The WISSH quasar project

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

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Dipartimento di Fisica e Astronomia “Augusto Righi”, Università degli Studi di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via P. Gobetti 93/3, 40129 Bologna, Italy
⁴ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, Italy
⁵ INAF–OAA, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50127 Firenze, Italy
⁶ Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
⁷ Dipartimento di Matematica e Fisica, Università Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
⁸ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, Via A. Corti 12, 20133 Milano, Italy
⁹ Dipartimento di Fisica, Università di Trieste, Via Alfonso Valerio 2, 34127 Trieste, Italy
¹⁰ INAF–OAT, Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34131 Trieste, Italy
¹¹ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Roma, Italy
¹² Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
¹³ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Trieste, Italy
¹⁴ Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
¹⁵ Department of Physics, Informatics and Mathematics, University of Modena and Reggio Emilia, 41125 Modena, Italy
¹⁶ Department of Physics, Middlebury College, Middlebury, VT, 05753 USA
¹⁷ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, 1200 California Boulevard, Pasadena, CA, 91125 USA
¹⁸ Physics Department, Tor Vergata University of Rome, Via della Ricerca Scientifica 1, 00133 Rome, Italy
¹⁹ INFN – Rome Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy
²⁰ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
²¹ Istituto Nazionale di Astrofisica INAF IASF Palermo, Via Ugo La Malfa 153, Palermo, 90146 Italy

^⋆ Corresponding author: cdagosti@mpifr-bonn.mpg.de

Received: 1 April 2025
Accepted: 31 July 2025

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 L_bol > 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 L_2 − 10; only 8 sources were not detected.

Results. We report a large dispersion in L_2 − 10 despite the narrow distribution in L_bol, 2500 Å intrinsic luminosity L_2500 Å, and 6 μm intrinsic luminosity λL_6 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 L_bol, and does not depend on the Eddington ratio (λ_Edd) 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 N_H ≤ 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(N_H)−Log(λ_Edd) plane, which is typically occupied by dust-reddened QSOs and is associated with intense feedback processes. Additionally, we confirm a significant correlation between L_2 − 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 k_bol and X-ray–to–optical indices α_OX of the WISSH quasars is strikingly broad, suggesting that caution should be exercised when using L_bol, L_2500 Å, and λL_6 μm to estimate the X-ray emission of individual luminous QSOs.