Researchers Reveal X-ray Illumination Explains 0.7 Variability in Active Galactic Nuclei Accretion Discs

Quasars, incredibly luminous active galactic nuclei, exhibit fluctuating brightness across the electromagnetic spectrum, and understanding the source of this variability remains a key challenge in astrophysics. Researchers, including M. Papoutsis, I. E. Papadakis, and C. Panagiotou, alongside colleagues E. Kammoun and M. Dovciak, investigate this phenomenon by modelling how X-ray light echoes off the accretion discs surrounding supermassive black holes. Their work demonstrates that X-ray reverberation, where X-rays emitted from a corona above the disc illuminate its surface, successfully explains the observed ultraviolet and optical power spectra of distant quasars across a wide range of wavelengths. By fitting their models to data from the SDSS Stripe-82 survey, the team infers that the X-ray corona is likely powered by the accretion disc itself, and constrains the black hole spin to below 0. 7, providing valuable insights into the physics of these energetic objects and corroborating previous findings from related studies.

X-ray Driven Variability in Active Galactic Nuclei

Over the past decade, significant effort has focused on understanding the ultraviolet and optical variability of active galactic nuclei (AGNs). Comprehensive studies demonstrate that X-ray illumination plays a crucial role in driving these observed changes in brightness, and recent advances in reverberation mapping have provided valuable insights into the size, density, and movement of gas within the broad-line region (BLR). The BLR is a complex system of clouds orbiting the central supermassive black hole, but the spatial distribution and physical properties of individual clouds remain largely unknown. This research addresses these limitations by combining high-cadence optical monitoring with advanced modelling techniques to constrain the geometry and physical properties of the BLR clouds. Focusing on the AGN Mrk 841, the team seeks to determine the size, density, and velocity dispersion of the BLR clouds, as well as their spatial distribution around the central black hole, with the ultimate goal of developing a more comprehensive model of the BLR.

AGN Corona Variability in Ultraviolet and Optical Light

This research investigates the variability of active galactic nuclei (AGNs) in ultraviolet and optical wavelengths, focusing on the corona, a hot, energetic region surrounding the supermassive black hole. The study aims to understand how the corona is powered and how its properties relate to the accretion disc, a swirling structure of gas and dust falling into the black hole, which powers the AGN. The team investigates how the black hole’s spin and the rate at which matter falls into it, known as the accretion rate, affect the corona, analysing how variability changes at different wavelengths. By using models to simulate AGN variability and fitting them to observational data, the researchers adjust parameters related to the corona’s temperature, density, and size, considering different black hole masses and analysing data from six wavelengths.

X-ray Reverberation Explains Quasar Brightness Variations

Researchers investigated the ultraviolet and optical variability of active galactic nuclei (AGNs), focusing on whether X-ray illumination of accretion discs can explain observed fluctuations in brightness. They developed a model based on X-ray reverberation, where variations in X-rays emitted from a corona above the disc cause corresponding changes in the light emitted by the disc itself. The X-ray reverberation model successfully fits the observed power spectra of quasars across a broad range of wavelengths, demonstrating a strong correlation between X-ray variations and subsequent changes in the optical and ultraviolet light emitted by the accretion disc. Analysis of the best-fit models suggests that the X-ray corona is powered by the accretion disc itself, and constrains the black hole spin to be lower than 0. 7, aligning with previous research and recent measurements of time lags.

X-ray Illumination Explains Quasar Variability

This research demonstrates that the observed variability in quasar brightness can be well explained by a model where X-ray light illuminates the accretion disc surrounding a black hole, a process known as X-ray reverberation. By modelling the transfer of energy within the disc, the team successfully fitted observed power spectra of quasars across a range of ultraviolet and optical wavelengths. The best-fit models indicate that the X-ray corona is powered by the accretion disc itself, suggesting a black hole spin likely less than 0. 7 and a corona height around 60 gravitational radii, reinforcing the connection between accretion processes and the emission of high-energy radiation.