Time-dependent modelling of short-term variability in the TeV-blazar VER J0521+211 during the major flare in 2020

TL;DR

This study models the short-term variability of the TeV-blazar VER J0521+211 during a 2020 flare using multiwavelength data, comparing leptonic and lepto-hadronic emission scenarios and predicting neutrino fluxes.

Contribution

It introduces a time-dependent spectral energy distribution model for a blazar flare, incorporating evolving parameters and comparing leptonic and lepto-hadronic mechanisms.

Findings

Both models successfully fit the observed SED and flux evolution.

The lepto-hadronic model predicts detectable ultra-high-energy neutrinos.

The redshift of the blazar is constrained to z ≤ 0.244.

Abstract

The BL Lacertae object VER J0521+211 underwent a notable flaring episode in February 2020. A short-term monitoring campaign, led by the MAGIC (Major Atmospheric Gamma Imaging Cherenkov) collaboration, covering a wide energy range from radio to very-high-energy (VHE, 100 GeV < E < 100 TeV) gamma rays was organised to study its evolution. These observations resulted in a consistent detection of the source over six consecutive nights in the VHE gamma-ray domain. Combining these nightly observations with an extensive set of multiwavelength data made modelling of the blazar's spectral energy distribution (SED) possible during the flare. This modelling was performed with a focus on two plausible emission mechanisms: i) a leptonic two-zone synchrotron-self-Compton scenario, and ii) a lepto-hadronic one-zone scenario. Both models effectively replicated the observed SED from radio to the VHE gamma-ray band. Furthermore, by introducing a set of evolving parameters, both models were successful in reproducing the evolution of the fluxes measured in different bands throughout the observing campaign. Notably, the lepto-hadronic model predicts enhanced photon and neutrino fluxes at ultra-high energies (E > 100 TeV). While the photon component, generated via decay of neutral pions, is not directly observable as it is subject to intense pair production (and therefore extinction) through interactions with the cosmic microwave background photons, neutrino detectors (e.g. IceCube) can probe the predicted neutrino component. Finally, the analysis of the gamma-ray spectra, as observed by MAGIC and the Fermi-LAT telescopes, yielded a conservative 95\% confidence upper limit of z \leq 0.244 for the redshift of this blazar.