Estimating the size of X-ray lamppost coronae in active galactic nuclei

Ursini, F.; Dovčiak, M.; Zhang, W.; Matt, G.; Petrucci, P.-O.; Done, C.

Abstract

Aims. We report estimates of the X-ray coronal size of active galactic nuclei in the lamppost geometry. In this commonly adopted scenario, the corona is assumed for simplicity to be a point-like X-ray source located on the axis of the accretion disc. However, the corona must intercept a number of optical/UV seed photons from the disc consistent with the observed X-ray flux, which constrains its size. Methods. We employ a relativistic ray-tracing code, originally developed by Dovčiak and Done, that calculates the size of a Comptonizing lamppost corona illuminated by a standard thin disc. We assume that the disc extends down to the innermost stable circular orbit of a non-spinning or a maximally spinning black hole. We apply this method to a sample of 20 Seyfert 1 galaxies using simultaneous optical/UV and X-ray archival data from XMM-Newton. Results. At least for the sources accreting below the Eddington limit, we find that a Comptonizing lamppost corona can generally exist, but with constraints on its size and height above the event horizon of the black hole depending on the spin. For a maximally spinning black hole, a solution can almost always be found at any height, while for a non-spinning black hole the height must generally be higher than 5 gravitational radii. This is because, for a given luminosity, a higher spin implies more seed photons illuminating the corona, which is due to a larger and hotter inner disc area. The maximal spin solution is favoured, as it predicts an X-ray photon index in better agreement with the observations.

Citation

Ursini, F., Dovčiak, M., Zhang, W., Matt, G., Petrucci, P.-O., & Done, C. (2020). Estimating the size of X-ray lamppost coronae in active galactic nuclei. Astronomy & Astrophysics, 644, Article A132. https://doi.org/10.1051/0004-6361/202039158

• Journal Article Type: Article
• Acceptance Date: Oct 12, 2020
• Online Publication Date: Dec 11, 2020
• Publication Date: 2020-12
• Deposit Date: Jun 24, 2021
• Publicly Available Date: Jun 24, 2021
• Journal: Astronomy and astrophysics.
• Print ISSN: 0004-6361
• Electronic ISSN: 1432-0746
• Publisher: EDP Sciences
• Peer Reviewed: Peer Reviewed
• Volume: 644
• Article Number: A132
• DOI: https://doi.org/10.1051/0004-6361/202039158
• Public URL: https://durham-repository.worktribe.com/output/1273679