A study published in The Astrophysical Journal and by Prof. MAO Jirong from Yunnan Observatories of the Chinese Academy of Sciences and international collaborators showed that the magnetic field of the gamma-ray burst (GRB) can be effectively measured by the high-resolution spectral observation or the spectropolarimetric observation of the GRB optical absorption lines.

GRB is the strongest explosion in the universe. The origin and the explosion mechanism are still under debate. It is difficult to measure magnetic field of high-energy celestial object in a direct way.

Polarization, the light propagating along a certain direction, is usually used to measure the magnetic field in the high-energy astrophysical field. Zeeman Effect, the triplet splitting of absorption lines in an optical spectrum, is also suggested to measure the magnetic field of the celestial objects in a direct way.

The South African Large Telescope (SLAT) is one of the largest optical/infrared telescopes in the world. It was used by Prof. MAO and the collaborators to take spectropolarimetric measurements of GRB 191221B.

The researchers investigated the polarization on the GRB optical absorption lines. They built the radiative transfer model of the GRB absorption lines, presented the polarization degree as a function of the magnetic field, and obtained the magnetic field strength by the spectropolarimetry.

Besides, the researchers used the Zeeman Effectis to observe the triplet structure in the optical absorption lines by the high-resolution spectrographs. They revealed the magnetic field properties of the GRB and the surrounding medium. High-resolution observation can be one vital issue for the future GRB research.

The results of this study are important for the magnetic field measurement in the high-energy astrophysics, and are helpful to explore the GRB origin and evolution.