Mechanism driving NGC 1566’s changing-look spectral transitions

Determine the physical mechanism responsible for the recurring spectral-shape changes in the active galactic nucleus NGC 1566, including the appearance and disappearance of broad emission lines, distinguishing among hypothesized scenarios such as broad-line region occultation, eccentric accretion disks, turbulent disk-dominated broad-line regions, or a binary supermassive black hole.

Background

NGC 1566 is a recurrent changing-look AGN that alternates between type-1 and type-2 spectral states, with broad emission lines appearing and disappearing in concert with continuum variability. Multiple explanations have been proposed in the literature—including obscuration, BLR/inner-flow restructuring, and binary SMBHs—but a definitive mechanism has not been established.

This paper presents new optical spectropolarimetry that challenges some proposed scenarios (e.g., pure occultation and binary SMBH) and supports accretion-driven photoionization and structural changes, while underscoring that only further observations during a type-1 phase can resolve the geometry and driving physics.