The shortest detected intra-day variability of active galactic nuclei in TESS survey

Abstract

AGNs are known to be variable in almost all wavelengths and timescales. The shortest variability timescale of AGNs can be used to probe the smallest scale structures within AGNs. We aim to measure the shortest detected variability timescale, $t_{min,ul}$, of type 1 radio-quiet Seyfert galaxies and analyse their characteristics. We extracted TESS light curves of 47 Seyfert 1 galaxies. We measured the PSDs of the sample, modelled by a power law model plus a constant noise, and constrained the shortest detected AGN variability timescale as the power law component exceeds the constant noise and systematic uncertainties indicated by the upper limits of non-variable quiescent galaxies' PSDs. We measured the upper limits of the shortest variability timescale to be $\log(t_{min,ul}/hrs)=0.85\pm0.55$. We compared these upper limits to a range of theoretical AGN variability timescales, and the natural interpretation of our measured $t_{min,ul}$ is the light crossing scale from a coherently varying region, where the measured $t_{min,ul}$ corresponds to the range from a few to thousands of gravitational radii. A significant fraction of these light crossing scales is smaller than the accretion disk emission sizes measured by quasar microlensing, reverberation mapping, or theoretical accretion disk models. Since we only measure the upper limits, the true physical shortest variability timescales are even shorter. We measure the power law index to be $2.0\pm0.2$, and find weak anticorrelations with the black hole mass and luminosity. Our analysis suggests that the shortest optical variability is driven by a compact region smaller than the accretion disk size, potentially by X-ray reprocessing. Alternatively, this shortest timescale variability suggests that the accretion disk can be inhomogeneous potentially caused by turbulence from magnetorotational instability or magnetic reconnections. (abridged)

Figures (8)

• Figure 1: Left: The systematics of TESS survey and light curve extraction technique indicated by the PSDs of non-variable galaxy light curves, where the y-axis is the scaled power relative to the constant white noise. Each gray line indicates the power law plus white noise fit for an individual galaxy's light curve, the black line represents the mean galaxy PSD, and the red and yellow shaded areas represent the 1-sigma and 3-sigma limits, respectively. Right: Comparison between Seyfert 1 AGN, ESO 362-G018, Sector 5 PSD, which has the measured $t_{\textrm{min,ul}}$ value equal to the median of the AGN sample, and the upper limits of galaxies as a representative to illustrate the measurement of the shortest detected variability time scale. The text next to the PSD lists the parameters for the best fit. PSDs in both panels are normalized by their respective noise levels.
• Figure 2: Left: The distribution of quiescent galaxies' structure functions, normalized by the noise level, similar to Figure \ref{['comparetogal']} left. Right: Comparison of structure functions of galaxies and NGC 931 sector 18 to demonstrate the smallest variability timescale measurement.
• Figure 3: Left: Distribution of the shortest detected TESS variability timescales of the sample. Right: Histogram showing the distribution of $c t_{\textrm{min,ul}}$ in terms of $r_g$.
• Figure 4: Distribution of the shortest detected TESS variability timescales of the sample, plotted against the black hole mass. The blue dashed lines indicate the different values of gravitational radii for the given black hole mass, and the green and red solid lines indicate the theoretical and measured accretion disk and corona sizes from literature shakura73morgan10dogruel20.
• Figure 5: Top left: Distribution of the power law indices for the PSD power law fit. Black hole mass (top right), $L_V/L_{\textrm{Edd}}$ (bottom left), and V-band luminosity (bottom right) plotted against the power law index.