When a cosmic-ray detector clocks an extremely speedy proton or nucleus, chances are that the particle was initially accelerated by a pulsar, black hole jet, or other powerful astronomical object that is located far beyond our galaxy. Cosmic-ray electrons and their antimatter counterparts, positrons, have different origin stories. Because they readily emit radiation as they interact with both photons and magnetic fields, their energies drop precipitously as they traverse interstellar space. As a result, high-energy cosmic electrons detected on Earth almost certainly began their journeys no more than several hundred light-years away.
Now researchers using data from the High Energy Stereoscopic System (HESS) observatory in Namibia have taken inventory of cosmic-ray electrons and positrons with energies up to 40 trillion eV, several times the maximum energy of past studies. The census was designed to evaluate the population of objects in our stellar neighborhood that can fling particles at tremendous speeds. Judging by the new HESS results, that population is extremely small.
In full operation since 2003, HESS consists of four 12-meter optical telescopes—and, since 2012, a 28-meter telescope—that indirectly detect cosmic gamma radiation (see Physics Today, November 2002, page 31). When an incoming gamma ray interacts with a particle in Earth’s atmosphere, it triggers a cascade of secondary particles that, as they slow down, emit faint flashes of blue Cherenkov radiation. Cosmic-ray electrons and positrons trigger similar air showers and Cherenkov flashes when they pelt the planet. By filtering out observations of the galactic plane, a region that is abundant in gamma-ray sources, and comparing their data with the output of simulations, the researchers isolated the Cherenkov events that were likely signatures of cosmic electrons and positrons.
Plotting events with energies of 300 GeV–40 TeV, the researchers ended up with a relatively smooth curve of cosmic electron and positron flux that falls with increasing energy, gradually at first but then sharply at energies greater than about 1.2 TeV. The findings are largely consistent with those from detectors fitted on balloons and satellites, though they do not include any of the bumps or shifts in the curve that had been hinted at in some experiments. Such features could suggest the presence of nearby pulsars or supernova remnants or of dark matter that decays into electrons and positrons (see, for example, Physics Today, June 2013, page 12).
The featureless curve and the steep drop-off at high energies imply that “our local neighborhood is a relatively peaceful place,” says Kathrin Egberts of the University of Potsdam, with no more than a few nearby cosmic objects acting as extreme particle accelerators. Egberts and her HESS colleagues next plan to analyze the arrival directions of the detected electrons and positrons. Although the particles are charged and thus deflected by the galaxy’s magnetic fields, the researchers still may find directional differences if the entire high-energy flux is the product of a single astronomical source. (F. Aharonian et al., Phys. Rev. Lett. 133, 221001, 2024.)
