Active Galactic Nuclei XVI: A Sea of Discoveries

(Highlight) The Quasar X-ray View at the Epoch of Reionization: Implications for Early Black Hole Accretion, AGN Emissivity, and Future Surveys

15 Jun 2026, 11:30

20m

Sala Conferenze presso Autorità di Sistema Portuale (Brindisi)

Talk S1 - Black Hole Accretion and Outflows: Focusing on the physics of accretion disks, the mechanisms driving high-velocity winds, and the geometry of circumnuclear regions

Speaker

Luca Zappacosta (Istituto Nazionale di Astrofisica (INAF))

Description

Shedding light on the nuclear properties of $z>6$ quasars (QSOs) powered by supermassive black holes (SMBHs) with masses $>10^9~M_\odot$​ is fundamental to understand their rapid formation during the Epoch of Reionization (EoR). Probing the innermost regions of these QSOs via X-ray emission, at just a few tens of gravitational radii, provides the most direct window into the growing SMBH.

We present results from HYPERION, an unprecedented $\sim$700-hour XMM-Newton Heritage Program targeting 18 luminous QSOs at $z\approx6-7.5$ selected for their extreme SMBH growth. Our findings reveal a strikingly new regime for the nuclear properties of these early-formed QSOs: their X-ray continuum slopes ($\Gamma$) are significantly steeper than those of analogous QSOs at $z<6$. By investigating correlations between $\Gamma$ and UV-derived properties, we identify a significant link between the X-ray slope and the velocity of nuclear AGN-driven ionized winds, potentially indicative of fast SMBH accretion.

This investigation is further expanded by a new 150-hour XMM-Newton Large Program which, combined with archival X-ray data, brings the total sample to 30 QSOs and extends to sources with slower growth rates. The data confirm that steep X-ray slopes are a ubiquitous feature of the $z\gtrsim6$ QSOs population at EoR, independent of the SMBH growth-rate selection.

The enhanced X-ray spectral steepness in these high-redshift quasars results in significantly fainter X-ray fluxes than those predicted by a canonical X-ray spectral slope. We show that if the fainter $z\gtrsim 5-6$ AGN population exhibits similarly steep spectra, it would provide a natural explanation for the widespread lack of X-ray emission observed for the AGN population emerging from the JWST surveys.

Furthermore, we discuss how the persistence of steep X-ray spectra in the whole AGN population would carry critical implications for future EoR X-ray surveys. Indeed, non-canonical spectral shapes if not accounted for properly, can significantly bias detectability and flux/luminosity estimates, necessitating a recalibration of current survey strategies.