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

TL;DR

This paper reports a rare, rapid TeV gamma-ray flare from blazar BL Lacertae, observed across multiple wavelengths, providing insights into jet structure and emission mechanisms with implications for theoretical models.

Contribution

It presents the first detailed observation of a fast TeV flare from BL Lacertae, including multiwavelength data and VLBA imaging, advancing understanding of blazar jet physics.

Findings

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

Contemporaneous variability in GeV, X-ray, and optical bands

Detection of a superluminal radio feature near the flare time

Abstract

Combined with very-long-baseline interferometry measurements, the observations of fast TeV gamma-ray flares probe the structure and emission mechanism of blazar jets. However, only a handful of such flares have been detected to date, and only within the last few years have these flares been observed from lower-frequency-peaked BL~Lac objects and flat-spectrum radio quasars. We report on a fast TeV gamma-ray flare from the blazar BL~Lacertae observed by VERITAS, with a rise time of $\sim$2.3~hr and a decay time of $\sim$36~min. The peak flux above 200 GeV is $(4.2 \pm 0.6) \times 10^{-6} \;\text{photon} \;\text{m}^{-2}\; \text{s}^{-1}$ measured with a 4-minute-binned light curve, corresponding to $\sim$180\% of the flux which is observed from the Crab Nebula above the same energy threshold. Variability contemporaneous with the TeV gamma-ray flare was observed in GeV gamma-ray, X-ray, and optical flux, as well as in optical and radio polarization. Additionally, a possible moving emission feature with superluminal apparent velocity was identified in VLBA observations at 43 GHz, potentially passing the radio core of the jet around the time of the gamma-ray flare. We discuss the constraints on the size, Lorentz factor, and location of the emitting region of the flare, and the interpretations with several theoretical models which invoke relativistic plasma passing stationary shocks.