The WISSH quasars project

III. X-ray properties of hyper-luminous quasars

¹ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
e-mail: silvia.martocchia@gmail.com
² INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00078 Monteporzio Catone, Italy
³ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
⁴ Università degli Studi di Roma “La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy
⁵ Dipartimento di Fisica e Astronomia, Università di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
⁶ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁷ Università degli Studi di Roma “Tor Vergata”, via Orazio Raimondo 18, 00173 Roma, Italy
⁸ Dipartimento di Fisica e Astronomia, Università di Firenze, via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
⁹ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹⁰ Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA
¹¹ Centro de Astrobiología (CSIC–INTA), Depto. de Astrofísica, ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Spain
¹² INAF–Istituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere 100, 00133 Rome, Italy
¹³ Max Planck Institute for Radio Astronomy, Auf dem Hugel 69, 53121 Bonn, Germany

Received: 5 June 2017
Accepted: 31 July 2017

Abstract

We performed a survey of the X-ray properties of 41 objects from the WISE/SDSS selected hyper-luminous (WISSH) quasars sample, which includes 86 broad-line quasars with bolometric luminosity L_Bol ≳ 2 × 10⁴⁷ erg s^-1 shining at z ~ 2–4. We used both proprietary and archival Chandra and XMM-Newton observations. Twenty-one quasars have sufficient quality data to perform a spectroscopic analysis, while for the remaining sources, X-ray properties are derived through hardness-ratio analysis (apart for six sources that result to be undetected). The bulk (~70%) of the detected WISSH quasars exhibit N_H <5 × 10²² cm^-2, in agreement with their optical Type 1 AGN classification. All but three quasars show unabsorbed 2–10 keV luminosities L₂₋₁₀≥ 10⁴⁵ erg s^-1. Thanks to their extreme radiative output across the mid-IR-to-X-ray range, WISSH quasars therefore offer the opportunity to significantly extend and validate the existing relations involving L₂₋₁₀. Specifically, we studied the X-ray luminosity as a function of (i) X-ray-to-optical (X/O) flux ratio; (ii) mid-IR luminosity (L_MIR); (iii) L_Bol, and (iv) α_OX versus 2500 Å luminosity. We find that the WISSH quasars show (i) unreported very low X/O ( <0.1) compared to typical AGN values; (ii) L₂₋₁₀/L_MIR ratios that are significantly smaller than those derived for AGN with lower luminosity; (iii) a large X-ray bolometric correction, k_Bol,X ≈ 100–1000; and (iv) steep −2≳α_OX≳−1.7. These results lead to a scenario in which the X-ray emission of hyper-luminous quasars is relatively weaker compared to lower luminosity AGN. Models predict that such an X-ray weakness can be relevant for the acceleration of powerful high-ionization, emission-line-driven winds, which are commonly detected in the UV spectra of WISSH quasars and can, in turn, perturb the X-ray corona and weaken its emission. Accordingly, hyper-luminous quasars represent the ideal laboratory to study the link between the AGN energy output and wind acceleration. Additionally, WISSH quasars exhibit very large SMBH masses (log [M_BH/M_⊙]≳ 9.5). This enables a more robust modeling of the Γ–M_BH relation by increasing the statistics at high masses. We derive a flatter Γ dependence than previously found over the broad range 5 ≲log (M_BH/M_⊙)≲ 11. Finally, we estimate that only 300 ks observations of X-IFU on board Athena will offer a detailed view of the properties of absorption features associated with powerful X-ray SMBH winds for a representative sample of WISSH quasars.