Multiwavelength observations of the blazar BL Lacertae: a new fast TeV $\gamma$-ray flare

TL;DR

This paper reports on a rapid TeV gamma-ray flare from blazar BL Lacertae, combining multiwavelength observations to explore the jet's structure, emission mechanisms, and the physical conditions during the flare.

Contribution

It presents the first detailed observation of a fast TeV gamma-ray flare from BL Lacertae, linking multiwavelength variability and jet features to constrain emission models.

Findings

Fast TeV gamma-ray flare with 2.3-hour rise time

Correlation of gamma-ray, X-ray, optical, and polarization variability

Identification of a superluminal knot in VLBA observations

Abstract

Observations of fast TeV $\gamma$-ray flares from blazars reveal the extreme compactness of emitting regions in blazar jets. Combined with very-long-baseline radio interferometry measurements, they probe the structure and emission mechanism of the jet. We report on a fast TeV $\gamma$-ray flare from BL Lacertae observed by VERITAS, with a rise time of about 2.3 hours and a decay time of about 36 minutes. The peak flux at $>$200 GeV measured with the 4-minute binned light curve is $(4.2 \pm 0.6) \times 10^{-6} \;\text{photons} \;\text{m}^{-2}\, \text{s}^{-1}$, or $\sim$180% the Crab Nebula flux. Variability in GeV $\gamma$-ray, X-ray, and optical flux, as well as in optical and radio polarization was observed around the time of the TeV $\gamma$-ray flare. A possible superluminal knot was identified in the VLBA observations at 43 GHz. The flare constrains the size of the emitting region, and is consistent with several theoretical models with stationary shocks.