Origin of the broadband emission from the transition blazar B2 1308+326

Abstract

Context. Transition blazars exhibit a shift from one subclass to the next during different flux states. It is therefore crucial to study them to understand the underlying physics of blazars. Aims. We probe the origin of the multi-wavelength emission from the transition blazar B2 1308+326 using the ∼14-year long γ-ray light curve from Fermi and the quasi-simultaneous data from Swift. Methods. We used the Bayesian block algorithm to identify epochs of flaring and quiescent flux states and modelled the broadband spectral energy distributions (SEDs) for these epochs. We employed the one-zone leptonic model in which the synchrotron emission causes the low-energy part of the SED and the high-energy part is produced by the inverse-Compton (IC) emission of external seed photons. We also investigated its multi-band variability properties and γ-ray flux distribution, and the correlation between optical and γ-ray emissions. Results. We observed a historically bright flare from B2 1308+326 across the optical to γ-ray bands in June and July 2022. The highest daily averaged γ-ray flux was (14.24 ± 2.36) × 10−7 ph cm−2 s −1 and was detected on 1 July 2022. For the entire period, the observed variability amplitude was higher at low (optical/UV) energies than at high (X-ray/γ-ray) energies. The γ-ray flux distribution was found to be log-normal. The optical and γ-ray emissions are well correlated with zero time lag. The synchrotron peak frequency changes from ∼8 × 1012 Hz (in the quiescent state) to ∼6 × 1014 Hz (in the flaring state), together with a decrease in the Compton dominance (the ratio of IC to the synchrotron peak luminosities), providing a hint that the source transitions from a low-synchrotron peaked blazar (LSP) to an intermediate-synchrotron peaked blazar (ISP). The SEDs for these two states are well fitted by one-zone leptonic models. The parameters in the model fits are essentially consistent between both SEDs, except for the Doppler-beaming factor, which changes from ∼15.6 to ∼27 during the transition. Conclusions. An increase in the Doppler factor might cause both the flare and the transition of B2 1308+326 from an LSP to an ISP blazar.