Microlensing of Quasar Broad Emission Lines: Constraints on Broad Line Region Size

TL;DR

This study uses gravitational microlensing to measure the sizes of quasar broad emission line regions, finding that high ionization regions are more compact and that larger quasars have bigger emission regions, consistent with photoionization models.

Contribution

It provides the first microlensing-based size estimates for broad line regions, confirming their dependence on ionization level and quasar luminosity.

Findings

High ionization lines are more strongly microlensed than low ionization lines.

Estimated BLR sizes are 24 and 55 light-days for high and low ionization lines.

BLR size scales with quasar luminosity as predicted by models.

Abstract

We measure the differential microlensing of the broad emission lines between 18 quasar image pairs in 16 gravitational lenses. We find that high ionization lines such as CIV are more strongly microlensed than low ionization lines, indicating that the high ionization line emission regions are more compact. If we statistically model the distribution of microlensing magnifications, we obtain estimates for the broad line region radius of 24 (-15/+22) and 55 (-35/+150) light-days (90% confidence) for the high and low ionization lines, respectively. When the sample is divided attending to quasar luminosity, we find that the line emission regions of more luminous quasars are larger, with a slope consistent with the expected scaling from photoionization models. Our estimates also agree well with the results from local reveberation mapping studies.