First Direct Evidence for Keplerian Rotation in Quasar Inner Broad Line Regions

TL;DR

This paper presents the first direct evidence of Keplerian rotation in the inner broad-line regions of quasars, using microlensing effects in gravitationally lensed quasars to resolve the kinematics at light-day scales.

Contribution

It introduces a novel microlensing-based method to measure the rotation curves of quasar inner regions with high spatial resolution, confirming disk-like Keplerian motion.

Findings

Microlensing magnification increases smoothly with velocity.

Velocity-size relationships match Keplerian disk models.

Provides direct evidence of Keplerian rotation in quasar inner regions.

Abstract

We introduce a novel method to derive rotation curves with light-day spatial resolution of the inner regions of lensed quasars. We aim to probe the kinematics of the inner part of the broad-line region (BLR) by resolving the microlensing response - a proxy for the size of the emitting region - in the wings of the broad emission lines (BELs). Specifically, we assess the strength of the microlensing effects in the wings of the high-ionization lines Si IV and C IV across various velocity bins in five gravitationally lensed quasars: SDSS J1001+5027, SDSS J1004+4112, HE 1104$-$1805, SDSS J1206+4332, and SDSS J1339+1310. Using Bayesian methods to estimate the dimensions of the corresponding emission regions and adopting a Keplerian model as our baseline, we examine the consistency of the hypothesis of disk-like rotation. Our results reveal a monotonic, smooth increase in microlensing magnification with velocity. The deduced velocity-size relationships inferred for the various quasars and emission lines closely conform to the Keplerian model of an inclined disk. This study provides the first direct evidence of Keplerian rotation in the innermost region of quasars across a range of radial distances spanning from $\sim$5 to 20 light-days.