Somewhere in our galaxy are engines capable of driving atomic fragments to velocities that come within a whisker of lightspeed.
The explosive deaths of stars seems like a natural place to search for sources of these highly energetic cosmic bullets, yet when it comes to the most powerful particles, researchers have had their doubts.
Numerical simulations by a small international team of physicists may yet save the supernova theory of cosmic ray emissions at the highest of energies, suggesting there is a brief period where a collapsing star could still become the Universe’s most extreme accelerator.
For more than a century, scientists have scanned the skies for phenomena that may be responsible for the relatively constant showers of atomic nuclei and occasional electrons that pepper our planet.
Simply following their trajectory would be like picking up a bottle on the beach and looking to the horizon for its home. The charges of most cosmic rays put them at the mercy of a turbulent ocean of magnetic fields across the galaxy and beyond, leaving researchers to search for other clues.
A mere few thousand light years away in our galactic backyard, the historic supernova known as Tycho’s star has been studied for signs of physics capable of accelerating charged particles.
In 1572, astronomers marveled at the star’s sudden brightening, now understood to be the final hoorah of a white dwarf ending its life in a thermonuclear catastrophe. As its core collapsed under its own weight, the burst of heat and radiation slammed into the shell of surrounding gases, generating immense magnetic fields.
In 2023 researchers published their analysis on those fields, finding their ability to generate cosmic rays was “significantly smaller” than those expected of existing models.
While this doesn’t rule out collapsing stars as potential particle accelerators, it does raise questions on just how much power they can provide.
Every now and then, Earth is struck by some true monsters – particles that are up to a thousand times more powerful than anything our own technology has been capable of generating. These peta-electronvolt (PeV) energies are the work of hypothetical cosmic engines dubbed PeVatrons.
According to astrophysicists Robert Brose from the University of Potsdam in Germany, Iurii Sushch from the Spanish Centre for Energy, Environmental and Technological Research, and Jonathan Mackey from the Dublin Institute for Advanced Studies, dying stars just might be the mysterious PeVatrons scientists have been searching for.
For it to work, the dying star first needs to cough up enough material to form a dense shell around itself. Then, at the moment of supernova the rapidly expanding shock wave smashes into this dense environment, generating the necessary magnetic turbulence to whip nuclei and electrons towards PeV-levels of acceleration.
The critical element, they claim, is timing – only within its first decade or two is the surrounding shell dense enough to provide the amount of turbulence required for particles to reach the highest of energies.
“It is possible that only very young supernova remnants evolving in dense environments may satisfy the necessary conditions to accelerate particles to PeV energies,” the team writes.
Had Tycho’s star held its breath for just another few centuries, astrophysicists may have recorded a shower of cosmic rays at the highest of magnitudes.
Perhaps in the near future, the violent end of another nearby star just might give us the opportunity they need to solve the perplexing mystery of PeVatrons once and for all.
This research has been accepted for publication in Astronomy & Astrophysics.
