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Published October 2025 | Version Published
Journal Article Open

The WISSH quasar project. XII. X-ray view of the most luminous quasi-stellar objects at Cosmic Noon

Creators

  • 1. ROR icon Max Planck Institute for Radio Astronomy
  • 2. ROR icon University of Bologna
  • 3. INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via P. Gobetti 93/3, 40129, Bologna, Italy
  • 4. ROR icon Astronomical Observatory of Rome
  • 5. ROR icon Arcetri Astrophysical Observatory
  • 6. ROR icon Agenzia Spaziale Italiana
  • 7. ROR icon Roma Tre University
  • 8. ROR icon Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano
  • 9. ROR icon University of Trieste
  • 10. ROR icon Trieste Astronomical Observatory
  • 11. ROR icon National Institute for Astrophysics
  • 12. ROR icon Scuola Normale Superiore di Pisa
  • 13. ROR icon Institute for Fundamental Physics of the Universe
  • 14. ROR icon University of Milano-Bicocca
  • 15. ROR icon University of Modena and Reggio Emilia
  • 16. ROR icon Middlebury College
  • 17. ROR icon California Institute of Technology
  • 18. ROR icon University of Rome Tor Vergata
  • 19. ROR icon National Institute for Nuclear Physics
  • 20. ROR icon Centro de Astrobiología
  • 21. Istituto Nazionale di Astrofisica INAF IASF Palermo, Via Ugo La Malfa 153, Palermo, 90146, Italy

Abstract

Aims. To improve our knowledge of the nuclear emission of luminous quasi-stellar objects (QSOs) at Cosmic Noon, we studied the X-ray emission of the WISE/SDSS-selected hyper-luminous (WISSH) QSO sample. It consists of 85 broad-line active galactic nuclei (AGN) with bolometric luminosities Lbol > few × 1047 erg s−1 at z ≈ 2 − 4. Our goal is to characterise their X-ray spectral properties and investigate the relation between the X-ray luminosity and the energy output in other bands. To this end, we compared the nuclear properties of powerful QSOs with those derived for the majority of the AGN population.

Methods. We were able to perform X-ray spectral analysis for about one-half of the sample. For 16 sources, we applied the hardness ratio analysis, while for the remaining sources we estimated their 2 − 10 keV intrinsic luminosity L2 − 10; only 8 sources were not detected.

Results. We report a large dispersion in L2 − 10 despite the narrow distribution in Lbol, 2500 Å intrinsic luminosity L2500 Å, and 6 μm intrinsic luminosity λL6 μm of WISSH QSOs (approximately one-third of the sources classified as X-ray-weak QSOs). This suggests that the properties of the X-ray corona and inner accretion flow in hyper-luminous QSOs can be significantly different from those of typical less powerful AGN. The distribution of the X-ray spectral index does not differ from that of AGN at lower redshift and lower Lbol, and does not depend on the Eddington ratio (λEdd) and X-ray weakness. The majority of WISSH QSOs, for which it was possible to estimate the presence of intrinsic absorption (≈65% of the sample), exhibit little to no obscuration (i.e. column density NH ≤ 5 × 1022 cm−2). Among the X-ray obscured sources, we find some blue QSOs without broad absorption lines (BALs) that fall within the ‘forbidden region’ of the Log(NH)−Log(λEdd) plane, which is typically occupied by dust-reddened QSOs and is associated with intense feedback processes. Additionally, we confirm a significant correlation between L2 − 10 and velocity shift of the CIV emission line, a tracer of nuclear ionised outflows.

Conclusions. Multi-wavelength observations of the broad-line WISSH quasars at Cosmic Noon and, in particular, their complete X-ray coverage, allow us to properly investigate the accretion disk–corona interplay to the highest luminosity regime. The distribution of bolometric corrections kbol and X-ray–to–optical indices αOX of the WISSH quasars is strikingly broad, suggesting that caution should be exercised when using Lbol, L2500 Å, and λL6 μm to estimate the X-ray emission of individual luminous QSOs.

Copyright and License

© The Authors 2025. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Open Access funding provided by Max Planck Society.

Acknowledgement

The scientific results reported in this article are based to a significant degree on observations made by the Chandra X-ray Observatory, and XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. The research has made use of data obtained from the Chandra Data Archive provided by the Chandra X-ray Center (CXC). Data analysis was performed with the XMM-Newton SAS and CXC CIAO software packages. Support for this work was provided by the National Aeronautics and Space Administration through Chandra Award Number GO2-23087X issued by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060. We acknowledge financial support from the Bando Ricerca Fondamentale INAF Large Grant 2022 “Toward an holistic view of the Titans: multi-band observations of z > 6 QSOs powered by greedy supermassive black holes”. We thank the anonymous referee for the constructive comments that helped to improve the manuscript. We also acknowledge E. Ros for the useful suggestions. FT and ML acknowledges funding from the European Union – Next Generation EU, PRIN/MUR 2022 (2022K9N5B4). MG acknowledges support from the ERC Consolidator Grant BlackHoleWeather (101086804). EB acknowledges financial support from INAF under the Large Grant 2022 “The metal circle: a new sharp view of the baryon cycle up to Cosmic Dawn with the latest generation IFU facilities”. FS acknowledges financial support from the PRIN MUR 2022 2022TKPB2P – BIG-z, Ricerca Fondamentale INAF 2023 Data Analysis grant “ARCHIE ARchive Cosmic HI & ISM Evolution”, Ricerca Fondamentale INAF 2024 under project “ECHOS” MINI-GRANTS RSN1. MB acknowledges support from INAF project 1.05.12.04.01 – MINI-GRANTS di RSN1 “Mini-feedback” and from UniTs under FVG LR 2/2011 project D55-microgrants23 “Hyper-gal”. GM is funded by Spanish MICIU/AEI/10.13039/501100011033 and ERDF/EU grant PID2023-147338NB-C21. CP acknowledge funding by the European Union – Next Generation EU, Mission 4 Component 1 CUP C53D23001330006. EG acknowledges the generous support of the Cottrell Scholar Award through the Research Corporation for Science Advancement. EG is grateful to the Mittelman Family Foundation for their generous support. LZ acknowledges support from the European Union – Next Generation EU, PRIN/MUR 2022 2022TKPB2P – BIG-z. LZ acknowledges partial support by grant NSF PHY-2309135 to the Kavli Institute for Theoretical Physics (KITP).

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Additional details

Related works

Is new version of
Discussion Paper: arXiv:2509.08055 (arXiv)

Funding

National Aeronautics and Space Administration
Chandra GO2-23087X
National Aeronautics and Space Administration
NAS8-03060
National Institute for Astrophysics
Bando Ricerca Fondamentale -
European Union
PRIN/MUR 2022 2022K9N5B4
European Research Council
BlackHoleWeather 101086804
European Union
2022TKPB2P
National Institute for Astrophysics
1.05.12.04.01
Ministerio de Ciencia, Innovación y Universidades
PID2023-147338NB-C21
European Union
C53D23001330006
Research Corporation for Science Advancement
Cottrell Scholar Award -
European Union
2022TKPB2P
National Science Foundation
PHY-2309135

Dates

Accepted
2025-07-31
Available
2025-10-14
Published online

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Published