The γ-Ray-emitting Blazar B3 1239+376 at z = 3.82 Identified in a Multiwavelength Context

Among thousands of extragalactic γ -ray emitters, only a handful of distant ( z > 3) sources are detected. Yet they are crucial probes, shedding light on the cosmic evolution of jets of active galactic nuclei and the initial phase of mass growth of supermassive black holes. Here, we report on a multiband study of the radio quasar B3 1239+376 at z = 3.82. By analyzing the Fermi Large Area Telescope data, a significant (globally 7.7 σ ) γ -ray source in its direction, with an estimated association probability of 0.91, is observed in a half-year period of 2025. The analysis also reveals the emergence of cospatial γ -ray residues in prior epochs. Moreover, the γ -ray and infrared light curves obtained from WISE and SPHEREx observations are likely correlated, as we observe that the emissions in both bands peak at the same time. The temporal coincidence establishes a firm association relationship between the γ -ray source and the quasar. Therefore, B3 1239+376 is proposed as the third-most-distant γ -ray detected blazar to date. Benefiting from the multiwavelength observations, broadband spectral energy distributions in different flux states are compiled and reproduced by the classic one-zone leptonic radiation model, to investigate the jet properties. Considering the recent brightening in γ rays, prompt follow-up observations are encouraged, especially radio interferometry observations that may catch the potential ejection of a new jet blob.