Are pulsars the source of galactic cosmic rays?

Are pulsars the source of galactic cosmic rays?

Press release from:
Posted: Wednesday September 29 2021

For Dr Alison Mitchell, who is transferring to the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) at ETH Zürich, this would be a dream come true. Starting in October, Dr Mitchell and his junior research group Emmy Noether at the Erlangen Center for Astroparticle Physics (ECAP) are to study the role pulsars play in creating high-energy galactic cosmic rays. The project has a duration of six years and has received nearly 1.5 million euros in funding.

The search for the origin of cosmic rays

Galactic cosmic rays are created in our galaxy, the Milky Way. They are mainly made up of charged particles, i.e. protons, ions, positrons and electrons which are accelerated under extreme conditions and arrive on Earth charged with high energy. As photons, or light particles, are created during the acceleration process, gamma rays can also provide clues about the nature of cosmic accelerators. The first step towards understanding this phenomenon was taken in 1912, when cosmic rays were discovered by Austrian physicist Victor Franz Hess. Charged particles are deflected on their long journey to Earth by interstellar magnetic fields. Research on the origin of cosmic rays therefore focuses on uncharged particles like photons or neutrinos, because they descend directly to Earth and can give us clues to their site of origin. Alison Mitchell is one of the world’s leading scientists for researching high-energy photons from space, known as gamma rays.

It is still unclear where cosmic rays come from and whether they originate from one or more source populations. The most promising candidates include supernova remnants, the area around rotating neutron stars, or pulsars, and black holes. “Many colleagues tend to favor supernova remnants, but experiments so far have failed to provide unambiguous evidence that this is the case,” says Dr Mitchell. The longer the starburst took place, the weaker the acceleration should be. Additionally, theoretical research has yet to deliver convincing evidence that particles in supernova remnants can be accelerated to the extremely high energy levels found in cosmic rays. Scientists are therefore looking for other explanations. Several research groups, for example in France, Poland and the United States, are working on theoretical models indicating that galactic cosmic rays originate from the area around pulsars.

It was only proven in 2019 that pulsar wind nebulae are able to accelerate positrons and electrons to energy levels of 1015 electron-volts. It follows that the main components of galactic cosmic rays, i.e. protons and ions, can also originate from the area around a pulsar. In his work at FAU, Dr Mitchell now hopes to provide the expected experimental evidence that protons are accelerated by pulsars and the area around them. “As far as I know, the large research program we are planning is the only one of its kind in the world,” she explains.

On the lookout with gigantic telescopes
Since high-energy particles are difficult to find using satellites, researchers are using the Earth’s atmosphere as a detector. Cherenkov telescopes intercept the faint glow emitted when a gamma-ray photon collides with Earth’s atmosphere. The five telescopes at the HESS Observatory in Namibia accurately trace the direction of the gamma rays. Alison Mitchell and other FAU researchers are also involved in the large international project to build a ground-based observatory for gamma-ray astronomy known as the Cherenkov Telescope Array (CTA). “As the project progresses, other CTA telescopes with higher resolution and a wider field of view will be erected in La Palma in the Canary Islands and Chile,” says Dr Mitchell.

Algorithms must be used to improve the resolution of telescopes. Currently, researchers are also working on new methods to detect sources of gamma rays distributed over a larger area. Mitchell believes that with its leadership position in theoretical astrophysics and neutrino, X-ray and gamma-ray astronomy, FAU offers a very wide range of possible avenues for his project. “It is extremely likely that supernovas and pulsars are responsible for galactic cosmic rays, but I believe pulsars can accelerate particles to energies thousands of times greater than those reached by remnants of supernovae.”

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