Widespread steep X-ray spectra of luminous $z\simeq 5.85-7.6$ quasars and their implications for future X-ray surveys

A. Tortosa, L. Zappacosta, E. Piconcelli +54

Luminous quasars (QSOs) at the Epoch of Reionization (EoR, $z\gtrsim 6$) host billion-solar-mass black holes assembled within the first Gyr of cosmic time. Their X-ray emission prob…