HYPerluminous quasars at the Epoch of ReionizatION (HYPERION): A new regime for the X-ray nuclear properties of the first quasars

¹ INAF – Osservatorio Astronomico di Roma, Via di Frascati 33, 00078 Monte Porzio Catone, Italy
e-mail: luca.zappacosta@inaf.it
² INAF – Osservatorio Astronomico di Trieste, Via G. Tiepolo 11, 34143 Trieste, Italy
³ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Trieste, Italy
⁴ Dipartimento di Matematica e Fisica, Università Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
⁵ INFN, Sezione Roma1, Dipartimento di Fisica, Università di Roma La Sapienza, Piazzale Aldo Moro 2, 00185 Roma, Italy
⁶ Dipartimento di Fisica e Astronomia ‘Augusto Righi’, Università degli Studi di Bologna, Via P. Gobetti, 93/2, 40129 Bologna, Italy
⁷ INAF-Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti, 93/3, 40129 Bologna, Italy
⁸ Institut d’Astrophysique de Paris, Sorbonne Université, CNRS, UMR 7095, 98 bis bd Arago, 75014 Paris, France
⁹ Dipartimento di Fisica, Sezione di Astronomia, Università di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
¹⁰ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
¹¹ Center for Astrophysics – Harvard & Smithsonian, Cambridge, MA 02138, USA
¹² INFN – Sezione di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹³ Space Science Data Center, SSDC, ASI, Via del Politecnico snc, 00133 Roma, Italy
¹⁴ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, ESAC campus, 28692 Villanueva de la Cañada, Spain
¹⁵ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁶ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
¹⁷ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁸ European Space Agency, ESTEC, Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
¹⁹ ASI – Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
²⁰ Dipartimento di Fisica, Università di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
²¹ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
²² INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, Via A. Corti 12, 20133 Milano, Italy
²³ INFN-National Institute for Nuclear Physics, Via Valerio 2, 34127 Trieste, Italy
²⁴ DiSAT, Università degli Studi dell’Insubria, Via Valleggio 11, 22100 Como, Italy
²⁵ INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
²⁶ INAF, Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
²⁷ Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Ave., Cambridge CB3 0HE, UK
²⁸ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
²⁹ Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
³⁰ Dipartimento di Fisica, Università di Roma La Sapienza, Piazzale Aldo Moro 2, 00185 Roma, Italy
³¹ Sapienza School for Advanced Studies, Viale Regina Elena 291, 00161 Roma, Italy

Received: 2 May 2023
Accepted: 5 July 2023

Abstract

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.