2025A&A...693A.157S

Aims. We aim to characterize the X-ray-to-optical/near-infrared(NIR) broad-band emission of luminous quasars (QSOs) in the first gigayear (Gyr) of cosmic evolution in order to decipher whether or not they exhibit differences compared to the lower-z QSO population. Our goal is also to provide a reliable and uniform catalog of derivable properties for these objects (from fitting their spectral energy distribution), such as bolometric and monochromatic luminosities, Eddington ratios, dust extinction, and the strength of the hot dust emission.
Methods. We gathered all available photometry -from XMM-Newton proprietary data in X-rays to rest-frame NIR wavelengths- for the 18 QSOs in the HYPERION samples (6.0 ≤ z ≤ 7.5). For sources lacking uniform NIR coverage, we conducted NIR observations in the J, H, and K bands. To increase the statistical robustness of our analysis across the UV-to-NIR region, we add 36 additional sources to our sample from the E-XQR-30 sample with 5.7 ≲ z ≲ 6.6. We characterized the X-ray/UV emission of each QSO using average SEDs from luminous Type 1 sources and calculated bolometric and monochromatic luminosities. Finally, we constructed a mean SED extending from the X-rays to the NIR bands.
Results. We find that the UV-optical emission of these QSOs can be modeled with templates of z ∼ 2 luminous QSOs. We observe that the bolometric luminosities derived while adopting some bolometric corrections at 3000 Å (BC_3000 Å_) largely used in the literature are slightly overestimated, by 0.13 dex, as they also include reprocessed IR emission. We estimate a revised value of BC_3000 Å_ = 3.3, which can be used to derive L_bol in z ≥ 6 QSOs. We provide a subsample of 11 QSOs with rest-frame NIR photometry; these show a broad range of hot dust emission strength, with two sources exhibiting low levels of emission. Despite potential observational biases arising from nonuniform photometric coverage and selection biases, we produce an X-ray-to-NIR mean SED for QSOs at z ≳ 6 that is a good match to templates of lower-redshift, luminous QSOs up to the UV-optical range, with a slightly enhanced contribution from hot dust in the NIR.