The most distant $\gamma$-ray flare to date: a multiwavelength campaign on the $z = 4.715$ blazar GB6 B1428+4217

TL;DR

This paper reports the detection of the most distant gamma-ray flare from a high-redshift blazar, GB6 B1428+4217, through a comprehensive multiwavelength campaign, revealing insights into jet physics and emission mechanisms in the early Universe.

Contribution

It presents the first detailed multiwavelength analysis of a gamma-ray flare from a $z=4.715$ blazar, highlighting its spectral energy distribution and jet properties at high redshift.

Findings

Gamma-ray flare with top 5% luminosity detected at high redshift.

Spectral modeling indicates external Compton scattering dominates high-energy emission.

Polarization measurements confirm synchrotron emission in optical band.

Abstract

In November 2023, the Fermi Large Area Telescope detected a $\gamma$-ray flare from the high-redshift blazar GB6 B1428+4217 ($z=4.715$). We initiated a multi-wavelength 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 ${\sim}8$\% 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 $\gamma$-ray luminosity among the top 5% of flaring events, GB6 B1428+4217 exemplifies a prototypical MeV blazar. Its Compton-dominated SED 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 COSI.