Blazars are active galactic nuclei (AGN) with relativistic jets whose non-thermal radiation is extremely variable on various timescales. This variability seems random primarily, although some quasi-periodic oscillations (QPOs), implying systematic processes, have been reported in blazars and other AGNs.
After its prototype, BL Lacertae, BL Lac is a type of active galactic nucleus (AGN) or a galaxy harboring such an AGN. BL Lacs are distinguished from other categories of active galactic nuclei by rapid, large-amplitude flux variability and considerable optical polarization. It is fuelled, like all blazars, by matter falling into a galaxy’s supermassive black hole (SMBH).
A team of 86 scientists from 13 countries, including Dr. Alok Chandra Gupta from Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, an autonomous institution of the Department of Science and Technology (DST), Government of India, carried out an extensive high time resolution optical monitoring of the blazar BL Lacertae (BL Lac). They spotted surprising rapid oscillations of brightness in the jet of a blazar during a strong multi-wavelength outburst in the second half of 2020, which was rich in gamma rays. They have attributed these cycles of brightness changes termed quasi-periodic oscillations (QPOs) to twists in the magnetic field in the jet.
To trace these, observations with the Whole Earth Blazar Telescope (WEBT) were organized by Drs. Claudia M. Raiteri and Massimo Villata of INAF-Osservatorio Astrofisico di Torino, Italy, to monitor variability of visible light in blazars that are bright in gamma-rays. Optical observations by the WEBT collaboration of astronomers, working together with 37 ground-based telescopes around the globe, found cycles of visible brightness changes as fast as about 13 hr in a jet of high-energy particles of BL Lac —a blazar powered by a black hole located roughly 1 billion light years away.
Dr. Svetlana Jorstad of Boston University, who led the team of astronomers participating in the study published in Nature, said, “The cycles of brightness changes termed quasi-periodic oscillations (QPOs) are more often seen in other systems called X-ray black hole binaries, which have black holes with smaller masses of 10 – 50 M and are generally explained by clumps of hot gas in the disk of accreting material orbiting the black hole.”
“However, in the case of BL Lac, the light is polarized, which is not the case for emission by hot gas in the disk, so interpreting such behavior is difficult.”
Prof. Alan Marscher of Boston University, an expert in blazar research and a co-author in the study, said, “a kink forms in the jet, twisting the magnetic field in such a way that it causes the brightness to oscillate. Furthermore, the polarization changes with a similar timescale as the brightness. Such polarized light comes from the jet, and the polarization can vary only if the magnetic field changes in the light-producing region. The magnetic field in the jet must be twisting to cause the oscillations.”
“The observations of BL Lac show a strong correlation between visible light and gamma ray variations without any delay, which places gamma-rays in the region where the visible light changes.”