A research group led by the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences (CAS) recently reported their discovery of the gamma-ray line up to 37 million electron-volts from an extremely bright gamma-ray burst, which represents the highest energy spectral line features emitted by celestial objects in the universe. In addition, they found that the gamma-ray line evolves as a power-law function of time, providing critical clues to unveil the mystery of the line. Their paper was published as a cover story in Science China Physics, Mechanics & Astronomy on July 25.

Gamma-ray bursts (GRBs) are the most energetic explosion phenomena in the universe, which can be produced by either the core collapse of a massive star or the collision of two compact stars, such as neutron stars and black holes. This discovery was made on the so-called brightest of all time GRB, named GRB 221009A, which belongs to the former category and arrived at the Earth on October 9th, 2022, and was observed by many telescopes all over the world.

Researchers from the IHEP of CAS, the Yunnan observatories of CAS, Hebei Normal University, and Guizhou Normal University have formed a collaboration team and executed a comprehensive analysis of the observation data from two space gamma-ray monitors named GECAM-C and Fermi/GBM. The CAS-funded GECAM-C provided an accurate measurement of the low energy band spectrum of this GRB, while the Fermi/GBM covered the high energy band where the gamma-ray line showed up.

In particular, this burst is so bright (too many gamma-ray photons in a short period of time) that the Fermi/GBM detector suffered data loss during the bright part of the burst, making reliable data analysis very difficult. Fortunately, GECAM-C did not experience such a problem thanks to its dedicated design of the instrument; thus, the GECAM-C data was used to correct the Fermi/GBM data.

After the very challenging analyses of the data issues, instrumental effects, and background modeling, the research group managed to derive the reliable spectra of this GRB and further identify a series of gamma-ray lines throughout the burst, including the surprising gamma-ray line with energy up to 37 million electron-volts detected during the bright part of the burst. Interestingly, they found the line energy varies as a power-law function of time, while the ratio of the line width to line energy remains almost constant. Such features provide solid proof of the reality and the GRB origin of these gamma-ray lines. These discoveries shed new and unique light on the physics of GRBs and their relativistic jets.

Paper Link: https://doi.org/10.1007/s11433-023-2381-0

Figure 1: Illustration of the observation of the brightest ever gamma-ray burst (GRB 221009A) by GECAM-C and Fermi/GBM, leading to the discovery of gamma-ray line up to 37 MeV. This work was published as a cover story in Science China Physics, Mechanics & Astronomy.

Figure 2: The top panel shows the power-law evolution of the energy of gamma-ray line in GRB 221009A, while the prompt emission and afterglow are plotted in the bottom panel.