Rapidly changing magnetic fields revealed in black hole jets

Supermassive black holes launch powerful collimated streams of highly magnetized plasma at relativistic speeds, we call jets. When those jets are pointed towards Earth, their emission is modulated by effects predicted by Einstein’s theory of special relativity, making them brighter and more variable than they really are. Those systems are called blazars, and are among the brightest objects in the Universe.

Light in blazars is generated by relativistic electrons accelerated in the jet while flowing along the magnetic field lines. The electrons produce synchrotron radiation which is highly directional. The directionality of the emission imposed by the magnetic field configuration aligns the electromagnetic waves that make up light, giving rise to what we call polarized emission. Therefore, by measuring the evolution with time of the polarization degree (i.e., how much of that light is polarized), and the polarization angle (the direction of the electromagnetic waves), astronomers can study the underlying magnetic field structure in those jets.

Skinakas observatory has pioneered the study of magnetic field evolution in jets with several experiments, BOOTES being the most recent one. All of our previous efforts provided us with indications that magnetic fields can vary rapidly, much faster what we had already observed, but such rapid variations were very hard to observe.

A recent study published in the journal "Astronomy and Astrophysics"  led by I. Liodakis and other members of the Institute of Astrophysics (IA) aimed to discover how fast can those magnetic fields really change. IA coordinated 15 telescopes across the world for a campaign they called the Non-stOp Polarization Experiment (NOPE). NOPE relies on the rotation of the Earth to continuously observe a blazar for more than 24 hours. If you strategically place your telescopes in different timezones, and with a little bit of good weather, you can guarantee that at any given time at least one telescope is observing the target, hence the observations can continue uninterrupted by the rising sun.
 

BLLac
Results from the first NOPE campaign on a famous blazar called BL Lacertae. The image shows the evolution with time (in modified julian date – 59000) of the total light (top panel), polarization degree (middle panel), and polarization angle (bottom panel). The range of polarization variability, i.e., the range of magnetic field variations, and the timescales on which they happened, were revealed only by the coordination of several telescopes across the world.

It took seven days and 33 astronomers in 11 countries, but the team found what they were looking for. This unprecedented dataset revealed the hypothesized rapid changes of the magnetic field, that provide unique opportunities to study jet physics and understand the particle acceleration that takes place. In fact, comparison of the observations with state-of-the-art models showed encouraging results, but also that neither of the existing models can fully describe the range of polarization variability seen in these still very enigmatic sources.
 

Article: “Testing particle acceleration in blazar jets with continuous high-cadence optical polarization observations”, I. Liodakis et al.,  2024, A&A, 689, 200 – September 2024