Physical origin of the cold coronae in luminous EoR quasars

Determine the physical origin of the systematically cold coronae inferred for luminous quasars at the Epoch of Reionization, including whether they result from intrinsically lower electron temperatures or enhanced radiative Compton cooling in extreme accretion flows.

Background

The sample exhibits unusually steep X-ray continua and low inferred high-energy cutoffs compared with lower-redshift active galactic nuclei. The paper discusses two possible explanations: altered magnetic or plasma properties that reduce coronal heating, and stronger ultraviolet and soft-X-ray seed-photon fields that increase Compton cooling. It does not determine which mechanism is responsible.

References

The physical origin of this coronal state, however, remains unclear.

— Widespread steep X-ray spectra of luminous $z\simeq5.85-7.5$ quasars and their implications for future X-ray surveys  (2610.10522 - Tortosa et al., 7 Oct 2026) in Section 4.1, X-ray spectral steepness

In the presence of limited quality spectra such as those we are reporting for EoR QSOs, it is not possible to distinguish between intrinsically steep power-laws and canonical power-laws ($\Gamma=1.9$) with a relatively low value of the energy cutoff energy ($E_{\rm c}\sim20$\,keV).

— Widespread steep X-ray spectra of luminous $z\simeq5.85-7.5$ quasars and their implications for future X-ray surveys  (2610.10522 - Tortosa et al., 7 Oct 2026) in Section 2.1, Spectral analysis