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

TL;DR

This work addresses the scarcity of γ-ray blazars at $z>3$ by conducting a coordinated, multi-wavelength analysis of B3 1239+376 ($z=3.82$). It combines long-term Fermi-LAT data with X-ray, optical/IR, and radio observations, identifying a significant γ-ray residue in 2025 that is spatially and temporally associated with the quasar, and revealing contemporaneous infrared brightening. The broadband SEDs are well described by a classic one-zone leptonic jet model, yielding a bulk Lorentz factor of $\Gamma \sim 15$ and a Compton-dominant jet ($\sim 10$), and implying Lyα external photons drive the EC process. These results demonstrate that powerful relativistic jets were already established in the early universe and highlight the importance of rapid, multi-wavelength follow-up to probe jet physics and SMBH growth in high-redshift systems.

Abstract

Among thousands of extragalactic $γ$-ray emitters, only a handful of distant ($z >$ 3) sources are detected, yet they are cruial 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 a radio quasar B3 1239+376 with $z$ = 3.82. By analyzing the Fermi-LAT 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 analyses also reveal the emergence of co-spatial $γ$-ray residues in prior epochs. Moreover, the $γ$-ray and infrared light curves are likely correlated, particularly, the two emissions climb to the peaking values 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 {\it third} most distant $γ$-ray-emitting blazar to date. Benefiting from the multi-wavelength observations, broadband spectral energy distributions in different flux states are drawn 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 which may catch the potential ejection of a new jet blob.

Figures (7)

• Figure 1: The Fermi-LAT $\gamma$-ray light curve of the source B3 1239+376 between August 2022 and August 2025, with each bin corresponding to a 2-month period. The blue circles represent flux estimates with $1\sigma$ statistical errors, while the red downward triangles are the upper limits. The pink shaded bars show the TS values for each bin. The green horizontal line indicates the averaged flux upper limit for the first 15 bins.
• Figure 2: $\gamma$-ray TS maps centered in the radio position of B3 1239+376 (shown as a black x-shape symbol) that the target source is not included in the analysis source model. They are in a $2^\circ \times 2^\circ$ scale, with an each pixel size of $0.1^\circ$. Panel (a): the TS map are derived by analyzing Fermi-LAT data between Feb. 2025 and Aug. 2025, from which the 95% C.L. localization uncertainty is also plotted by the green solid line. Panel (b): The TS map is based by a joint-likelihood analysis combining two time intervals, from Sep. 2017 to May 2018 and from Nov. 2019 to May 2020, respectively. The corresponding 95% C.L. localization uncertainty of the joint analysis is drawn in the blue solid line.
• Figure 3: Multi-wavelength long-term light curves of B3 1239+376. Top panel: Fermi-LAT 1-year time bin $\gamma$-ray light curve. Blue points denote flux measurements, red triangles are 95% CL upper limits, and the pink histogram shows the corresponding TS values. The green horizontal line marks the averaged flux over the first 9-year Fermi-LAT observation, and the green triangle is the upper limit for a four-year quiescent state. Upper middle panel: Infrared light curves in the W1 ($3.4~\mu\mathrm{m}$) and W2 ($4.6~\mu\mathrm{m}$) bands, observed by WISE/NEOWISE and SPHEREx. The vertical filled regions in color olive marks the time ranges for the joint Fermi-LAT analysis, of which the TS map is presented in Figure \ref{['pfjmap']}. While the filled region in color cyan represents the half-year $\gamma$-ray flaring period in 2025, corresponding to the TS map in Figure \ref{['fmap']}. Lower middle panel: VLBA K-band ($24~\mathrm{GHz}$) flux densities of the parsec-scale jet and compact core components, shown in blue circles and orange squares, respectively. Bottom panel: Radio light curves from RATAN-600 at 2.3–22 GHz together with VLASS 3 GHz flux measurements.
• Figure 4: The folded X-ray spectra of B3 1239+376 from two Chandra observations (top panel), as well as fit residuals (lower panel). The solid lines in the top panel represents the best-fit models.
• Figure 5: Fitting of the SDSS spectrum of B3 1239+376. A global scope of the spectrum (in grey) and the modeled continuum component (in orange) are shown in the upper panel. The distinct emission lines are marked as well. In the lower panel, zoomed-in views on the Ly$\alpha$+N v (left) and C iv (right) line regions are provided especially.