Test whether steep spectra extend to lower-luminosity EoR AGN

Determine whether the steep X-ray spectra observed in luminous quasars at the Epoch of Reionization also characterize lower-luminosity active galactic nuclei at the same epochs.

Background

The study is limited to luminous quasars, although the authors note that lower-redshift active galactic nuclei show little dependence of photon index on luminosity. If that behavior persists in the early Universe, steep spectra could also occur in lower-luminosity EoR AGN. This extrapolation has not yet been observationally tested.

References

Although this conclusion is currently limited to the luminous QSOs population, the lack of a significant dependence of $\Gamma$ on AGN luminosity observed at lower redshift \citep[e.g.][]{Piconcelli2005,Ricci2017} suggests that a similar behaviour for EoR AGN with lower luminosities is plausible, although it remains to be tested observationally.

— 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