Black hole shadow images of giant galaxy Messier 87 (M87) observed the Event Horizon Telescope (EHT) collaboration provides rich information for astronomy and gravity. What else can we learn from those beautiful images? Using the variations of polarization image, an international team of scientists including Yue Zhao, assistant professor at the University of Utah, gives a new constraint of the coupling between axion and photon to the previously unexplored region.
In 2019, combining the observations from telescopes all around the Earth, the EHT collaboration published a photo of a supermassive black hole M87 with extremely high resolution. The shining donut-like structure comes from the radiation of the accretion flow around the black hole. The black hole swallows the light in the central region, creating a big shadow inside the donut. The EHT collaboration updates the same photo with more delicate structures two years later. The sweeping lines, showing the linear polarization orientation (electric vector position angles), transform the donut into a cruller, a pastry that resembles a donut with ridges. These first-ever images give the most direct evidence of black holes and reveal the magnetic fields outside M87.
A view of the M87* supermassive black hole in polarised light, taken by Event Horizon Telescope and revealed on 24 March 2021. The direction of lines atop the total intensity mark the direction of the electromagnetic wave electric vector oscillations.
"The axion is a hypothetical, ultralight particle. If the axion exists and it has the right mass, a spinning supermassive black hole will generate billions of billions of axions in its neighborhood and form a gigantic axion cloud," said Zhao. "The cloud will change the orientation of the light being emitted from the accretion disk surrounding the black hole. EHT is measuring how the polarization changes over time. If it changes in certain pattern, it may be an indication that there's an axion cloud."
Dr. Yifan Chen comments: "We are fascinated by the idea that ultralight particles can accumulate outside the black hole. We realized that if ultralight axion exists and stays outside the black hole, they would make the cruller dance! Using the four days variations of the crullers, we can constrain the coupling between axion and photon to the previously unexplored region."
How to turn supermassive black holes into detectors for ultralight particles? It dates to a thought experiment by Roger Penrose in 1969. Imagine someone throwing a rock into a fast-spinning black hole, and the stone has a certain chance to escape with a larger velocity than the previous. The additional energy it carries is taken from the rotation of the black hole. Now think about the particle-wave duality in quantum mechanics. We can replace the rock with a wave outside the spinning black hole. It can form a dense cloud by extracting energy and angular momentum from the black bole, called the superradiance mechanism. To make this process sufficiently fast, it requires the Compton wavelength of the boson to be comparable with the horizon size of the black hole. Thus, supermassive black holes become natural detectors for ultralight particles!
Illustration of the polarised emission from a Kerr black hole surrounded by an axion cloud. Different colors on the electric vector position angles (EVPA) quivers, which range from red through to purple, represent the time variation of the EVPA in the presence of the axion-photon coupling. White quivers are the EVPAs when the axion field is absent. The intensity scale is normalized so that the brightest pixel is unity.
Among different types of ultralight fields beyond the standard model of particle physics, axion is one of the most well-motivated candidates. Searching for the axion is among the top priorities in particle physics. It naturally appears in many fundamental theories with extra dimensions, like the string theory. Axion is also a perfect cold dark matter candidate. In the ultralight mass window, some small-scale problems of the galaxy can be potentially solved by those fields forming a core in the center.
Once the ultralight axion exists within the right mass window, a dense axion cloud and the center black hole form a bound state similar to the hydrogen atom and being called the gravitational atom. Prof. Jing Shu said: "Besides purely gravitational effects, the existence of axion can rotate the orientation of the linear polarization periodically as well, with a period between 5 to 20 days. The variations of the polarization angle behave as a propagating wave along the bright photon ring, which the dance of the crullers has a particular pattern instead of a random walk by a drunken man."
The EHT's polarimetric measurements provide the spatial distribution of the linear polarization orientations for four days, precisely the information we need to search for axion. Prof. Yosuke Mizuno remarks: "To suppress the turbulent variations of the accretion flow, we introduced a novel analysis strategy where the difference between two sequential daysare used as observables to constrain the axion-induced EVPA variations. A much larger parameter space can be probed with more detailed data provided in the future, especially the more sequential time observation and better spatial resolutions."
Publication: Yifan Chen, Yuxin Liu, Ru-Sen Lu, Yosuke Mizuno, Jing Shu, Xiao Xue, Qiang Yuan, Yue Zhao: Stringent axion constraints with Event Horizon Telescope polarimetric measurements of M87⋆. Nature Astronomy 2022, March https://www.nature.com/articles/s41550-022-01620-3
