Studying the light emitted by black holes helps narrow the search for axions

The search for a hypothetical subatomic particle (particle that makes up atoms) will be a new signal that will help narrow the path of physics research - thanks to the light swirling around a giant black hole in a galaxy. other ha.

The light particle - called an axion - has been proposed as a solution to the mystery of why the universe has so little antimatter and is a candidate for the elusive dark matter that fills the universe. The swirling and chaotic surroundings of the central black hole of galaxy M87 are also thought to encode information about such particles.

According to a report in the journal Nature Astronomy on March 17, researchers have detailed how directing light around the black hole M87 can rule out the possibility of axions appearing in a wide range. specific micro-volume.

Lines trace the polarization or orientation of light waves emitted by the region around the supermassive black hole in galaxy M87.

This study also shows that scientists can use a similar method in upcoming astrophysics observations to look for these particles with diverse masses.

'It's a very interesting idea,' said physicist Benjamin Safdi of the University of California, Berkeley. They've come up with a new method and they've proven that it can work in principle'.

First proposed in the late 1970s, axions have yet to be found in experiments. Theoretical work since that initial proposal has shown that there could be an extended family of axes, each with different masses but all of which interact weakly with ordinary matter. In 2020, physicist Yifan Chen of the Chinese Academy of Sciences in Beijing and colleagues described a way to search for axes using observations of light around black holes.

In theory, a rapidly rotating black hole could form a dense clump of axions in the immediate surroundings. Exactly what kind of axis is formed depends on the width of the black hole. And the supermassive black hole at M87 is just the right size to cook a stew of ultralight axon-like particles. If the black hole did create such a cloud, that would change the direction, or polarization, of the light coming from that region. In particular, the polarization will be shaken over time.

Unfortunately, no one had any images of polarized light from a black hole to test - until last year. That's when the Event Horizon Telescope, or EHT, a network of radio telescopes stretching across the Earth, revealed images of polarized light around the supermassive black hole at the center of M87 (SN: 3 /24/21).

'This is exactly the information we need to make this theoretical proposal,' said particle physicist Yue Zhao of the University of Utah in Salt Lake City. We have a very harsh condition that can produce a large number of axes, and we have the right tool to study the signatures of the axes'.

So, Zhao, Chen and colleagues examined the EHT data for the change over time in the direction of the polarization. While an axion cloud would change direction, so would the active and turbulent region around the black hole. "This is a sort of unavoidable background that we have to deal with. After removing that signal from the total number of signals, they found that there wasn't enough additional fluctuations to say that any one signal," Zhao said. any signal that might come from the axion cloud". They rule out the existence of ultralight axes with masses of about 10 billionths of a billionth of the mass of an electron.

But the same technique can be used to hunt for other shaft seeds as well. Zhao said: 'The larger you have a black hole, the lighter your mass. Physicists hope to use future EHT observations of other black holes to look for axes of different masses. Zhao notes that a black hole on the EHT's radar is massive at the center of our galaxy, one-thousandth the mass of M87 (SN: 6/5/19). If our galaxy's monster black hole had an axion cloud, it would be heavier particles."

"The idea of ​​finding these axial seeds is, in my opinion, the most exciting thing happening in particle physics at the moment," Safdi said.