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 $\log (t_{min,ul}/hrs)=0.85\pm 0.55$. 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 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 $2.0\pm 0.2$, 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)

10 pages, 8 figures, accepted for publication in A&A

References in corpus (18)

• The All-Sky Automated Survey for Supernovae (ASAS-SN) Light Curve Server v1.0
• Modeling the Time Variability of SDSS Stripe 82 Quasars as a Damped Random Walk
• The AGN Black Hole Mass Database
• BAT AGN Spectroscopic Survey I: Spectral Measurements, Derived Quantities, and AGN Demographics
• Space Telescope and Optical Reverberation Mapping Project. II. Swift and HST Reverberation Mapping of the Accretion Disk of NGC 5548
• Swift BAT Survey of AGN
• Magnetic Reconnection and Hot Spot Formation in Black Hole Accretion Disks
• A characteristic optical variability timescale in astrophysical accretion disks
• X-Ray and Optical Microlensing in the Lensed Quasar PG 1115+080
• A local baseline of the black hole mass scaling relations for active galaxies. IV. Correlations between $M_{\mathrm{BH}}$ and host galaxy $\sigma$, stellar mass, and luminosity
• Accretion Disk Sizes from Continuum Reverberation Mapping of AGN Selected from the ZTF Survey
• A long term study of AGN X-ray variability. Structure function analysis on a ROSAT-XMM quasar sample
• High-Cadence, Early-Time Observations of Core-Collapse Supernovae From the TESS Prime Mission
• Optical Variability of the Dwarf AGN NGC 4395 from the Transiting Exoplanet Survey Satellite
• The K-band properties of Seyfert 2 galaxies
• Intranight Optical Variability of Radio-Quiet Weak Emission Line Quasars-III
• The Variability of the Narrow-line Seyfert 1 Galaxies from the PanSTARRS's View
• High frequency breaks in optical active galactic nuclei power spectral density