Gamma-ray burst (GRB) is the violent explosion in the universe. It is extragalactic emission and its redshift can be constrained by the detection of the X-ray emission line.

The detection of the GRB X-ray emission line started at the end of last century. Even though some observations on the X-ray line-like feature were performed, the significance remains controversial to date. What are the physical issues that make the emission line only marginally detected or even not observed?

Dr. LIU Jieying and Prof. MAO Jirong from the Yunnan Observatories of the Chinese Academy of Sciences utilized the down-Comptonization mechanism and found that it significantly decreased the intensity of the X-ray emission line in GRB and the significance of line detection.

The study was published in The Astrophysical Journal on August 27.

Comptonization is the energy transfer process by the scattering between the photons and plasma. This changes the strength and shape of the radiation spectrum, and also causes changes to the plasma temperature. When the photons go through cold environment, down-Comptonization will take place, resulting in decrease of intensity and deviation of the Gaussian profile of the initial emission line.

According to the effect of down-Comptonization, the intensity of spectrum decreases more quickly in denser environment. Since Swift-XRT can perform a normal observation in the X-ray band begins 50-100 seconds after a GRB trigger, the nondetection of emission line might imply the density of the environment of GRB is dense.

In this study, Dr. LIU and Prof. MAO presented the time evolution of the intensity and profile of the Fe line emitted near the GRB central engine. They compared the time for line becoming insignificant in the enviroment of different density.

They found that the emission line generally became insignificant enough to be detected after 100 seconds when it penetrated the environment with electron density above 10¹²cm^-3 and temperature of 1 keV. After efficient scattering, the line-like profile deviated from the Gaussian form, and it finally changed to be similar to a blackbody shape at the time of the thermal equilibrium between the line photons and the surrounding material.

In addition, when the GRB X-ray flux decreased very quickly in total, X-ray emission lines were sustained in a shorter time as deduced by researchers. Thus, it was even harder to detect X-ray emission lines.

This research sheds light on the X-ray emission line detection of GRBs by high-energy satellites being developed in China and the world, and can promote the development of instrument in high-energy satellite for X-ray detection.