VLTI/AMBER differential interferometry of the broad-line region of the quasar 3C273

TL;DR

This study used differential interferometry with VLTI/AMBER to resolve the broad-line region of quasar 3C273, providing new size estimates that challenge previous reverberation mapping results and offer insights into BLR structure.

Contribution

It presents the first spectrally resolved interferometric observation of a quasar BLR using a novel 'blind' method, advancing direct measurement techniques for AGN structures.

Findings

BLR size estimated at over 1000 light days, larger than reverberation mapping results.

Differential visibility decreases with baseline in the Pa-alpha line.

Angular size of the BLR is larger than previously measured by reverberation mapping.

Abstract

Unveiling the structure of the Broad Line Region (BLR) of AGNs is critical to understand the quasar phenomenon. Resolving a few BLRs by optical interferometry will bring decisive information to confront, complement and calibrate the reverberation mapping technique, basis of the mass-luminosity relation in quasars. BLRs are much smaller than the angular resolution of the VLT and Keck interferometers and they can be resolved only by differential interferometry very accurate measurements of differential visibility and phase as a function of wavelength. The latter yields the photocenter variation with wavelength, and constrains the size, position and velocity law of various regions of the BLR. AGNs are below the magnitude limit for spectrally resolved interferometry set by currently available fringe trackers. A new "blind" observation method and a data processing based on the accumulation of 2D Fourier power and cross spectra permitted us to obtain the first spectrally resolved interferometric observation of a BLR, on the K=10 quasar 3C273. A careful bias analysis is still in progress, but we report strong evidence that, as the baseline increases, the differential visibility decreases in the Pa-alpha line. Combined with a differential phase smaller than 3 degree, this yields an angular equivalent radius of the BLR larger than 0.4 milliarcseconds, or 1000 light days at the distance of 3C273, much larger than the reverberation mapping radius of 300 light days. Explaining the coexistence of these two different sizes, and possibly structures and mechanisms, implies very new insights into the BLR of 3C273