The highest-energy photons ever seen hail from the Crab Nebula

Physicists have spotted the highest-energy light ever seen. It emanated from the roiling remains left behind when a star exploded.

This light made its way to Earth from the Crab Nebula, a remnant of a stellar explosion, or supernova, about 6,500 light-years away in the Milky Way. The Tibet AS-gamma experiment caught multiple particles of light — or photons — from the nebula with energies higher than 100 trillion electron volts, researchers report in a study accepted in Physical Review Letters. Visible light, for comparison, has just a few electron volts of energy.

Although scientists have searched for photons at these energies before, they haven’t succeeded in detecting such energetic photons until now, says astrophysicist Petra Huentemeyer of Michigan Technological University in Houghton, who was not involved with the research. For physicists who study this high-energy light, known as gamma rays, “it’s an exciting time,” she says.

In space, supernova remnants and other cosmic accelerators can boost subatomic particles such as electrons, photons and protons to extreme energies, much higher than those achieved in the most powerful earthly particle accelerators (SN: 10/1/05, p. 213). Protons in the Large Hadron Collider in Geneva, for example, reach a comparatively wimpy 6.5 trillion electron volts. Somehow, the cosmic accelerators vastly outperform humankind’s most advanced machines.

“The question is: How does nature do it?” says physicist David Hanna of McGill University in Montreal.

In the Crab Nebula, the initial explosion set up the conditions for acceleration, with magnetic fields and shock waves plowing through space, giving an energy boost to charged particles such as electrons. Low-energy photons in the vicinity get kicked to high energies when they collide with the speedy electrons, and ultimately, some of those photons make their way to Earth.

When a high-energy photon hits Earth’s atmosphere, it creates a shower of other subatomic particles that can be detected on the ground. To capture that resulting deluge, Tibet AS-gamma uses nearly 600 particle detectors spread across an area of more than 65,000 square meters in Tibet. From the information recorded by the detectors, researchers can calculate the energy of the initial photon.

But other kinds of spacefaring particles known as cosmic rays create particle showers that are much more plentiful. To select photons, cosmic rays, which are mainly composed of protons and atomic nuclei, need to be weeded out. So the researchers used underground detectors to look for muons — heavier relatives of electrons that are created in cosmic ray showers, but not in showers created by photons.

Previous experiments have glimpsed photons with nearly 100 TeV, or trillion electron volts. Now, after about three years of gathering data, the researchers found 24 seemingly photon-initiated showers above 100 TeV, and some with energies as high as 450 TeV. Because the weeding out process isn’t perfect, the researchers estimate that around six of those showers could have come from cosmic rays mimicking photons, but the rest are the real deal.

Researchers with Tibet AS-gamma declined to comment for this story, as the study has not yet been published.

Looking for photons of ever higher energies could help scientists nail down the details of how the particles are accelerated. “There has to be a limit to how high the energy of the photons can go,” Hanna says. If scientists can pinpoint that maximum energy, that could help distinguish between various theoretical tweaks to how the particles get their oomph.

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Editor’s note: This story was updated June 28, 2019, to clarify Petra Huentemeyer’s comments about previous efforts to find high-energy photons.