A research team at the Chinese Academy of Sciences realized that gamma-ray bursts (GRB) offer a unique opportunity to search for gamma-ray lines in the spectrum, which are crucial but have never been detected in a GRB before, given its record-breaking brightness and rich observation data sets. The study has been published in Science China Physics, Mechanics & Astronomy.
Joint analysis of the GECAM-C and Fermi/GBM observation data on the brightest ever gamma-ray burst (GRB 221009A) reveals the discovery of a gamma-ray line up to 37 MeV, the highest-energy gamma-ray line seen from any celestial object in the universe. Image Credit: Art by Jingchuan Yu.
Professors Jirong Mao (Yunnan Observatories, Chinese Academy of Sciences), Shuang-Nan Zhang (Institute of High Energy Physics, Chinese Academy of Sciences), and Shaolin Xiong (Institute of High Energy Physics, Chinese Academy of Sciences) headed this research.
The most powerful explosion phenomena, gamma-ray bursts (GRBs) hold significant secrets about stars, galaxies, and the universe. On October 9, 2022, the brightest known GRB (designated GRB 221009A) ever observed swept the Earth; its intense brightness made it difficult for many gamma-ray telescopes to observe regularly. This was the first known GRB since its discovery in the 1960s.
Due to its specific design for bright events, GECAM-C, the third instrument in Shaolin Xiong's GECAM series, offered a precise and high-resolution measurement of the burst.
The research team executed a comprehensive spectral analysis of the joint observation data from two space gamma-ray monitors.
GECAM-C provided an accurate spectral measurement for the full course of this burst, while Fermi/GBM could extend the spectrum to a higher energy band. They together can give a very wide range of spectrum measurement and line search.
Shaolin Xiong, Study Lead, Institute of High Energy Physics, Chinese Academy of Sciences
“Soon after we started this analysis, we noticed that, above the continuum spectrum that is usually seen in GRBs, there are some interesting excess features in some time intervals of this burst. We thought they might be the grail we’ve been looking for,” said Yanqiu Zhang, Ph.D. Student and Study First Author, Institute of High Energy Physics, Chinese Academy of Sciences.
Shaolin Xiong said, “But we knew there are so many things to check out before we claim it, because many elusive factors, such as background subtraction, instrumental effects, and systematic errors in detector response, can lead to fake features in the spectrum for bright bursts; thus, we have to investigate all these issues as much as possible. Having two telescopes to cross-check each other helped a lot in this study.”
The research team found a series of emission lines over the typical GRB spectra. It was able to derive reliable spectra of this GRB after months of extremely difficult analyses of these issues. Surprisingly, the team also discovered that the flux and line energy change as a power law function of time.
Shaolin Xiong said, “Such a physical evolution cannot be produced by any factors we can think of and thus provides a solid proof of the reality and the GRB origin of these gamma-ray lines.”
Interestingly, during the bright portion of the burst, the team measured line energy of up to 37 million electron volts.
To our knowledge, a significant gamma-ray line with such high energy is never seen before; thus, we believe it is the highest-energy spectral line observed in the universe so far.
Shuang-Nan Zhang, Study Lead, Institute of High Energy Physics, Chinese Academy of Sciences
The team also discussed a few potential explanations for the observed gamma-ray line features in this work, such as the nearly constant ratio of line width to line energy and the power-law evolution of line energy and flux.
“These discoveries shed new and unique light on the physics of GRB and its relativistic jet. More theoretical studies are required to fully understand the observed gamma-ray line,” said Jirong Mao.
Journal Reference:
Zhang, Y., Observation of spectral lines in the exceptional GRB 221009A. Science China Physics, Mechanics & Astronomy.doi.org/10.1007/s11433-023-2381-0