Hard X-ray bolometric correction as a function of the bolometric luminosity. XID439 is indicated by the red star. Other X-ray QSOs at z ~ 1–3.5 for which ionised gas outflows on the kiloparsec scale have been detected and for which a solid estimate of the bolometric luminosity from SED fitting is available are also plotted as labelled. In most cases, the X-ray spectral analysis and SED fitting were performed on data of similar quality (in terms of counting statistics, depth of the images, and photometric errors) of the analysis of XID439, and we expect that the associated error bar would be similar. The solid and dotted lines are the analytic expressions for the hard X-ray bolometric correction and its scatter derived in Duras et al. (2020).