Size and kinematics of the low-ionization broad emission line region from microlensing-induced line profile distortions in gravitationally lensed quasars

TL;DR

This study uses microlensing-induced line profile distortions in gravitationally lensed quasars to measure the size and kinematics of low-ionization broad emission line regions, comparing observations with models to determine their geometry and dynamics.

Contribution

It demonstrates that microlensing effects can distinguish between different BLR geometries and kinematics, providing size estimates that challenge existing radius-luminosity relations.

Findings

BLR sizes range from 3 to 25 light-days.

Low-ionization BLRs are larger than high-ionization regions by a factor of four.

Microlensing radii are systematically below R-L relation predictions.

Abstract

MgII or H$\alpha$ line profile distortions observed in five gravitationally lensed quasars have been compared with simulated ones. The simulations are based on three BLR models, a Keplerian disk (KD), an equatorial wind (EW), and a polar wind (PW). We find that the wide variety of observed line profile distortions can be reproduced with microlensing-induced distortions of line profiles generated by our BLR models. For three quasars, the most likely model is either KD or EW, depending on the orientation of the magnification map with respect to the BLR axis. This shows that the line profile distortions depend on the position and orientation of the BLR with respect to the caustic network, and not only on their different effective sizes. In the other quasars, the EW model is preferred. For all objects, the PW model has a lower probability. We conclude that disk geometries with kinematics dominated by either Keplerian rotation or equatorial outflow best reproduce the microlensing effects on the MgII and H$\alpha$ emission lines. The half-light radii of the MgII and H$\alpha$ BLRs are measured in the range of 3 to 25 light-days. The size of the region emitting the low-ionization lines is larger than the region emitting the high-ionization lines, with a factor of four measured between the sizes of the MgII and CIV emitting regions. The microlensing radii of the BLRs are found to be systematically below the radius-luminosity ($R -L$) relations derived from reverberation mapping, confirming that the intrinsic dispersion of the BLR radii with respect to the $R-L$ relations is large, but also revealing a selection bias that affects microlensing-based BLR size measurements.