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

Abstract

NGC 1566 has shown dramatic X-ray spectral changes during its recent changing-look outburst, but the evolution of its broadband X-ray timing properties remains poorly constrained. We combine long-term Swift/XRT monitoring with high-time-resolution XMM-Newton observations to construct one pre-outburst Dim broadband PSD and two outburst broadband reconstructions associated with the O1 peak and O2 decay observations. In the outburst reconstructions, the same Swift/XRT up-state monitoring segment provides the low-frequency constraint, while the O1 and O2 XMM-Newton observations provide phase-specific high-frequency constraints. Using PSRESP forward modelling with the observed sampling windows, we test bending-power-law PSD models in the soft (0.3–2 keV) and hard (2–10 keV) bands. The Dim and O2-associated reconstructions are acceptably described by bending-power-law solutions, whereas the O1 peak observation does not yield a robust bend-frequency measurement. For the accepted Dim and O2-associated solutions, the preferred bend frequency shifts from ~2.0 × 10−5 to ~2.7 × 10−7 Hz in the soft band, and from ~2.1 × 10−5 to ~2.7 × 10−7 Hz in the hard band, implying a substantially longer characteristic variability timescale in the O2-associated reconstruction. This consistent shift in both energy bands suggests that the timing evolution is not confined to the soft-excess component alone, but reflects a broader change in the X-ray variability structure. Together with previous spectral studies, these results point to a transient reconfiguration of the disc–corona variability timescale during the changing-look transition in NGC 1566.