Spingola, C.C.SpingolaDALLACASA, DANIELEDANIELEDALLACASABelladitta, S.S.BelladittaCACCIANIGA, AlessandroAlessandroCACCIANIGAGIROLETTI, MARCELLOMARCELLOGIROLETTIMORETTI, AlbertoAlbertoMORETTIORIENTI, MonicaMonicaORIENTI2021-09-202021-09-2020200004-6361http://hdl.handle.net/20.500.12386/31046We present Director's Discretionary Time multi-frequency observations obtained with the Jansky Very Large Array (VLA) and the Very Long Baseline Array (VLBA) of the blazar PSO J030947.49+271757.31 (hereafter PSO J0309+27) at z = 6.10+/-0.03. The milliarcsecond angular resolution of our VLBA observations at 1.5, 5 and 8.4 GHz unveils a bright one-sided jet extended for $\sim500$ parsecs in projection. This high-z radio-loud AGN is resolved into multiple compact sub-components, embedded in a more diffuse and faint radio emission, which enshrouds them in a continuous jet structure. We derive limits on some physical parameters directly from the observable quantities, such as viewing angle, Lorentz and Doppler factors. If PSO J0309+27 is a genuine blazar, as suggested by its X-ray properties, then we find that its bulk Lorentz factor must be relatively low (less than 5). Such value would be in favour of a scenario currently proposed to reconcile the paucity of high-z blazars with respect to current predictions. Nevertheless, we cannot exclude that PSO J0309+27 is seen under a larger viewing angle, which would imply that the X-ray emission must be enhanced, for example, by inverse Compton with the Cosmic Microwave Background. More stringent constraints on the bulk Lorentz factor in PSO J0309+27 and the other high-z blazars are necessary to test whether their properties are intrinsically different with respect to the low-z blazar population.STAMPAenArticle10.1051/0004-6361/2020394582-s2.0-85096175949WOS:000593929300001https://www.aanda.org/articles/aa/full_html/2020/11/aa39458-20/aa39458-20.htmlhttp://arxiv.org/abs/2010.11193v1FIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation::PE9_11 Relativistic astrophysics