Gamma-ray variability and multi-wavelength insights into the unprecedented outburst from 4C 31.03

Abstract

The blazar 4C 31.03 recently underwent a major γ-ray outburst at the beginning of 2023 after a prolonged quiescent phase. Fermi-LAT reported a daily average flux of $5\times {10}^{-6}$ phs cm⁻² s⁻¹, which is about 60 times its average value. We investigated this extraordinary outbreak through temporal and multi-wavelength analysis. From the statistical analysis of the γ-ray lightcurves using Bayesian blocks, we identified 3 epochs of prominent flares. The fastest flux decay during this major outburst was observed within $5.5\pm 0.7$ hours. The highest energy of γ-ray photons found from the source during the active phase is ∼82 GeV. Using the transparency of γ-rays against pair production and light crossing time argument, we could obtain the minimum jet Doppler factor as 17 corresponding to the flaring state. The broadband spectral energy distribution study performed using synchrotron, SSC and EC emission processes supports the external Compton scattering of IR photons as the likely mechanism for the γ-ray emission from the source. The results of this study support the scenario of the emission region in 4C 31.03, being located beyond the Broad line region from the central blackhole. The long-term γ-ray flux distribution depicts a double log-normal variability, indicating that two distinct flux states are active in this energy band. The index distribution also reveals a two distinct variability pattern and hints that the γ-ray spectrum can be more precisely described by two photon indices.