Cosmic-Ray Electrons Slamming Earth Are The Most Powerful Ever Seen

To identify high-energy CRe, the researchers had to pore over H.E.S.S. data, identifying CRe candidates. Their final list of candidate events probably also includes some gamma rays; but the pool is large enough to draw some statistical inferences.

The energies range up to 40 teraelectronvolts, more powerful than any CRe we’ve detected hitting Earth to date.

CRe detections with energies higher than a teraelectronvolt are very rare. That’s because, as they move through space, they lose energy rapidly.

“In synchrotron radiation, charged particles interact with the interstellar, galactic field. They acquire a spiral trajectory around the magnetic field lines, and radiate electromagnetic radiation, from radio up to X-rays. By doing so, they lose energy,” de Naurois said.

“In the so-called ‘Inverse Compton Scattering’, a charged particle interacts with ambient light. They interact with a low-energy photon and give most of their energy to it. The process is called ‘Inverse Compton’ because it is the reverse of the Compton scattering, in which a high-energy photon scatters off an electron from the medium and boosts it to high energy.”

Since the CRe lose energy so quickly, the candidate events must have traveled from nearby space to remain so powerful by the time they reach Earth. We can’t track them to a source; their trajectories are too unpredictable; but there’s something else about their energies that may be a clue. There’s a distinct lower cut-off point at 1.17 teraelectronvolts.

“The fact that the change of slope is sharp indicates that it’s only a handful of cosmic sources, if not only one, that produces these electrons,” de Naurois explained.

“Otherwise the energy spectrum would be the superposition of the contributions of different sources with breaks at different energy, resulting in a much smoother curve.”

Because the volume of space from which these CRe could have emerged is so small, that means the pool of potential sources is also small. Candidates include a supernova remnant called the Monogem Ring; a dying star of the Wolf-Rayet type called γ² Velorum; or a pulsar like Vela or Geminga.

But it’s also possible that the source is a supernova remnant so old that it has dissipated and faded from view. We just have no way of knowing right now.

Nevertheless, this extraordinary work brings us a step closer to understanding how the Universe is energized. The team plans to investigate further to see if they can identify a preferred direction from which the CRe arrive.

It’s going to be tricky, but the potential rewards are high, and the increased candidate pool will be invaluable to the study of CRe going forward.

“Our measurement does not only provide data in a crucial and previously unexplored energy range, impacting our understanding of the local neighborhood, but it is also likely to remain a benchmark for the coming years,” de Naurois says.