Pulsars may unleash high-energy cosmic rays (Image: NASA/Goddard Space Flight Center Conceptual Image Lab)

Talk about enfants terribles. Baby pulsars may unleash torrents of the highest energy particles known, explaining the provenance of the ultra-high-energy cosmic rays that hit Earth.

Charged particles with energies of at least 1019 electronvolts slam into our atmosphere from time to time – since 2008, 5000 have been detected by the Auger observatory in Argentina.

Their source has been a mystery. Pulsars – ultradense stars formed during supernova blasts – are one candidate, but it has not been clear if the particles they shed could make it through the dense shroud of stellar shrapnel that surrounds them.


Now Ke Fang and colleagues at the University of Chicago have modelled these particles and found that they can escape within the first year of a pulsar’s life.

At that time, the pulsar’s spin, which gradually slows, is still fast enough to shoot out high-energy particles, and the supernova debris has spread out enough to allow those particles to escape. “The most energetic particles may come from the smallest stars,” says Fang.

‘Violent fellows’

If pulsars are the source of ultra-high-energy cosmic rays, it could explain a puzzling finding by Auger. Unlike the majority of low-energy cosmic rays, ultra-high-energy cosmic rays seem to be made up of the nuclei of heavy atoms, such as iron.

Pulsars – whose cores are made of neutrons – are thought to have charged particles such as iron nuclei in their crusts. When the stars spin, their electromagnetic fields accelerate these heavy, charged particles outwards. Because each iron nuclei has 26 protons, each of which feels the pull of the field, the resulting cosmic rays will be of a higher energy than any single protons thrown out by the pulsar.

“We set out to search for energetic cosmic accelerators that have iron, and [pulsars] seem to be an obvious place,” says team member Angela Olinto, also at the University of Chicago.

“Pulsars are certainly violent little fellows,” agrees Jim Matthews of Louisiana State University in Baton Rouge, a spokesman for the Auger observatory.

Nearby pulsar

The plot could thicken further, however. Auger doesn’t detect cosmic rays directly, but the shower of secondary particles created when these rays slam into the atmosphere. So it’s possible that the signals being picked up aren’t from iron nuclei but single protons interacting with atmospheric particles in a previously unknown way.

We may one day get a chance to test whether pulsars really are the source of ultra-high-energy cosmic rays, says Matthews.

To date, the highest energy cosmic rays measured by Auger seem to be scattered fairly evenly across the sky, suggesting they originated beyond our galaxy and were deflected by cosmic magnetic fields before they hit Auger’s detectors. This makes it impossible to trace them back to a source such as a pulsar.

But if a supernova one day explodes in our galaxy, producing a pulsar, it would be close enough that Auger could trace any cosmic rays back to it. “Then it should just be shining iron nuclei on us,” says Matthews.

Journal reference: arxiv.org/abs/1201.5197