Rapid quasi-periodic oscillations in the relativistic jet of BL Lacertae.

Blazars are active galactic nuclei (AGN) with relativistic jets whose non-thermal radiation is extremely variable on various timescales ^1-3 . This variability seems mostly random, although some quasi-periodic oscillations (QPOs), implying systematic processes, have been reported in blazars and other AGN. QPOs with timescales of days or hours are especially rare ⁴ in AGN and their nature is highly debated, explained by emitting plasma moving helically inside the jet ⁵ , plasma instabilities ^6,7 or orbital motion in an accretion disc ^7,8 . Here we report results of intense optical and γ-ray flux monitoring of BL Lacertae (BL Lac) during a dramatic outburst in 2020 (ref.  ⁹ ). BL Lac, the prototype of a subclass of blazars ¹⁰ , is powered by a 1.7 × 10 ⁸  M _Sun (ref.  ¹¹ ) black hole in an elliptical galaxy (distance = 313 megaparsecs (ref.  ¹² )). Our observations show QPOs of optical flux and linear polarization, and γ-ray flux, with cycles as short as approximately 13 h during the highest state of the outburst. The QPO properties match the expectations of current-driven kink instabilities ⁶ near a recollimation shock about 5 parsecs (pc) from the black hole in the wake of an apparent superluminal feature moving down the jet. Such a kink is apparent in a microwave Very Long Baseline Array (VLBA) image.
(© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)