Abstract:
Blazars are active galactic nuclei (AGN) with relativistic jets whose non-thermal
radiation is extremely variable on various timescales1–3
. 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 rare4
in AGN and their nature is highly debated, explained by
emitting plasma moving helically inside the jet5
, plasma instabilities6,7
or orbital
motion in an accretion disc7,8
. Here we report results of intense optical and γ-ray fux
monitoring of BL Lacertae (BL Lac) during a dramatic outburst in 2020 (ref. 9
). BL Lac,
the prototype of a subclass of blazars10, is powered by a 1.7 × 108
MSun (ref. 11) black hole
in an elliptical galaxy (distance = 313 megaparsecs (ref. 12)). Our observations show
QPOs of optical fux and linear polarization, and γ-ray fux, 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 instabilities6
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.
