A collaboration of Chinese and Japanese astrophysicists has reported the highest energy photons ever seen: gamma rays with energies up to 450 trillion electron volts (TeV).
The particles of light were traced back to the Crab nebula, the remnant of a stellar explosion observed by Chinese astronomers nearly 1000 years ago, and the powerful pulsar, a dense neutron star, that now sits at the nebula’s heart. “We know the environment of a pulsar is extreme,” says Geraint Lewis, an astrophysicist at the University of Sydney in Australia who was not involved in the research. The question raised by the finding “is just how extreme,” he says. He says the results will help constrain ideas about how the photons are boosted to such extraordinary energies.
The Tibet ASgamma experiment spotted the photons using an array of nearly 600 scintillation detectors, sensors that turn particle strikes into electronic signals. The detectors are spread out across 66,000 square meters in a valley 4300 meters above sea level on the Tibetan Plateau in China. When gamma rays strike Earth’s atmosphere they create air showers—spreading cascades of electrons and other subatomic particles. As these particles hit the detectors, the timing and energy of the strikes are recorded—enabling astronomers to reconstruct the energy and trajectory of the original gamma ray.
The problem is distinguishing gamma rays from cosmic rays, charged particles that can also reach these colossal energies and create similar air showers. Fortunately, the air showers sparked by cosmic rays contain a higher proportion of muons, short-lived cousins of the electrons, than the showers from gamma rays. The muons can be detected in underground water chambers and used to distinguish between gamma ray and cosmic ray events. Gamma rays are prized because they travel through the cosmos in straight lines, and thus point back to their sources. Cosmic rays, in contrast, get pulled into corkscrew trajectories by magnetic fields, making their origins obscure.
To improve muon detection, the Tibet ASGamma team buried water tank detectors several meters below ground at 64 locations around the site in Yangbajing, a town northwest of Lhasa on the Tibetan Plateau, giving the array “the world’s highest sensitivity to gamma rays in the 100-TeV region,” says Masato Takita, an experimental physicist for the project at the University of Tokyo’s Institute for Cosmic Ray Research in Kashiwa, Japan. With the enhanced capabilities, “I believed we could find results that no one ever found before,” adds Huang Jing, an astrophysicist at the Chinese Academy of Science’s Institute of High Energy Physics in Beijing.
And that they did. From February 2014 to May 2017, the array caught 24 gamma rays ranging from 100 TeV to 450 TeV coming from the Crab nebula, the team of 90 researchers from two dozen institutions reports in a paper accepted at Physical Review Letters. The strikes shatter the previous record holder: 75-TeV gamma rays observed by the High Energy Gamma Ray Astronomy experiment located on La Palma, one of Spain’s Canary Islands.
Modeling had predicted the existence of such high energy gamma rays, so although the finding isn’t a surprise, it still provides valuable confirmation for assumptions thinly supported by observations, says Felix Aharonian, an astrophysicist at the Dublin Institute for Advanced Studies.
The models point to a process called inverse Compton scattering, in which the pulsar’s magnetic field whips up electrons to energies far higher than achieved in particle accelerators on Earth. The electrons then smash into the ambient photons that pervade the universe as a part of the cosmic microwave background and send them speeding through the galaxy. The photons “receive a huge amount of energy in that kick,” Lewis says. The results show “the Crab nebula is the most powerful natural electron accelerator known so far in our galaxy,” Huang says.
Lewis adds that the observed energies of gamma ray photons have been steadily going up thanks to improved detectors. He believes supermassive black holes that sit at the centers of galaxies might prove to be another source of high energy gamma rays.
More evidence may be on the way with the opening of new observatories. The multinational Cherenkov Telescope Array may be completed by 2025, and the Large High Altitude Air Shower Observatory, a partially completed facility also on the Tibetan Plateau, started observations in April and should be up to full speed next year.
For the moment, however, the Tibet ASgamma experiment is leading the hunt for “PeVatrons”—astrophysical sources capable of accelerating gamma ray photons and cosmic rays up to one petaelectronvolt, or 1000 TeV. “We expect to identify a lot of PeVatrons,” Huang says. Signals from other 100-TeV sources besides the Crab nebula may already be hidden in the Tibet ASgamma experiment data, a possibility Takita says is “currently under analysis.”