The X-ray properties of the most luminous quasars with strong emission-line outflows

¹ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
² Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy
⁵ Dipartimento di Matematica e Fisica, Univeristà di Roma 3, Via della Vasca Navale, 84, 00146 Roma RM, Italy
⁶ European Space Agency (ESA), European Space Research and Technology Centre, Noordwijk, The Netherlands
⁷ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D Arica, Chile
⁸ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁹ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁰ Department of Physics, Drexel University, 32 S. 32nd Street, Philadelphia, PA 19104, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 May 2025
Accepted: 19 November 2025

Abstract

Context. Strong outflows from active galactic nuclei are frequently observed in objects with lower coronal X-ray luminosity. This intrinsic X-ray weakness is considered a requirement for the formation of radiatively driven winds.

Aims. To obtain an unbiased view on the connection between X-ray emission and the presence of powerful winds in the most luminous quasar phase, we present an X-ray analysis of a sample of extremely luminous, radio-quiet quasars with signatures of strong outflows in their rest-frame ultraviolet (UV) emission spectra.

Methods. We study the Chandra X-ray spectral properties of 10 objects, selected from the Sloan Digital Sky Survey Data Release 16 quasar catalogue based on their UV luminosities and C IV emission line blueshifts, comparing them to typical optically blue quasars.

Results. Our analysis reveals that seven out of 10 quasars in our sample have photon indices Γ > 1.7. Only two out of 10 objects exhibiting outflows with velocities exceeding 1400 km/s are X-ray ‘weak’, consistent with the fraction of X-ray ‘weak’ objects generally observed in quasar populations. Notably, one of the objects identified as X-ray ‘weak’ is likely an intrinsically X-ray ‘normal’ quasar that is heavily obscured. We observe a tentative indication at a ∼2σ confidence level that the correlation between the excessively low X-ray flux level and the presence of C IV emission-line outflows might emerge at wind velocities greater than 3000 km/s.

Conclusions. Our study provides additional evidence that the relationship between X-ray emission and the presence of winds is intricate. Our findings emphasise the need for X-ray observations of a larger sample of UV-selected quasars with confirmed strong emission-line outflows to unravel the nuanced interplay between winds and X-ray emission.