Spiral Arms in Broad-line Regions of Active Galactic Nuclei. I. Reverberation and Differential Interferometric Signals of Tightly Wound Cases

TL;DR

This paper models the observational signatures of spiral arms in the broad-line regions of active galactic nuclei, predicting detectable reverberation and interferometric signals that can inform about the gas dynamics near supermassive black holes.

Contribution

It introduces a WKB-based model for spiral arms in BLRs and predicts their reverberation and interferometric signatures, aiding future observational detection.

Findings

Spiral arms cause distinct broad emission line profiles.

Predicted reverberation mapping signals can be observed with current instruments.

Interferometric features of spiral arms are detectable with high-quality data.

Abstract

As a major feature in spectra of active galactic nuclei, broad emission lines deliver information of kinematics and spatial distributions of ionized gas surrounding the central supermassive black holes (SMBHs), that is the so-called broad-line regions (BLRs). There is growing evidence for appearance of spiral arms in the BLRs. It has been shown by reverberation mapping (RM) campaigns that the characterized radius of BLRs overlaps with that of self-gravitating regions of accretion disks. In the framework of the WKB approximation, we show robust properties of observational features of the spiral arms. The resulting spiral arms lead to various profiles of the broad emission line. We calculate RM and differential interferometric features of BLRs with $m=1$ mode spiral arms. These features can be detected with high-quality RM and differential interferometric observations via such as GRAVITY onboard Very Large Telescope Interferometer. The WKB approximation will be relaxed and universalized in the future to explore more general cases of density wave signals in RM campaigns and differential spectroastrometry observations.