Event Horizon telescope revealed magnetic field lines around central M87 black hole

In 2019, astronomers captured the first direct image of a black hole. It was an image of the supermassive black hole at the heart of M87. And when a lot of people saw it, their reaction was “is that it?” Which is understandable, considering the image is just a donut-shaped blur. It’s not much to watch. But an astronomical image is only a small fraction of the data collected by astronomers. Recently, more of this data has been analyzed, including both the polarization of light and the magnetic field surrounding the black hole.

Polarization is a basic property of light, just like wavelength or intensity. If you imagine light as a wave that oscillates as it moves through space, then polarization is the orientation of that oscillation. Light waves can oscillate up and down, left and right, or even spiral clockwise or clockwise. When light comes from a hot source, such as the material surrounding a black hole, many polarizations are mixed so that the light is basically unpolarized. But when light passes through an ionized gas, different polarizations interact with the gas more or more weakly. As a result, the light that reaches the Earth is polarized. By studying the polarization of light near the M87 black hole, we can learn more about the surrounding material.

In the case of radio astronomy, there is also a source of polarized light called synchrotron radiation. This happens when electrons are trapped by a magnetic field and travel along tight spiral field lines. The polarization of sychrotron radiation tells us the orientation of the magnetic field lines.

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

But what is interesting is that most of the light observed is not polarized. Only about 15% of light is polarized. Most of the light coming from near the black hole is unpolarized. This is unexpected, because the ionized gas near the black hole must be strongly magnetized, so we would expect the light reaching us to be strongly polarized. So what gives?

It seems the gas near the black hole is magnetized, but rather than having a large and simple magnetic structure, magnetization is a chaotic jumble at smaller scales. The scale at which the magnetization has a random type orientation is lower than the resolution of the Event Horizon telescope. So things fade away. All small-scale polarizations fade to appear unpolarized.

Results like these are important because they give us tremendous insight into the material and magnetic fields near black holes. As we gain a better understanding, we can discover the complex processes that create active black holes and how they interact with the surrounding galaxy. All of this information is buried in the data, and it’s more than what you see.

Reference: Akiyama, Kazunori et al. “Results of the first M87 Event Horizon telescope. VII. Polarization of the ring. ” Letters from the astrophysical journal 910.1 (2021): L12.

Reference: Akiyama, Kazunori et al. “Results of the first M87 Event Horizon telescope. VIII. Structure of the magnetic field near the event horizon. ” Letters from the astrophysical journal 910.1 (2021): L13.