X-ray Quasi-Periodic Oscillations in Active Galactic Nuclei and Their Implications for the Changing Look Phenomenon

Abstract

X-ray timing of active galactic nuclei (AGN) provides a unique probe of gas accretion onto supermassive black holes (SMBHs). Quasi-periodic oscillations (QPOs), which trace gas dynamics in the strongly curved spacetime around SMBHs, are rare in AGN. These signals often are analogs of high-frequency QPOs occasionally seen in some black-hole X-ray binaries, and their scarcity in AGN can partly be attributed to the low frequencies expected for typical SMBH masses. Intriguingly, robust X-ray QPO detections in SMBH systems have so far been reported only in narrow-line Seyfert 1 galaxies (NLS1s) and tidal disruption events (TDEs). Here we report the discovery of a QPO candidate during the 2018 outburst of the changing-look AGN (CL-AGN) NGC 1566. Numerical simulations indicate that the disk epicyclic oscillations responsible for high-frequency QPOs are damped by magnetohydrodynamic turbulence unless the accretion flow is misaligned and/or eccentric. In TDEs, the stellar debris stream is naturally misaligned with the SMBH spin, while NLS1s may host misaligned disks due to their youth. Motivated by the QPO candidate in NGC 1566, we propose that CL-AGN accretion is also misaligned -- potentially fueled by captured, free-falling broad-line region clouds. This model naturally explains why CL-AGN transition timescales are much shorter than the standard disk viscous timescale. This picture can be tested by searching for QPOs or quasi-periodic eruptions in other CL-AGN.

Figures (3)

• Figure 1: The GP fitting results of the QPO+DRW model for NGC 1566. Top-left: the background-subtracted XMM-Newton EPIC light curve of NGC 1566 in the $0.2$--$10$ keV band (black dots with error bars), binned at $100$ s. The light curve corresponds to the obs-ID $0800840201$, for which NGC 1566 was in the outburst phase. The green curves represent the long-term trend modeled by an shewed Gaussian function, which well describes the outburst profile. The yellow curve is the sum of the best-fitting QPO+DRW model and the trend. Top-right: the same as the top panel, but with the long-term trend (i.e., the green curve in the top panel) being subtracted. Bottom-left: the posterior distribution of the QPO period, yielding $P_{\mathrm{QPO}}=(1.78^{+0.17}_{-0.14})\times 10^4\ \mathrm{s}$. Bottom-right: the best-fitting PSDs for the QPO+DRW model (blue curve) and the noise model (purple dots); the shaded regions indicate the $1\sigma$ confidence intervals.
• Figure 2: The observed and best-fitting PSDs. The black curve shows the observed PSD estimated via the FFT method. The purple curve corresponds to the best-fitting model, which consists of a power law (red noise; the pink dashed line) and a constant (measurement noise; the orange dotted line). The shaded regions correspond to the $1\sigma$ confidence intervals. Note that the confidence interval for the measurement noise is too small to be visible.
• Figure 3: The FFT analysis of NGC 1566. Left: the FFT PSD of the observed light curve with obs-ID $0800840201$ (black curve). The pink curve is the average FFT PSD of the $5\times 10^5$ mock light curves (see text), with the shaded pink regions indicating its $3\sigma$ and $4\sigma$ confidence intervals. The QPO signal has a local $p$-value of $1.8\times 10^{-4}$ ($3.6\sigma$) for the period $P_{\mathrm{QPO}}=1.32\times 10^4\ \mathrm{s}$. Note that the y-axis is the product of the PSD and frequency. Right: the pink histogram represents the global $p$-value (i.e., the survival function, accounting for the "look elsewhere" effect) derived from our red-noise Monte Carlo simulations. The blue dashed lines indicate the $2\sigma$ and $3\sigma$ significance levels. The black line represents the observed QPO signal strength, resulting in a global $p$-value of $1.5\times 10^{-3}$ (i.e., $2.96\sigma$).