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We present a multi-wavelength study on three new z5. 6 quasi stellar objects (QSOs) selected based on their radio and optical/near-infrared properties in publicly available surveys and then identified with dedicated spectroscopic observations. These are among the radio-brightest QSOs currently known at z>5. 5, having R=S ₅₆₇ₙ/S ₄₄₀₀₀>100. In this work we present their identification and we also discuss their multi-wavelength properties (from the radio to the X-ray band) based on the detection in public surveys as well as dedicated radio and X-ray observations. The three sources present a wide range of properties, in terms of relative intensity and spectral shape, highlighting the importance of multi-wavelength observations in order to accurately characterise these high-z objects. In particular, from our analysis we found one source, at z=5. 61, that presents clear blazars properties (strong and flat radio and X-ray emission), making it one of the most distant currently known in this class. Moreover, from the fit of the optical/near-infrared photometric measurements with an accretion disc model, as well as the analysis of the CIV broad emission line in one case, we were able to estimate the mass and the accretion rate of the central black holes in these systems, finding M ₁₇1-1010⁹ M_ accreting at a rate ₄₃₃0. 1-0. 2. With this work we increase the number of very-high redshift radio-powerful QSOs characterised with multi-wavelength observations, essential in order to understand the evolution and the properties of this still poorly constrained class of sources.
Ighina et al. (Thu,) studied this question.