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 line emission in X-ray. 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 Yunnan Observatories utilize the down-Comptonization and find that it significantly decreases the intensity of the X-ray line emission in GRB and the significance of line detection. This result is published in The Astrophysical Journal recently.
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 the cold environment, since the average photon energy is larger than the electron thermal energy, down-Comptonization will take place, which results in the decreasing 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 research, Dr. LIU and Prof. MAO present the time evolution of the intensity and profile of the Fe line emitted near the GRB central engine. They compare the time for line becoming insignificant in the enviroment of different density.
They find that the emission line generally becomes insignificant enough to be detected after 100 seconds when it penetrates the environment with electron density above 1012cm-3 and temperature of 1 keV. After efficient scattering, the line-like profile deviates from the Gaussian form, and it finally changes 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 decreases very quickly in total, X-ray emission lines are sustained in a shorter time as deduced by researchers. Thus, it is even harder to detect X-ray emission lines.
This research has great scientific significance for 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.
Contact:
MAO Jirong, Yunnan Observatories, CAS
jirongmao@mail.ynao.ac.cn
