Spiral arms in broad-line regions of active galactic nuclei II. Loosely wound cases: Reverberation properties

There is growing evidence that broad-line regions (BLRs) in active galactic nuclei (AGNs) have regular substructures, such as spiral arms. This is supported by the fact that the radii of BLRs measured by reverberation mapping (RM) observations are generally consistent with the self-gravitating regions of accretion disks. We showed in Paper I that the spiral arms excited by the gravitational instabilities in these regions may exist in some disk-like BLRs. Here, in the second paper of the series, we investigate the loosely wound spiral arms excited by gravitational instabilities in disk-like BLRs and present their observational characteristics. We solve the governing integro-differential equation by a matrix scheme. The emission-line profiles, velocity-delay maps, and velocity-resolved lags of the BLR spiral arms are calculated. We find that the spiral arms can explain some of the phenomena seen in observations: (1) different asymmetries in the emission-line profiles in the mean and rms spectra; (2) complex subfeatures (incomplete ellipse) in some velocity-delay maps, for example that of NGC 5548; and (3) the short timescales of the asymmetry changes in emission-line profiles (rms spectra). These features are attractive for modeling the observed line profiles and the properties of reverberation, and for revealing the details of the BLR geometry and kinematics.

Automated summary

There is evidence of regular substructures, such as spiral arms, in broad-line regions (BLRs) of active galactic nuclei (AGNs), which are consistent with self-gravitating regions of accretion disks. This study investigates the loosely wound spiral arms excited by gravitational instabilities and their observational characteristics in disk-like BLRs. The results show that these spiral arms can explain various phenomena observed, including asymmetries in emission-line profiles, complex subfeatures in velocity-delay maps, and short timescales of asymmetry changes in emission-line profiles.