The $\gamma$-ray-emitting blazar B3 1239+376 at $z$ = 3.82 identified in a multi-wavelength context

TL;DR

This study identifies B3 1239+376 as the third most distant gamma-ray detected blazar at z=3.82, using multi-wavelength data to analyze its jet properties and emission mechanisms.

Contribution

First multi-wavelength confirmation of a gamma-ray emitting blazar at such a high redshift, with detailed spectral modeling of its jet characteristics.

Findings

Significant gamma-ray detection with 7.7σ significance in 2025.

Correlated gamma-ray and infrared light curves indicating co-spatial emission.

Broadband spectral energy distributions modeled successfully with a one-zone leptonic model.

Abstract

Among thousands of extragalactic $\gamma$-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 multi-band study of the radio quasar B3 1239+376 with $z$ = 3.82. By analyzing the Fermi-LAT data, a significant (globally 7.7$\sigma$) $\gamma$-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 co-spatial $\gamma$-ray residues in prior epochs. Moreover, the $\gamma$-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 $\gamma$-ray source and the quasar. Therefore, B3 1239+376 is proposed as the third most distant $\gamma$-ray detected blazar to date. Benefiting from the multi-wavelength 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 $\gamma$ rays, prompt follow-up observations are encouraged, especially radio interferometry observations which may catch the potential ejection of a new jet blob.