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Telescopes map magnetic fields near binary black hole candidate
Event Horizon Telescope (EHT) observations of OJ 287 show interactions between shock waves, producing polarization rotations in opposite directions and generating energy beams with twisted trajectories
Observations of the energy flow took place over five days – Image taken from the article
For the first time, observations from the Event Horizon Telescope (EHT) global network of radio telescopes reveal the cosmic ballet shaping the powerful energy beams launched from the heart of a distant galaxy. Scientists directly observed the magnetic field arising from the dynamic interaction between shock waves – regions of compressed plasma (ionized gas) – and helical pressure waves in a supermassive black hole jet in OJ 287, a likely binary black hole system located 1.6 billion light-years from Earth. Analysis of the observations reveals that the waves forming the jet move in consistent twisted trajectories, rather than just in straight lines or simple curves.
The EHT is a global network of radio telescopes acting as an Earth-sized virtual telescope, involving collaboration among various institutions, including USP. Capable of identifying a ping-pong ball on the Moon, its resolution allowed for the capture of two bright shock waves traveling through the jet at different speeds. As they move, these waves interact in a wave pattern known as Kelvin-Helmholtz (which forms vortices and ripples similar to ocean waves) within a helical, or helix-shaped, magnetic field. This produces a remarkable phenomenon: the polarization of light (the direction of wave oscillation) rotates in opposite directions.
The study results were published in the journal Astronomy & Astrophysics.
Jet physics
The observations lasted five days – between April 5 and 10, 2017 – and exhibited distinct polarization behaviors: the faster wave rotates counter-clockwise at 3.7° per day, while the slower one rotates clockwise at 2.5° per day.
The observations showed rapid changes in both structure and polarization. According to Efthalia Traianou, coordinator of the EHT AGN Working Group, “this is the first time we have directly observed the interaction of shocks and instabilities in a black hole jet”. Detailed modeling shows that jet components move in consistent twisted trajectories, not just in straight lines or simple curves.
The instabilities of the Kelvin-Helmholtz wave pattern are caused by velocity shear between the jet plasma and the surrounding medium, creating a helical wave pattern with a wavelength of 100 microarcseconds (1.5 light-years). As the shocks travel through this pattern, they illuminate different phases of the helical magnetic field, naturally explaining the observed opposite rotations.
“These rotations in opposite directions are the definitive evidence”, explains José L. Gómez, a researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) and lead author of the paper. “When the shocks interact with the Kelvin-Helmholtz instability, they reveal the helical structure of the magnetic field”, he adds.
Binary black hole laboratory
Known for its periodic and dramatic eruptions, OJ 287 offers a unique laboratory for studying black hole physics. The EHT observes the jet at scales of 10 to 100 gravitational radii from the primary black hole – exactly where jets are launched and where magnetic fields play a crucial role.
“These measurements allow us to directly map the magnetic field geometry in the jet’s launch and collimation region“, explains Ilje Cho, from the Korea Astronomy and Space Science Institute (KASI).
EHT observations of OJ 287 on April 5 and 10, 2017, showing the jet structure with unprecedented resolution, just 0.75 light-years from the supermassive black hole – Photo: EHT/E. Traianou Collaboration
Brazilian collaboration
USP’s Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) formally joined the EHT collaboration in 2022 with the arrival of professor Ciriaco Goddi, who has been an EHT collaborator since 2014.
The professor led the group responsible for calibrating data from the Atacama Large Millimeter Array (ALMA), one of the pillars of the global telescope network. ALMA is the world’s largest and most powerful radio telescope, located at an altitude of 5,000 meters in the Atacama Desert, Chile. “ALMA data, obtained simultaneously with the EHT, are essential to calibrate the data and detect the weak polarized emission along the jet of OJ 287”, explains Goddi.
In a partnership between Brazil and Argentina, USP co-leads the construction of a new telescope that will soon join the EHT: the Large Latin American Millimeter Array (LLAMA), in the Puna de Atacama desert, Argentina. “Located just 180 km from ALMA, LLAMA will provide sensitive intermediate baselines with ALMA”, adds Goddi. “These baselines will be crucial to detect the weak polarized emission at much larger scales, allowing us to map the complex magnetic field configuration for tens of light-years along the OJ 287 jet”.
The article is available online and can be read at this link.
*Written with information provided by the EHT Collaboration. Adapted for Jornal da USP
**Intern under the supervision of Moisés Dorado
English version: Translated and edited by Denis Pacheco
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A reprodução de matérias e fotografias é livre mediante a citação do Jornal da USP e do autor. No caso dos arquivos de áudio, deverão constar dos créditos a Rádio USP e, em sendo explicitados, os autores. Para uso de arquivos de vídeo, esses créditos deverão mencionar a TV USP e, caso estejam explicitados, os autores. Fotos devem ser creditadas como USP Imagens e o nome do fotógrafo.