A new observation from the Event Horizon Telescope (EHT), in which the Astronomical Observatory of the University of Valencia (OAUV), reveals in detail the turbulent structure of the magnetic field in the jet of accelerated material from a supermassive black hole. This finding provides an unprecedented insight into the physics of supermassive black holes, considered the most powerful "motors" in the Universe
The Event Horizon Telescope (EHT) has just published the observation of a relativistic jet (an enormous structure formed by a collimated beam of plasma, strongly accelerated by a supermassive black hole) in which it can be seen how internal shock waves interact with the turbulent magnetic field of the same jet. This discovery, recently published in the journal Astronomy & Astrophysics, provides a close and unusual picture of a rapidly evolving region near a black hole where relativistic jets are forming and accelerating.
The observed relativistic jet is associated with the central black hole of blazar OJ 287, located about 1.6 billion light years away in the constellation of Cancer. Blazars are an extreme type of active galaxy whose jet points almost directly at the Earth, making them one of the most variable and energetic objects in the Universe. Using the extraordinary sharpness achievable with EHT (distinguishing structures the size of a tennis ball on the Moon), the team detected two bright components in the jet, which behave like shock waves moving at different speeds outward. In addition, the light emitted by these shock waves is strongly polarized (polarization of light is the preference of its electric field to vibrate in a certain direction). In this case, the polarization is due to plasma-related effects and the direction of the magnetic field embedded in the jet. Therefore, as these bright shock waves travel along the jet, their polarization changes, allowing you to "scan" the magnetic field structure as if you were using a tomograph.
"This is a common effect in which the speed difference between adjacent layers of fluid or gas generates waves and vortices, similar to the ripples and waves that are formed when two winds meet, but which in this case are produced in a plasma jet moving at speeds close to the speed of light", Manuel Perucho, OAUV researcher
The pattern of rotation in polarization shown by these components (with one component rotating opposite to the other) provides a direct signal that the jet is traversed by a helical magnetic field, with the field lines winding along the jet. Beyond these two shock waves, the images reveal that the jet is not simply straight and smooth. Instead, it shows a twisted wave-like structure, probably related to the so-called "Kelvin-Helmholtz instabilities".
According to the researcher of the Astronomical Observatory of the University of Valencia (OAUV), located in the scientific-academic area of the University of Valencia Science Park (PCUV), Manel Perucho, expert in dynamics of relativistic jets and numerical simulations, "is a common effect in which the speed difference between adjacent layers of fluid or gas generates waves and vortices, similar to the undulations and waves that are formed when two winds meet, but which in this case are produced in a plasma jet moving at speeds close to the speed of light".
In addition, José L. Gómez, principal author of the work and researcher at the Institute of Astrophysics of Andalusia-CSIC, states that "these rotations of polarization in opposite directions are the irrefutable proof of our finding. By propagating along the jet and passing through the Kelvin-Helmholtz wave, the shocks illuminate different phases of the helical magnetic field structure, producing the polarization changes we observe".
Accurate images of polarized jet light
Measuring the polarization of light with EHT is a technical challenge. The polarization signals are weak and can be easily distorted by small instrumental effects in each telescope. Extraction of reliable polarization maps requires extremely careful calibration and cross-checking throughout the analysis.
"Polarization is one of the richest signals in information that we can measure, but it is also one of the most fragile," says Iván Martí-Vidal, a specialist in high-precision VLBI polarimetry and one of the developers of the key calibration techniques used by the EHT. "To be sure that these polarization rotations actually occur near the black hole and are not due to instrumental effects, we need to model and correct the data with extreme care. The fact that completely independent algorithms recover the same polarization characteristics from the data is a solid validation of the result," he says.
"To be sure that these polarization rotations actually occur near the black hole and are not due to instrumental effects, we need to model and correct the data with extreme care. The fact that completely independent algorithms recover the same polarization characteristics from the data is a solid validation of the result", Iván Martí-Vidal, OAUV researcher
The observations used for the current analysis were made over five days in April 2017 and reveal surprising changes on an exceptionally short time scale for this source. In just a few days, both the jet structure and its polarized light have evolved significantly, demonstrating that the indoor jet is a very dynamic environment.
"We observed substantial changes over the course of five days. With longer and continuous monitoring, we were able to follow the interaction step by step and build a real three-dimensional image of the jet’s magnetic structure," says Efthalia Traianou from the University of Heidelberg and the Max Planck Institute for Radio Astronomy.
A cosmic laboratory
The OJ 287 jet has been the scene of spectacular cataclysmic events for more than a century of observations. Astronomers have long debated whether these events could be related to the presence of a second supermassive black hole orbiting around the main black hole. Whatever the ultimate cause of its long-term variability, OJ 287 remains an important "laboratory" for studying how black holes are fed and how their jets respond.
The new EHT images focus on the region where the jet is organized and energized, with a sharpness that directly elucidates its polarized structure, whose changes can be followed over time.
New frontiers in jet physics and latest simulations
Because of their very high spatial resolution, EHT observations allow researchers to test physical models of jet behaviour with spatially resolved measurements.
"When we combine EHT images with state-of-the-art simulations, we can ask very direct questions: how shock waves propagate, how Kelvin-Helmholtz instabilities shape flow or how magnetic fields influence particle acceleration. This is exactly the kind of synergy between data and simulations that we need to understand what makes jets stable, turbulent or radiatively efficient", José María Martí, OAUV researcher
According to OAUV researcher José María Martí, expert in the physics of relativistic jets and Numerical Relativity, "these observations finally allow us to separate in space processes that previously appeared superimposed". And he clarifies: "When we combine EHT images with state-of-the-art simulations, we can ask very direct questions: how shock waves propagate, how Kelvin-Helmholtz instabilities shape flow or how magnetic fields influence particle acceleration. This is exactly the kind of synergy between data and simulations that we need to understand what makes jets stable, turbulent or radiatively efficient".
A new window to the Universe
This discovery is a breakthrough in understanding how black holes feed and shape their jets. By resolving rapid polarization changes in the different jet characteristics, EHT offers a new way of testing ideas about magnetic fields, shock waves, instabilities and acceleration of particles in one of the most extreme environments in the known universe.
The result also points towards what could come next: a longer and denser time coverage to capture the evolution of the jet, not as a small succession of snapshots, but as an actual sequence (a "film"), revealing how the magnetic structure and plasma dynamics develop in great detail.
Don’t miss Iván Martí-Vidal’s presentation in Expoinnova 2024
Source: UV News
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