Gamma-Ray burst offers new insights into cosmic explosions 

A new study conducted by researchers in China found what may be the most significant cosmic explosion since the Big Bang, a collision between matter and antimatter occurred at nearly the speed of light, resulting in mutual annihilation and the release of immense energy back into the universe. 

Cosmic explosions are among the most dramatic events in the universe, often signaling the death of stars or the merging of stellar remnants. These phenomena release an enormous amount of energy and can have far-reaching effects on their surroundings. 

By examining data from both Chinese and American space telescopes, a team led by scientists from the Chinese Academy of Sciences (CAS) identified a spectral line peaking at an energy level of 37 million electron volts (MeV) during a particularly powerful gamma-ray burst, now dubbed the Brightest Of All Time (BOAT). 

This gamma-ray burst included high-energy particles such as electrons and positrons (the antimatter counterparts of electrons), which likely underwent annihilation, releasing gamma-ray photons and creating the observed spectral line. The findings were published last week in the journal Science China: Physics, Mechanics & Astronomy.

Gamma-ray bursts are the most energetic events observed in the universe, capable of releasing more energy in a few seconds than the Sun will emit in its entire lifetime. Associated with the collapse of massive stars into black holes. There’s two types of Gamma-ray bursts (GRBs) – long and short. Long GRBs often occur in star-forming regions and are typically followed by a supernova explosion. Short GRBs are believed to result from the merger of neutron stars or a neutron star with a black hole.

These events are less common but incredibly powerful, producing intense gamma radiation. The Big Bang Theory is a cornerstone of modern cosmology and explains the origin and evolution of the universe. It posits that the universe began as a singularity approximately 13.8 billion years ago and has been expanding ever since. The theory has profound implications for our understanding of the universe, its structure, and its future. 

Scientists now understand that the BOAT was caused by the death of a massive star in the direction of the constellation Sagitta over two billion years ago. As the star exhausted its fuel, it collapsed into a black hole, emitting jets of material into space. When one of these jets reached Earth in October 2022, it was detected by several space telescopes, including NASA’s Fermi Space Telescope and China’s GECAM-C (Gravitational Wave High-energy Electromagnetic Counterpart All-sky Monitor).

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The burst was so intense that it overwhelmed the detectors of some telescopes, causing them to register completely white pixels. However, thanks to the GECAM-C’s unique design and special operational mode, it managed to capture high-resolution, unsaturated data, providing an exceptionally accurate measurement of this rare event, which occurs approximately once every 10,000 years.

In the previous year, an Italian team had discovered a gamma-ray spectral line from Fermi data, which appeared about five minutes after the burst was detected, starting at an energy of about 12 MeV and decreasing over time. The GECAM team processed the Fermi data independently and conducted a joint analysis with GECAM-C data. This allowed them to confirm the spectral line within the timeframe identified by the Italian team and to discover an earlier line (four minutes post-detection) at a higher energy level of 37 MeV.

The combined analysis allowed the Chinese team to recover the line emission with higher significance over a wider range of timescales. Although China’s Large High Altitude Air Shower Observatory on the Tibetan Plateau has recorded gamma-ray photons with energies exceeding 10 tera-electron volts, these individual photons differ from a spectral line. A spectral line requires the accumulation of numerous photons around a specific energy, such as the 37 MeV identified in this study.

Photons at energies above or below this are less remarkable because they can be produced by known radiation mechanisms. Only specific mechanisms can generate photons at particular energy levels, Zhang elaborated. In this case, the 37 MeV line observed by both teams was produced by the annihilation of electron-positron pairs, typically expected to generate 0.511 MeV lines.

The higher energy measurement of 37 MeV allows scientists to gauge the Doppler effect, which describes the change in the wavelength of light as an object moves towards or away from the observer.A Doppler factor of 72 (37/0.511) indicates that the gamma-ray burst ejecta was moving at least 99.98 percent of the speed of light towards Earth, Zhang concluded.

Cosmic explosions play a crucial role in the evolution of the universe. They distribute heavy elements necessary for the formation of planets and life, influence galaxy dynamics, and can trigger the formation of new stars. The study of these events helps astronomers understand fundamental processes in astrophysics and the lifecycle of celestial objects.

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