Towards an informed quest for accretion disc winds in quasars: the intriguing case of Ton 28

We report on the detection of a blueshiftedFe K absorption feature in two consecutive XMM-Newton observations of the luminous blue quasarTon 28, at the 4$\sigma$ cumulative significance. The rest energy of 9.2 keV implies the presence of an accretion discwind with bulk outflow velocity of $\sim$0.28$c$, while the kinetic power is most likely a few per cent of the quasarluminosity. Remarkably, Ton 28 had been specifically selected as an optimal target to reveal an ultra-fast X-ray wind based on its total luminosity ($L_{\rm bol} > 10^{46}$ erg s$^{-1}$) and [OIII] $\lambda$5007\AA\ equivalent width(EW < 6 \AA), suggestive of high accretion rate and low inclination, respectively. Other peculiar optical/UV emission-line properties include narrow H$\beta$, strong FeII and blueshiftedCIV. These are key parameters in the Eigenvector 1 formalism, and are frequently found in active galaxies with ongoing accretion discwinds, hinting at a common physical explanation. Provided that the effectiveness of our selection method is confirmed with similar sources, this result could represent the first step towards the characterization of black-hole winds through multiwavelength indicators in the absence of high-quality X-ray spectra.