Spiral arms in broad-line regions of active galactic nuclei I. Reverberation and differential interferometric signals of tightly wound cases

As a major feature of the spectra of active galactic nuclei, broad emission lines deliver information on the kinematics and spatial distributions of ionized gas located in the so-called broad-line regions (BLRs) surrounding the central supermassive black holes (SMBHs). There is growing evidence for the appearance of spiral arms in BLRs. It has been shown through 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 the characterization of various profiles of the broad emission line. We calculated the 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 instruments as GRAVITY on board the Very Large Telescope Interferometer (VLTI). 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 spectroastrometric observations.