Explaining line–continuum timing in NGC 1566 outbursts

Establish the physical process that causes the broad emission lines in NGC 1566 to vary before the optical continuum during outbursts, contrary to standard photoionization expectations in which continuum changes precede line variability.

Background

Historical monitoring of NGC 1566 revealed that broad lines responded prior to the optical continuum during outbursts, a behavior opposite to canonical photoionization reverberation where the continuum leads and the lines lag. This timing inversion has remained unexplained for decades and is central to understanding the coupling between the ionizing source and line-emitting gas.

The present spectropolarimetric results reference this long-standing issue in discussing how polarization behavior may point to additional or alternative scattering/illumination processes linking the continuum and line emission.

References

However, an intriguing -- and yet to be explained -- feature of the outbursts is that the broad emission lines appear to vary before the optical continuum (i.e., the exact opposite of photoionization models), suggesting that the optical and ionizing non-stellar flux may not be directly linked, but instead coupled via slower secondary processes.

— Spectropolarimetry of the changing-look active galactic nucleus NGC 1566 and its potential link to supermassive black hole binaries  (2604.01872 - Marin et al., 2 Apr 2026) in Section 1 (Introduction)