2023A&A...678A.201Z

The existence of luminous quasars (QSOs) at the epoch of reionization (EoR; i.e., z > 6) powered by well-grown supermassive black holes (SMBHs) with masses ≳10⁹ M_⊙ challenges models of early SMBH formation and growth. To shed light on the nature of these sources, we started a multiwavelength program based on a sample of 18 HYPerluminous quasars at the Epoch of ReionizatION (HYPERION). These are the luminous QSOs whose SMBHs must have had the most rapid mass growth during the first gigayear of the Universe and therefore acquired the largest mass at their respective epochs. In this paper, we present the HYPERION sample and report results from the first year of the planned three years of observations of the 2.4 Ms XMM-Newton Multi-Year Heritage program on which HYPERION is based. The goal of this program is to accurately characterise the X-ray nuclear properties of QSOs at the EoR. Through a joint X-ray spectral analysis of ten sources, covering the rest-frame ∼2 - 50 keV energy range, we report a steep average photon index (Γ ≈ 2.4 ± 0.1). No absorption is required at levels of 10²¹ - 10²² cm^–2. The measured average Γ is inconsistent at ≥4σ level with the canonical value (Γ = 1.8 - 2) measured in QSOs at z < 6. Such a steep spectral slope is also significantly steeper than that reported in lower-z analog QSOs with similar luminosity or accretion rate, suggesting genuine redshift evolution. Alternatively, we can interpret this result as suggesting the presence of a very low energy cutoff E_cut ≈ 20 keV on a standard Γ = 1.9 power-law, the likes of which is rarely reported at lower z. We also report mild indications that, on average, HYPERION QSOs show higher levels of coronal soft X-rays at 2 keV compared to the accretion disk UV at 2500 Å than expected for lower-z AGN in the high-luminosity regime. We speculate that either a redshift-dependent coupling between the X-ray corona and accretion disk or intrinsically different coronal properties account for the steepness of the X-ray spectral slope, especially in the presence of powerful winds. The reported steep slopes, if confirmed in lower-luminosity regimes, may have an important impact on the design of next-generation X-ray facilities and future surveys designed to investigate the early Universe.