Pulsars may provide cosmic rays with the highest energies known in the universe

 Their magnetic environments are ideal for accelerating particles to ultrahigh energies.

Pulsars may provide cosmic rays with the highest energies known in the universe
cosmic rays

The windy and chaotic remnants of recently exploded stars may be launching the universe's fastest particles.

Pulsars are highly magnetic neutron stars that generate a strong magnetic wind. When charged particles, specifically electrons, become trapped in those turbulent conditions, they can be accelerated to extreme energies, astrophysicists report in the Astrophysical Journal Letters on April 28. 

Furthermore, those speedy electrons can then boost some ambient light to equally extreme energies, potentially producing the very-high-energy gamma-ray photons that led astronomers to discover these particle launchers in the first place

"This is the first step in investigating the link between pulsars and ultrahigh-energy emissions," says Ke Fang  University of Wisconsin, Madison, who was not involved in the new research.

Researchers from China's Large High Altitude Air Shower Observatory, or LHAASO, announced last year the discovery of the highest-energy gamma rays ever detected, with energies reaching 1.4 quadrillion electron volts (SN: 2/2/21). 

This is roughly 100 times more energetic than the highest energies attained by the world's most powerful particle accelerator, the Large Hadron Collider near Geneva. Identifying the source of these and other extremely high-energy gamma rays could lead to the location of cosmic rays — the active protons, heavier atomic nuclei, and electrons that bombard Earth from beyond our solar system.

Some gamma rays are thought to come from the same place as cosmic rays. Cosmic rays, shortly after being launched, can collide with relatively low-energy ambient photons, boosting them to high-energy gamma rays. 

However, because galactic magnetic fields buffet the electrically-charged cosmic rays, they do not travel in a straight line, complicating efforts to trace the zippy particles back to their source. However, because gamma rays are immune to magnetic fields, astrophysicists can follow their unwavering paths back to their origins — and figure out where cosmic rays are created.

To that end, the LHAASO team tracked hundreds of gamma-ray photons to 12 different locations in the sky. While the team identified one spot as the Crab Nebula, a supernova remnant about 6,500 light-years from Earth, the rest could be associated with other stellar explosion sites or even young massive star clusters (SN: 6/24/19).

Astrophysicist Emma de Oa Wilhelmi and colleagues focused on one of those possible origins in their new study: pulsar wind nebulas, which are clouds of turbulence and charged particles surrounding a pulsar. 

The researchers were skeptical that such locations could produce such high-energy particles and light, so they set out to prove that pulsar wind nebulas were not the source of extreme gamma rays through calculations. "However, to our surprise, we saw that under very extreme conditions, you can explain all the sources [that LHAASO saw]," says de Oa Wilhelmi of the German Electron Synchrotron in Hamburg.

Because of their ultrastrong magnetic fields, which create a turbulent magnetic bubble called a magnetosphere, the young pulsars at the heart of these nebulas — no more than 200,000 years old — can provide all that oomph.

According to de Oa Wilhelmi, any charged particles moving in a strong magnetic field are accelerated. This is how the Large Hadron Collider accelerates particles to extreme energies (SN: 4/22/22). The team calculates that a pulsar-powered accelerator can boost particles to even higher energies. This is due to electrons escaping the pulsar's magnetosphere and colliding with material and magnetic fields from the stellar explosion that created the pulsar. 

The team discovers that these magnetic fields can further accelerate electrons to even higher energies and that if those electrons collide with ambient photons, they can boost those light particles to ultrahigh energies, transforming them into gamma rays.

"Pulsars are definitely very powerful accelerators," says Fang, with "multiple places where particle acceleration can occur."

And this could cause some confusion. Gamma-ray telescopes have poor vision. LHASSO, for example, can only discern details as small as half the size of the full moon. According to de Oa Wilhelmi, the gamma-ray sources detected by the telescope appear as blobs or bubbles. There could be multiple energetic sources within those blobs, which are currently unresolved by existing observatories.

"We should be able to identify what [and] where the accelerator is with better angular resolution and sensitivity," she says. A few future observatories, such as the Cherenkov Telescope Array and the Southern Wide-field Gamma-ray Observatory, may be of assistance, but they are several years away.


E. de Oña Wilhelmi et al. On the potential of bright, young pulsars to power ultrahigh gamma-ray sources. Astrophysical Journal Letters. Vol. 930, April 28, 2022, p. L2. DOI: 10.3847/2041-8213/ac66cf

R. López-Coto et al. Gamma-ray haloes around pulsars as the key to understanding cosmic-ray transport in the Galaxy. Nature Astronomy. Vol. 6. February 2022. DOI: 10.1038/s41550-021-01580-0.


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