In 2019 astronauts captured the first direct image of a black hole. It was an image of the supermassive black hole at the heart of the M87. And when many people saw it, their answer was “is that it?” Which is understandable, since the image is just like a blurry smudge, donut shape. There is not much to see. But celestial imagery is a small fraction of the data collected by astronomers. More recently, more of these data have been analyzed, including both the polarization of the light and the magnetic field around the black hole.
Polarization is a fundamental property of light, just as a wavelength or intensity. If you think of light as an oscillating wave as it travels through space, polarization is the direction of that oscillation. Light waves can rise up and down, left and right, or even spin clockwise or widdershins. When light comes from a hot source, such as the material around a black hole, a lot of polarization goes together so that the light is essentially undisturbed. But when light passes through ionized gas, different polarization interacts with the stronger or weaker gas. As a result, the light that reaches the Earth is polar. By studying the polarization of light near the black hole of the M87, we can learn about the surrounding material.
In radio astronomy, there is also a polarized light source called synchrotron radiation. This occurs when electrons are trapped by a magnetic field and move forward on the field lines in tight spirals. The polarization of sychrotron radiation tells us the direction of the magnetic field lines.
In this latest work, astronomers measured the polarization of light seen near the black hole of the M87, and found a complex spiral pattern. Expect this a bit, because we know the black hole is circling. As it is, it drags a nearby space around it. The overall pattern is indicative of the gravity structure of the black hole.
But the interesting thing is that he saw most of the light which is not polarized. Only about 15% or so of the light is polar. Most of the light from near the black hole is undisturbed. That is invisible, because ionized gas near the black hole should be heavily attracted, so we would expect the light that reaches us to be strongly polarized. So what gives?
There appears to be gas near the black hole is magnetization, but instead of a large and simple magnetic structure, the magnetization is a chaotic leap at smaller blades. The scale at which the external magnetization appears is at random less than the resolution of the Horizon Event Telescope. So things get darker. All the small polarities scrub together to appear polar-free.
Results like this are important because they give us a terrific view of the materials and magnetic fields near black holes. As we understand more, we will be able to understand the complex processes that create active black holes and how they interact with the surrounding galaxy. All that information is buried in the data, and it is more than meets the eye.
Information: Akiyama, Kazunori, et al. “First Horizon Telescope Results M87 Event. VII. Polarization of the ring. ” Letters of the Astrophysical Journal 910.1 (2021): L12.
Information: Akiyama, Kazunori, et al. “First Horizon Telescope Results M87 Event. VIII. Magnetic field structure near the Horizon event. ” Letters of the Astrophysical Journal 910.1 (2021): L13.