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

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.

💡 Research Summary

This paper reports the discovery and multi‑wavelength characterization of the radio‑loud quasar B3 1239 376 at a redshift of z = 3.82, establishing it as the third most distant γ‑ray‑emitting blazar known to date. The authors performed a comprehensive analysis of 17 years of Fermi‑LAT Pass 8 data (2008 – 2025) and identified a highly significant γ‑ray excess in a half‑year interval in 2025. The test statistic (TS) of 80 corresponds to a global significance of 7.7 σ after accounting for 34 trial periods, and the derived photon flux (3.2 ± 0.6 × 10⁻⁸ ph cm⁻² s⁻¹) and photon index (Γ = 2.60 ± 0.14) indicate a relatively soft spectrum typical of high‑z blazars. A separate analysis limited to photons above 500 MeV still yields TS = 39 (global 4.7 σ), confirming that the detection is not an artifact of nearby bright background sources.

Localization of the γ‑ray source places it at RA = 190.6411°, Dec = 37.2587° with a 95 % confidence radius of 0.22°, only 0.11° from the known radio position of B3 1239 376. Bayesian association using the CRATES catalog gives an association probability of 0.91, well above the usual 0.8 threshold. The authors also searched for low‑energy counterparts in blazar candidate catalogs (BZCAT, WIBRaLS2) and VLBI surveys, finding B3 1239 376 as the sole plausible counterpart.

Temporal analysis shows clear variability. A 1‑year binned γ‑ray light curve over the full 17‑year span exhibits a variability index significant at 5.4 σ. The most recent half‑year bin (MJD 60710‑60892) displays a five‑fold flux increase relative to the long‑term average, coincident with a simultaneous peak in WISE infrared data. Earlier modest excesses (TS ≈ 15–17) are found in 2017 and 2020; a joint analysis of these epochs yields TS = 31 and a position consistent with B3 1239 376, suggesting that the source has undergone multiple γ‑ray active phases.

X‑ray observations with Chandra ACIS (2007 + 5.2 ks, 2022 + 20.1 ks) reveal a hard spectrum (Γₓ = 1.14 ± 0.33 in 2007, Γₓ = 1.51 ± 0.25 in 2022) and 0.5–7 keV fluxes of (2.4 ± 0.74) × 10⁻¹³ erg cm⁻² s⁻¹ and (1.8 ± 0.22) × 10⁻¹³ erg cm⁻² s⁻¹, respectively. These values are consistent with the typical X‑ray properties of flat‑spectrum radio quasars (FSRQs) and support the presence of a powerful jet.

Optical spectroscopy from SDSS DR16 provides a precise redshift measurement (z = 3.82 ± 0.01) based on prominent Ly α and C IV emission lines. The Ly α line luminosity is L_{Lyα} = 1.52 × 10⁴⁵ erg s⁻¹, which can be used to infer the accretion disk luminosity and black‑hole mass.

The authors construct broadband spectral energy distributions (SEDs) for low‑ and high‑state epochs, incorporating radio, infrared, optical, X‑ray, and γ‑ray data. They model the SEDs with a classic one‑zone leptonic scenario, where a single population of relativistic electrons in a spherical blob produces synchrotron radiation (radio–optical), synchrotron self‑Compton (SSC) emission, and external Compton (EC) scattering of photons from the broad‑line region (BLR) and infrared torus. The best‑fit parameters are: electron minimum Lorentz factor γ_min ≈ 1, maximum γ_max ≈ 10⁴, electron spectral index p ≈ 2.2, magnetic field B ≈ 0.3 G, blob radius R ≈ 5 × 10¹⁶ cm, and Doppler factor δ ≈ 15. These values are in line with those derived for other high‑z blazars and indicate a highly relativistic, magnetically modest jet where EC dominates the γ‑ray output.

Given the recent γ‑ray flare, the authors advocate for rapid follow‑up observations, especially high‑resolution VLBI imaging, to capture any newly ejected jet component that may be associated with the flare. Detecting such a component would provide direct evidence linking γ‑ray activity to jet kinematics at early cosmic epochs. Moreover, the detection of a soft γ‑ray spectrum from a source at z = 3.82 offers an additional probe of the extragalactic background light (EBL) attenuation at high redshift.

In summary, the paper presents a robust multi‑wavelength case for B3 1239 376 being a bona‑fide γ‑ray blazar at z = 3.82, expands the sample of known high‑z γ‑ray blazars, refines jet physical parameters through SED modeling, and outlines future observational strategies to further elucidate jet physics and black‑hole growth in the early universe.