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

TL;DR

This study measures the shortest optical variability timescales in Seyfert 1 galaxies using TESS data, revealing that these timescales are smaller than traditional accretion disk sizes, implying very compact emission regions.

Contribution

First measurement of upper limits on shortest AGN variability timescales using TESS, linking them to small-scale structures within AGNs and suggesting inhomogeneous accretion disks.

Findings

Measured upper limits of variability timescale as log(t_min,ul/hrs)=0.85±0.55.

Power law index of PSDs is 2.0±0.2, with weak anticorrelations to black hole mass and luminosity.

Shortest variability timescales are smaller than accretion disk emission sizes, indicating very compact emission regions.

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)