Abstract of the proposal for the experiment EA069B

Blazars are an ideal tool to study the cosmological evolution of supermassive black holes hosted by radio-loud AGN and to understand the role played by relativistic jets in their growth. But so far, there are only two blazars (J0906+6930, PSO J0309+27) discovered at z>=5.5, far below the predictions of theoretical models. Finding more blazars at z>~5.5 is crucial to constrain the number density of blazars in the early Universe. Recently eROSITA discovered a z=5.5 X-ray luminous quasar J1702+1301 in its first half-year survey and detected X-ray variability over a time span of ~1 month. The radio loudness is in the range of 300-1200, making it a blazar candidate. Our RATAN-600 total flux density monitoring suggests that the source is in a radio flaring state recently. To confirm or refute its blazar nature and to image its radio structure, we apply for 1.7 and 5 GHz EVN+e-MERLIN and 22 GHz EVN-only observations.