The Occasion Horizon Telescope uncovered the magnetic subject traces across the central black gap of the M87

In 2019, astronomers captured the first direct image of a black hole. It was a picture of the supermassive black hole in the heart of M87. And when a lot of people saw it, their reaction was “is that it?” This is understandable as the image is just a blurry, donut-shaped blotch. There is not much to see. However, an astronomical image is only a small fraction of the data astronomers collect. More of this data has recently been analyzed, including the polarization of light and the magnetic field surrounding the black hole.

Polarization is a fundamental property of light, just like wavelength or intensity. If you think of light as a wave that vibrates on its way through space, then the polarization is the direction of that vibration. Light waves can oscillate up and down, left and right, or even spiral clockwise or in ramshins. When light comes from a hot spring such as B. the material that surrounds a black hole, many polarizations are thrown together so that the light is basically unpolarized. However, when light passes through ionized gas, different polarizations interact more or less with the gas. As a result, the light that reaches the earth is polarized. By studying the polarization of light near black hole M87, we can learn something about the material around it.

M51 (Hubble) superimposed with 6 cm radio intensity contours and polarization vectors (Effelsberg and VLA) Photo credits: MPIfR Bonn

In the case of radio astronomy, there is also a polarized light source known as synchrotron radiation. This occurs when electrons are captured by a magnetic field and move in tight spirals along the field lines. The polarization of the sychrotron radiation provides information about the alignment of the magnetic field lines.

In this latest work, astronomers measured the polarization of light observed near black hole M87 and found it to have a twisted spiral pattern. This is somewhat expected as we know that the black hole is spinning. In doing so, it drags the nearby space around itself. The overall pattern shows the gravitational structure of the black hole.

An image of the black hole M87 with polarization is displayed. Photo credit: EHT Collaboration

What is interesting, however, is that most of the light observed is not polarized. Only about 15% of the light is polarized. Most of the light near the black hole is unpolarized. This is unexpected as ionized gas near the black hole should be highly magnetized, so we would expect the light reaching us to be highly polarized. So what’s up?

It seems that gas near the black hole is magnetized, but instead of having a large and simple magnetic structure, the magnetization on a smaller scale is a messy mess. The scale on which the magnetization has a random orientation is smaller than the resolution of the Event Horizon Telescope. That’s how things get blurred. All small-scale polarizations blur and appear unpolarized.

Results like these are important because they give us tremendous insight into the material and magnetic fields around black holes. If we understand more, we will be able to understand the complex processes that active black holes create and how they interact with the surrounding galaxy. All of this information is buried in the data and more than intended.

Reference: Akiyama, Kazunori, et al. “First results of the M87 Event Horizon Telescope. VII. Polarization of the ring. “The Astrophysical Journal Letters 910.1 (2021): L12.

Reference: Akiyama, Kazunori, et al. “First results of the M87 Event Horizon Telescope. VIII. Magnetic field structure near the event horizon. “The Astrophysical Journal Letters 910.1 (2021): L13.

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