State-dependent broadband X-ray timing reconfiguration in the changing-look AGN NGC 1566

TL;DR

This study investigates how the broadband X-ray timing properties of the changing-look AGN NGC 1566 evolve between dim and bright states, revealing a reconfiguration of its inner radiative structure.

Contribution

It provides the first detailed modeling of broadband X-ray PSD evolution across different states in NGC 1566, highlighting state-dependent timing reconfiguration.

Findings

PSD bend frequency shifts by ~1 dex between states

Soft-hard timing relation changes from stratified to coupled

Supports inner radiative structure reconfiguration during state change

Abstract

NGC 1566 has exhibited dramatic state changes in its X-ray spectrum, but the evolution of its broadband timing properties remains poorly constrained. We combine long-term Swift monitoring with high-time-resolution XMM-Newton observations to model the broadband X-ray power spectral density (PSD) in the dim and bright states. In the hard band, the PSD bend frequency shifts by about 1 dex between the two states, implying a substantially longer characteristic variability timescale in the bright state. The relative timing behaviour of the soft and hard bands also changes with state. In the dim state, the soft-band bend frequency is higher than the hard-band value by about 0.49 dex, whereas in the bright state the two become broadly consistent. The broadband variability evolution of NGC 1566 therefore involves not only an overall shift in characteristic timescale, but also a state-dependent change in the soft-hard timing relation, from a more stratified to a more tightly coupled configuration. Combined with previous spectral results, this supports a genuine reconfiguration of the inner radiative structure during the changing-look transition.