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

 Title A New Approach to Constrain Black Hole Spins in Active Galaxies Using Optical Reverberation Mapping Publication Type Journal Article Year of Publication 2014 Authors Wang, J-M, Du, P, Li, Y-R, Ho, L C, Hu, C, Bai, J-M Journal \apjl Volume 792 Pagination L13 Keywords accretion, accretion disks, galaxies: active 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 H{$\beta$} 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 H{$\beta$} to the varying continuum depends very sensitively on 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 H{$\beta$} 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. } DOI 10.1088/2041-8205/792/1/L13