In the wake of what might have been the biggest cosmic explosion since the Big Bang, matter and antimatter collided at nearly the speed of light, annihilating each other and releasing their energy back into the universe, a new study from China has suggested.
By analysing data from Chinese and US space telescopes, a team led by researchers from the Chinese Academy of Sciences (CAS) identified a spectral line with energy levels peaking at 37 million electron volts (MeV) during an exceptionally powerful gamma-ray burst, now known as the Brightest Of All Time, or BOAT.
As the BOAT comprised various high-energy particles, including electrons and their antimatter counterparts, positrons, it is likely these particles underwent annihilation, released gamma-ray photons and led to the observed spectral line, they reported in the journal, Science China: Physics, Mechanics & Astronomy, last week.
“Our findings have important and unique value for studying the physical properties and production mechanisms of gamma-ray bursts,” Xiong Shaolin, the paper’s lead author from the Institute of High Energy Physics, told Chinese media on June 26.
The 37 MeV line energy is “the highest detected from any gamma-ray bursts – and any object – so far,” said Bing Zhang from the University of Nevada, Las Vegas, who is not involved in the research.
It means that the gamma-ray burst ejecta was moving at least 99.98 per cent the speed of light, he told the South China Morning Post on Tuesday.
Scientists now know that the BOAT was caused by the death of a massive star in the direction of the constellation Sagitta more than two billion years ago. The star ran out of fuel and collapsed into a black hole, spewing out a pair of jets into space.
When one of the jets reached Earth in October 2022, it was first spotted by a number of space telescopes, including Nasa’s Fermi Space Telescope and China’s GECAM-C (Gravitational Wave High-energy Electromagnetic Counterpart All-sky Monitor).
In fact, the burst was so bright that it blinded the detectors of some telescopes and left them with completely white pixels, astronomers found later.
Thanks to GECAM-C’s design and a special operational mode setting, it managed to record high-resolution, unsaturated data, and provide a uniquely accurate measurement of this once-in-10,000-years event, according to the Chinese team.
Last year, a team from Italy first discovered a gamma-ray spectral line from the Fermi data, with a starting time about five minutes after the detection of the burst, and an initial energy of about 12 MeV which decreased with time, Zhang said.
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The GECAM team processed the Fermi data independently, and jointly analysed it with the GECAM-C data. This allowed the researchers not only to confirm the line in the time range claimed by the Italian team, but also to recover a line at an earlier time (four minutes after the detection) and a higher energy at 37 MeV, he said.
“The combined analysis allowed the Chinese team to recover the line emission with a higher significance over a wider range of timescale,” Zhang said.
While China’s Large High Altitude Air Shower Observatory on the Tibetan Plateau had recorded gamma ray photons as energetic as more than 10 tera-electron volts, individual photons are different from a spectral line, Zhang explained.
“A line needs to accumulate a large enough number of photons around a particular energy, such as mostly around 37 MeV. Photons above or below the energy are not as impressive because they are easier to make with known radiation mechanisms,” he said.
Notably, only specific mechanisms can make photons at a particular energy, Zhang said. In this case, the line identified by both teams was caused by the annihilation of electron and positron pairs, a process which is supposed to produce 0.511 MeV lines.
Therefore, the measured 37 MeV line can help scientists to measure the so-called Doppler effect, or the change in the wavelength of light that occurs when an object is moving towards or away from the observer.
A Doppler factor of 72 (37/0.511) means that the gamma-ray burst ejecta was moving at least 99.98 per cent the speed of light towards Earth, Zhang said.
