Dr. LIU Jieying and Prof. MAO Jirong from Yunnan Observatories of the Chinese Academy of Sciences reported a new finding in The Astrophysical Journal. By theoretically analysing the origin of the thermal X-ray emission of ultral-long gamma-ray burst (GRB) source- GRB 130925A, they proposed that bremsstrahlung radiation in a metal-rich and dusty environment can be one possible mechanism to explain the thermal component above 10 keV in X-ray afterglow.

Gamma-ray burst is the most violent phenomenon in the universe. The emission lasts from a few milliseconds to kiloseconds. The emission during the burst phase is called prompt emission. Following the burst, the emission called afterglow lasts long time in the low-energy band such as X-ray, optical, and radio band. X-ray afterglow spectrum is commonly non-thermal power-law, which comes from the synchrotron emission of the relativistic electrons.

However, the thermal emission is also observed in some X-ray afterglows. Though the black body radiation is suggested to the component, the afterglow is hardly produced in the optical-thick region.

In this study, scientists theoretically analysed the X-ray thermal component of GRB 130925A through the bremsstrahlung. GRB 130925A is an ultra-long burst, with the first episode at a duration of about 900s and the second episode at the time of 2-3 ks after the trigger.

In order to explain the long duration, blue/red supergiant (BSG) as its progenitor has been proposed. This means that the radiation region of the afterglow can be at very large fireball radius. Besides, the host galaxy has supersolar metallicity and large extinction in the line-of-sight. This implies that the surrounding medium of GRB 130925A may provide abundant ionized particles.

Scientists analysed the X-ray data of GRB 130925A observed by Swift-XRT and NuSTAR in 1.8 days after the Swift-BAT trigger. They proposed that the internal shock accelerates the wind mediums at about 10¹⁸ cm. This results in the increase of temperature and efficient ionization.

They then liberated that the matter in the wind is equilibrium by bremsstrahlung cooling and X-ray. Compared with the observational data, they obtained the number density and electron temperature in the surrounding wind.

This study implied that NuSTAR and other similar X-ray telescopes can perform more ultra-long GRB observations to further reveal the physics on the GRB progenitor and the GRB environment.