Distinguish intrinsically steep spectra from absorbed flat spectra

Establish whether quasars displaying canonical or relatively flat X-ray photon indices are intrinsically steep-spectrum systems affected by moderate intrinsic absorption, rather than members of the flat-spectrum tail or a distinct flat-spectrum subpopulation.

Background

A subset of the quasars has canonical or relatively flat photon indices, but the available low-count X-ray spectra do not meaningfully distinguish intrinsic spectral shape from moderate absorption. Absorbed models are not statistically preferred, yet the data are insufficient to rule out the possibility that some apparently flat sources are intrinsically steep and obscured.

References

We further investigated whether the few QSOs with canonical photon indices could instead be intrinsically steep-spectrum sources affected by moderate intrinsic absorption. The results of these tests are presented in App.~\ref{app:test_steep_spectra}. Absorbed models are not statistically preferred over a simple unabsorbed power-law, the current data quality does not allow us to establish whether these apparently flat-spectrum QSOs are intrinsically steep but moderately obscured systems.

— 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

As a consequence, the current data do not allow us to robustly assess whether an additional population of QSOs with canonical X-ray slopes is present.

— 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 Appendix, Section Uncertainty-propagated distribution