2025ApJ...990..206G

In 2023 November, the Fermi Large Area Telescope detected a γ-ray flare from the high-redshift blazar GB6 B1428+4217 (z = 4.715). We initiated a multiwavelength follow-up campaign involving Swift, NuSTAR, the Sierra Nevada and Perkins Observatories, and the Effelsberg 100 m radio telescope. This source, also known as 5BZQ J1430+4204, has shown an anomalous soft X-ray spectrum in previous observations, including possible ionized absorption features or signatures of bulk Comptonization of thermal electrons, which are also detected during the flaring episode. Simultaneous optical data revealed a polarization fraction of ∼8% in the R band, confirming that synchrotron emission dominated over thermal emission from the accretion disk. The hard X-ray flux was enhanced during the flare. Modeling of the broadband spectral energy distribution suggests that the high-energy component is dominated by Compton scattering by external seed photons from the accretion disk. The origin of the flare is consistent with the injection of a hard-spectrum electron population in the emission region. With a γ-ray luminosity among the top 5% of flaring events, GB6 B1428+4217 exemplifies a prototypical MeV blazar. Its Compton-dominated spectral energy distribution and extreme luminosity are in line with expectations from the blazar sequence. High-redshift flares like this are critical for understanding jet physics in the early Universe and may improve detection prospects with future missions such as the Compton Spectrometer and Imager.