Widespread steep X-ray spectra of luminous z≃5.85−7.6 quasars and their implications for future X-ray surveys

@misc{tortosa2026widespread,
  title = {{Widespread steep X-ray spectra of luminous $z\simeq5.85-7.6$ quasars and their implications for future X-ray surveys}},
  author = {A Tortosa and L Zappacosta and E Piconcelli and I Saccheo and G Lanzuisi and E Bertola and F Vito and M Bischetti and M Brazzini and M Castellano and S Carniani and A Comastri and S Gallerani and R Gilli and F La Franca and P Madau and R Maiolino and G Miniutti and B Musiimenta and C Piscitelli and F Salvestrini and F Tombesi and R Tripodi and C Vignali and M Volonteri and A Bongiorno and L Borrelli and M Brusa and I.V Chilingaria and F Civano and S Cristiani and V d'Odorico and D de Cicco and C Done and M Elvis and X Fan and C Feruglio and F Fiore and A Grazian and M Guainazzi and F Haardt and A Luminari and N Menci and R Middei and F Nicastro and M Paolillo and M Perna and S Puccetti and R Schneider and V Testa and R Valiante and L Vallini and E Vanzella and G Vietri},
  howpublished = {HAL},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.10522},
  eprint = {2610.10522},
  archivePrefix = {arXiv},
  url = {https://hal.science/hal-05842229v1},
  note = {CC BY 4.0},
}

TY  - UNPB
TI  - Widespread steep X-ray spectra of luminous z≃5.85−7.6 quasars and their implications for future X-ray surveys
AU  - A Tortosa
AU  - L Zappacosta
AU  - E Piconcelli
AU  - I Saccheo
AU  - G Lanzuisi
AU  - E Bertola
AU  - F Vito
AU  - M Bischetti
AU  - M Brazzini
AU  - M Castellano
AU  - S Carniani
AU  - A Comastri
AU  - S Gallerani
AU  - R Gilli
AU  - F La Franca
AU  - P Madau
AU  - R Maiolino
AU  - G Miniutti
AU  - B Musiimenta
AU  - C Piscitelli
AU  - F Salvestrini
AU  - F Tombesi
AU  - R Tripodi
AU  - C Vignali
AU  - M Volonteri
AU  - A Bongiorno
AU  - L Borrelli
AU  - M Brusa
AU  - I.V Chilingaria
AU  - F Civano
AU  - S Cristiani
AU  - V d'Odorico
AU  - D de Cicco
AU  - C Done
AU  - M Elvis
AU  - X Fan
AU  - C Feruglio
AU  - F Fiore
AU  - A Grazian
AU  - M Guainazzi
AU  - F Haardt
AU  - A Luminari
AU  - N Menci
AU  - R Middei
AU  - F Nicastro
AU  - M Paolillo
AU  - M Perna
AU  - S Puccetti
AU  - R Schneider
AU  - V Testa
AU  - R Valiante
AU  - L Vallini
AU  - E Vanzella
AU  - G Vietri
PY  - 2026
DA  - 2026/10/28
PB  - HAL
DO  - 10.48550/arxiv.2610.10522
UR  - https://hal.science/hal-05842229v1
AB  - Luminous quasars (QSOs) at the Epoch of Reionization (EoR, z≳6) host billion-solar-mass black holes assembled within the first Gyr of cosmic time. Their X-ray emission probes the innermost accreting regions and constrains the physical state of the corona during early supermassive black hole (SMBH) growth. Previous results from the HYPerluminous quasars at the Epoch of ReionizatION (HYPERION) sample of z ≳ 6 QSOs, powered by the most rapidly accreted SMBHs, suggested a link between X-ray spectral slope and long-term growth efficiency. We test whether the steep X-ray spectra of luminous z>6 QSOs are a general property of early SMBH growth or a peculiarity of the most efficiently grown SMBHs, using a sample extended mainly to the lowest growth efficiencies. We analysed the largest and most X-ray sensitive sample of 28 luminous QSOs at z = 5.85−7.64 (26 used in the correlation analysis), combining HYPERION targets, luminous non-HYPERION sources, and QSOs powered by moderately accreting SMBHs newly observed in our XMM-Newton Large Programme. We re-derived black hole masses and growth indicators for the entire sample homogeneously, incorporating the latest JWST-based spectroscopic measurements to ensure the most consistent and high-quality characterization across all targets. We found no statistically significant correlation between coronal spectral parameters and BH growth indicators: luminous z ≳ 6 QSOs systematically show steep X-ray continua, with high photon indices and/or low high-energy cutoffs, consistent with cold coronae, possibly linked to rapid accretion. Finally, if steep X-ray slopes are common among early AGN, they may significantly reduce count rates, at fixed intrinsic luminosity, in future surveys of the EoR, affecting the detectability of steep-spectrum X-ray sources and thus the census of the early AGN population.
ER  -