Images from the Event Horizon Telescope (EHT) team show that the black hole Sagittarius A* (Sgr A*) in the centre of our Milky Way is surrounded by strong, spiral-shaped magnetic fields. The team led by FIAS Senior Fellow Luciano Rezzolla played a key role in this study.

The first image of the black hole Sgr A* - roughly 27,000 light years away from Earth - was published by the collaboration in 2022, revealing that the supermassive black hole at the centre of our Milky Way is more than a thousand times smaller and less massive than that of the galaxy M87, of which the EHT collaboration published the first image of a black hole in 2019. Nevertheless, Sgr A* and M87* look remarkably similar. To find out whether the two black holes share any other common features, the EHT team decided to study Sgr A* in polarised light. It was already known from M87* that the magnetic fields around the giant black hole enable it to send a strong jet of particles into space. The new images show that the same could also apply to Sgr A*.

Imaging black holes in polarised light is not easy, especially for Sgr A*. This is because the gas, or plasma, in the vicinity of the black hole orbits Sgr A* in just a few minutes, and because the particles of the plasma swirl around the magnetic field lines, the magnetic field structures change rapidly during the recording of the radio waves by the EHT. Sophisticated instruments and procedures were therefore required to image the supermassive black hole.

Luciano Rezzolla, theoretical astrophysicist at Goethe University Frankfurt and FIAS Fellow, explains that magnetic fields influence the polarised radio waves. Examining the degree of polarisation of the observed light shows how the magnetic fields of the black hole are distributed. However, in contrast to a standard image, which only requires information about the intensity of the light, it is much more difficult to show the polarisation. "In fact, our polarised image of Sgr A* is the result of a careful comparison between the actual measurements and the hundreds of thousands of possible image variants that we can create using advanced supercomputer simulations," explains Rezzolla. "Similar to the first image of Sgr A*, these polarised images represent a kind of average of all measurements."

Publications:

EHT collaboration: First Sagittarius A* Event Horizon Telescope Results. VII. Polarization of the Ring. Astrophysical Journal Letters (2024) https://iopscience.iop.org/article/10.3847/2041-8213/ad2df0

EHT collaboration: First Sagittarius A* Event Horizon Telescope Results. VIII. Physical Interpretation of the Polarized Ring. Astrophysical Journal Letters (2024) https://iopscience.iop.org/article/10.3847/2041-8213/ad2df1

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