Spectroastrometric Reverberation Mapping of Broad-line Regions

TL;DR

This paper introduces a novel spectroastrometric reverberation mapping technique to spatially resolve broad-line region kinematics in active galactic nuclei, offering a complementary approach to traditional methods and demonstrating its feasibility with simulations and real data.

Contribution

It develops a mathematical formalism and Bayesian analysis framework for spectroastrometric reverberation mapping, enabling detailed mapping of BLR geometry and black hole mass estimation.

Findings

Spectroastrometric signals vary by several to tens of microarcseconds.

The method can spatially resolve BLR kinematics and geometry.

Application to 3C 273 shows promising results for future observations.

Abstract

Spectroastrometry measures source astrometry as a function of wavelength/velocity. Reverberations of spectroastrometric signals naturally arise in broad-line regions (BLRs) of active galactic nuclei (AGNs) as a result of the continuum variations that drive responses of the broad emission lines with time delays. Such signals provide a new diagnostic for mapping BLR kinematics and geometry, complementary to the traditional intensity reverberation mapping (RM) technique. We present the generic mathematical formalism for spectroastrometric RM and show that under realistic parameters of a phenomenological BLR model, the spectroastrometric reverberation signals vary on a level of several to tens of microarcseconds, depending on the BLR size, continuum variability, and angular-size distance. We also derive the analytical expressions of spectroastrometric RM for an inclined ring-like BLR. We develop a Bayesian framework with a sophisticated Monte Carlo sampling technique to analyze spectroastrometric data and infer the BLR properties, including the central black hole mass and angular-size distance. We demonstrate the potential of spectroastrometric RM in spatially resolving BLR kinematics and geometry through a suite of simulation tests. The application to realistic observation data of 3C~273 obtains tentative, but enlightening results, reinforcing the practical feasibility of conducting spectroastrometric RM experiments on bright AGNs with the operating Very Large Telescope Interferometer as well as possibly with the planned next-generation 30 m class telescopes.