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

Context. Active galactic nuclei (AGNs) are known to be variable in almost all wavelengths and timescales. The shortest AGN variability timescale can be used to probe the smallest scale structures within AGNs. Aims. We aim to measure the shortest detected variability timescale, t min, ul , of type 1 radio-quiet Seyfert galaxies and to analyse their characteristics. Methods. We extracted the Transiting Exoplanet Survey Satellite (TESS) light curves of 47 Seyfert 1 galaxies. We measured the power spectral densities (PSDs) of the sample, modelled by a power law model plus constant noise. We constrained the shortest detected AGN variability timescale, whereby the power law component exceeds the constant noise and systematic uncertainties indicated by the upper limits of non-variable quiescent galaxies’ PSDs. Results. We measured the upper limits of the shortest variability timescale to be log( t min, ul /h) = 0.85 ± 0.55. 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 disc emission sizes measured by quasar microlensing, reverberation mapping, or theoretical accretion disc models. Since we only measured the upper limits, the true physical shortest variability timescales are even shorter. We measure the power law index as α = 2.0 ± 0.2 and we found weak anti-correlations with the black hole mass and luminosity. Conclusions. Our analysis suggests that the shortest optical variability is driven by a compact region that is smaller than the accretion disc size, potentially as a result of X-ray reprocessing. Alternatively, this shortest timescale variability suggests that the accretion disc can be inhomogeneous. This could potentially be caused by turbulence from magnetorotational instability or magnetic reconnections.