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

Jorstad, S. G.; Marscher, A. P.; Raiteri, C. M.; Villata, M.; Weaver, Z. R.; Zhang, H.; Dong, L.; Gómez, J. L.; Perel, M. V.; Savchenko, S. S.; Larionov, V. M.; Carosati, D.; Chen, W. P.; Kurtanidze, O. M.; Marchini, A.; Matsumoto, K.; Mortari, F.; Aceti, P.; Acosta-Pulido, J. A.; Andreeva, T.; Apolonio, G.; Arena, C.; Arkharov, A.; Bachev, R.; Banfi, M.; Bonnoli, G.; Borman, G. A.; Bozhilov, V.; Carnerero, M. I.; Damljanovic, G.; Ehgamberdiev, S. A.; Elsässer, D.; Frasca, A.; Gabellini, D.; Grishina, T. S.; Gupta, A. C.; Hagen-Thorn, V. A.; Hallum, M. K.; Hart, M.; Hasuda, K.; Hemrich, F.; Hsiao, H. Y.; Ibryamov, S.; Irsmambetova, T. R.; Ivanov, D. V.; Joner, M. D.; Kimeridze, G. N.; Klimanov, S. A.; Knött, J.; Kopatskaya, E. N.; Kurtanidze, S. O.; Kurtenkov, A.; Kuutma, T.; Larionova, E. G.; Leonini, S.; Lin, H. C.; Lorey, C.; Mannheim, K.; Marino, G.; Minev, M.; Mirzaqulov, D. O.; Morozova, D. A.; Nikiforova, A. A.; Nikolashvili, M. G.; Ovcharov, E.; Papini, R.; Pursimo, T.; Rahimov, I.; Reinhart, D.; Sakamoto, T.; Salvaggio, F.; Semkov, E.; Shakhovskoy, D. N.; Sigua, L. A.; Steineke, R.; Stojanovic, M.; Strigachev, A.; Troitskaya, Y. V.; Troitskiy, I. S.; Tsai, A.; Valcheva, A.; Vasilyev, A. A.; Vince, O.; Waller, L.; Zaharieva, E.; Chatterjee, R.

doi:10.1038/s41586-022-05038-9

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 gamma-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 x 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 gamma-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.