Probing the fundamental properties of AGN
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X-ray and Optical-UV Properties of super-Eddington accreting AGNs
The accretion disk around the central black hole glows brightly in optical, UV, and soft X-rays. Hard X-rays are produced by up scattering of disk photons by hot electrons in the inner part of the nucleus called the corona. Hence X-rays are a probe of accretion and carry direct information on the physics of the inner regions of AGN.
Fig 1. Adapted from Tr’Ehnl & Brandt (2017). The X-ray spectrum above 2 keV is dominated by a single power-law emission with a hard photon index ~ 2. Therefore the hard x-ray photon index gives direct information about the energy distribution of the electrons in the corona. The spectral energy distribution of AGN peaks in UV emitted from the inner accretion disk. The relative strength of the disk and corona is shown by the relative strength of the energy emitted by the two regions. This is parameterized by the two-point spectral index between 2500A and 2 keV.
In Maithil et al. 2024, I characterized the X-ray and optical-UV properties and their relationship with accretion rates of some of the most extreme super-Eddington accreting sources (LBol/LEdd > 0.3) I extended key correlations between the X-ray photon index (2–8 keV) and accretion rates to these extreme accretors, revealing their steeper X-ray spectral slopes, indicative of the significant impact of high accretion rates on X-ray emissions (Fig 1.). The connection between the accretion disk and corona, evidenced by optically bright AGNs emitting relatively fewer X-rays than fainter ones, holds steady even in super-Eddington sources. For the first time, my results show X-ray weakness in a super-Eddington AGN, pointing towards orientation effects from the puffed-up slim disk structure.
Fig 1. Hard photon index vs dimensionless accretion rate parameter. Super- and sub-Eddington sample is from Liu et al. (2021).
Fig 2. Two point spectral slope between optical/UV and X-rays as a function of 2500 Å luminosity.
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