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

Abstract

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. 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. 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 ${\rm C}_{\rm IV}$ emission line blueshifts, comparing them to typical optically blue quasars. Our analysis reveals that seven out of 10 quasars in our sample have photon indices $\Gamma>1.7$. 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 $\sim$2$\sigma$ confidence level that the correlation between the excessively low X-ray flux level and the presence of ${\rm C}_{\rm IV}$ emission-line outflows might emerge at wind velocities greater than 3000 km/s. 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.