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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,tmin, 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(tmin, 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 measuredtmin, ulis the light-crossing scale from a coherently varying region, where the measuredtmin, ulcorresponds 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 asα = 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.
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High-frequency breaks in the optical active galactic nucleus power spectral density
Context.Variability is a ubiquitous feature of active galactic nuclei (AGNs), and the characterisation of this variability is crucial to constraining its physical mechanism and proper applications in AGN studies. The advent of all-sky and high-cadence optical surveys allows more accurate measurements of AGN variability down to short timescales as well as direct comparisons with X-ray variability from the same sample of sources. Aims.We aim to analyse the optical power spectral density (PSD) of AGNs with measured X-ray PSDs. Methods.We used light curves from the All-Sky Automated Survey for SuperNovae (ASAS-SN) and the Transiting Exoplanet Survey Satellite (TESS) and used the Lomb-Scargle periodogram to obtain PSDs. The joint optical PSD is measured over up to six orders of magnitude in frequency space on timescales of minutes to a decade. We fitted either a damped random walk (DRW) or a broken power law (BPL) model to constrain the PSD model and break frequency. Results.We find a set of break frequencies (≲10−2day−1) from DRW and BPL fits that generally confirm previously reported correlations between break frequencies and the black hole mass. In addition, we find a second set of break frequencies at higher frequencies (> 10−2day−1). We observe a potential weak correlation between the high-frequency breaks with the X-ray break frequencies and the black hole mass. We further explored the dependence of the correlations on other AGN parameters, finding that adding X-ray, optical, or bolometric luminosity as the third correlation parameter can substantially improve the correlation significances. The newly identified high-frequency optical breaks can constrain different aspects of the physics of AGNs.
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- Award ID(s):
- 2307802
- PAR ID:
- 10634779
- Publisher / Repository:
- Astronomy and Astrophysics
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 698
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- A105
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1051/0004-6361/202452469
