberation-mapped active galaxies implies that the majority of these sources have rapidly spinning black holes.