Presentation #100.58 in the session AGN.
High-redshift gamma-ray blazars (z>3) offer the possibility to study black hole growth, accretion processes and jet acceleration in the early Universe. The most luminous blazars, detectable out to high redshifts, tend to have the peaks in their non-thermal spectral energy distributions at relatively low frequencies, with the high-energy peak often appearing at MeV energies. In addition, the cosmological redshift further shifts the SED peaks towards lower frequencies. Because of their low fluxes and soft gamma-ray spectra, the detection of gamma-ray emission from these sources is difficult and only about a dozen have been detected by the Large Area Telescope (LAT) onboard the Fermi satellite. Flaring events provide a unique opportunity to detect and characterize the gamma-ray emission from high-z blazars and to gather contemporaneous multiwavelength observations that are necessary to interpret their spectral energy distribution. For this reason, we have designed a program to find flares in high-z blazars by using the public Fermi-LAT data, which is suitable to trigger multi-wavelength observations. In February 2022, we detected a long-term flare by the very distant blazar GB 1508+5714 (z=4.31), whose detection at gamma-ray energies was reported in 2017. The flaring event was accompanied by a significant spectral hardening, indicating considerable changes in the gamma-ray spectrum. We obtained follow-up observations across the electromagnetic spectrum and put a focus on observations at radio frequencies. We launched a dense, long-term monitoring campaign with the Effelsberg radio telescope to study the radio-gamma correlation for this high-redshift blazar. Previous VLBI observations revealed an extended jet emission being visible for GB 1508+5714. Hence, we acquired three VLBI observations with the VLBA and Effelsberg, which were taken with a cadence of five months in order to search for potential changes in the appearance of the jet. We will present the VLBI maps and broadband modelling of GB 1508+5714 with simultaneous multi-wavelength data taken during the flare, and put our findings in the context of future MeV missions.