Decoding the jet of BL Lacertae using relativistic magneto-hydrodynamics

Blazars are a highly variable subclass of active galactic nuclei whose relativistic jets point nearly along our line of sight, producing multi-band flares. BL Lacertae, the namesake of a blazar subclass, recently exhibited its highest recorded linearly polarized optical flare, prompting an investigation into its origin.

Using very-long-baseline interferometry (VLBI) observations, the team found that the sweeping, helical motion of the BL Lac jet—known to exhibit kink-like instabilities—can explain the observed flux density spike and polarization angle rotation. This was confirmed by state-of-the-art relativistic magneto-hydrodynamic simulations, which also showed that baryon loading of the jet is required to optimally replicate the observed jet morphology.