The nature of dark matter, the substance that accounts for more than 85% of all matter in the Universe, remains a mystery. Many theories predict that dark matter is made up from yet undiscovered fundamental particles and a plethora of experiments on Earth and in Space are looking for traces of them. A new study by Manuel Meyer of the Erlangen Center of Astroparticle Physics at FAU and Tanja Petrushevska of the Center for Astrophysics and Cosmology at the University of Nova Gorica suggests how researchers can use explosions of stars outside the Milky Way to search for a particular class of dark matter particles.
One popular hypothesis is that dark matter consists at least partially of hypothetical particles called axions or axion-like particles. These particles are potentially lighter than any other known fundamental particle (neutrinos might be an exception), and can “interchange personality” with photons in electromagnetic fields. In other words, when flying through a magnetic field or close to charged particles, a beam of axions could actually transform into photons or vice versa.
This special property leads to an intriguing possibility, where axions could be created during the explosion of a massive star at the end of its lifetime, an event commonly known as a supernova. In the core of the explosion, ions and protons are densely squeezed together, allowing energetic photons, called gamma rays, to transform into axions. In this ghostly axion form, they can quickly escape the dense core and slowly return to gamma-ray form on their long path through the magnetic fields in space.
Upon reaching Earth, the resulting short burst of gamma rays could be detected with the Large Area Telescope (LAT) on board the Fermi satellite, which constantly scans the entire sky for gamma rays and sees roughly 20% of the sky at any given moment. The challenge is to know when exactly to look for the burst, which, according to theory, is only tens of seconds long.
A new study uses the wealth of data collected with the Fermi LAT to search for the axion-induced bursts by correlating it with, for the first time, the results of dedicated supernova surveys. These surveys, which use traditional optical telescopes, detect hundreds of supernovae each year. By modeling the fading glow of the explosions, Manuel Meyer from the Erlangen Center of Astroparticle Physics and Tanja Petrushevska from the University Nova Gorica were able to make predictions for the time window of the explosion and search for the expected gamma-ray burst in the LAT data. Since they did not find any gamma-ray signal during the time windows in question, they could constrain the interaction strength between axion-like particles and photons.
“Even though our study does not rule out the possibility of axion-like particles making up all of the dark matter, we were able to test a so-far unexplored region of the axion-like-particle parameter space”, says Meyer, who was responsible for the gamma-ray analysis. “Our study improves previous limits from the non-observation of the gamma-ray burst from the supernova that occurred in 1987 in the Large Magellanic Cloud by a factor of two”.
There still remains a 10% chance that the LAT wasn’t looking at the right spot in the sky when the explosions occurred. “With on-going surveys such as the Zwicky Transient Facility and the up-coming Rubin Observatory, the chance that we observe at least one supernova explosion with Fermi should be almost 100%,” adds Petrushevska, who studies optical supernova observations. She continues, “this is one exciting part of the study: it opens up a new way to use supernova explosions in other galaxies to search for fundamental physics.”
The two researchers, who know each other from their common days at Stockholm University, hope that their new results will trigger a closer collaboration between the teams running the optical surveys and operating gamma-ray telescopes. There has not been much overlap between the two communities in the past, but the pair are optimistic that their research can contribute to changing that.
Link to the article: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.231101
Link to the preprint: https://arxiv.org/abs/2006.06722