Newswise —
Dr. XIN Liping from the Space-based Multi-band Astronomical Variable Objects Monitor (SVOM) research team, National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), and their team have used the Ground Wide Angle Camera Array (GWAC) located at Xinglong Observatory of NAOC to detect the prompt optical emission and its transition to the early afterglow of a gamma-ray burst (GRB 201223A).
The study was published in Nature Astronomy on April 10.
Gamma-ray bursts (GRBs) happen when massive stars collapse or binary neutron stars merge. They release powerful relativistic jets that emit a huge amount of energy in just a few seconds. There are two phases to the phenomenon: the prompt emission caused by the shock within the jet, and the afterglow caused by the interaction between the jet and the surrounding external medium.
It's hard to study gamma-ray bursts (GRBs) in real-time because they last only a few milliseconds to tens of seconds, and it's challenging to follow them up with ground-based optical telescopes. So far, only a few cases of optical emission have been detected before the end of prompt high-energy emission, and these GRBs have longer duration of high-energy emission (>30 seconds). However, these measurements were all affected by reverse shock, which made it difficult to clearly observe the transition from prompt emission to afterglow.
GWAC is a ground-based telescope proposed and led by Prof. WEI Jianyan, who is the principal investigator of the SVOM mission. It has a large coverage area of the sky and can detect objects with a magnitude of up to 16. It has a high temporal resolution of 15 seconds and is used to study the prompt optical emission of gamma-ray bursts discovered by the SVOM mission.
The researchers used the GWAC telescope to observe the entire process of the gamma-ray burst, including before, during, and after it occurred. The high-energy emission lasted for 29 seconds, and the telescope detected the optical and gamma-ray emissions at the same time.
"The prompt optical emission is far brighter than expected by about four orders of magnitude, if only gamma-ray emission is analyzed, which requires a special physical interpretation for these measurements," said by Dr. XIN.
The researchers were able to clearly observe the transition from the prompt optical emission to the afterglow, without any interference from reverse shock, by analyzing the follow-up observations made by F60A, an optical telescope operated by NAOC and Guangxi University.
The data obtained by GWAC helped researchers determine the characteristics of the progenitor, which is the source of the gamma-ray burst. Massive stars with strong stellar winds are typically considered to be the ideal progenitors of gamma-ray bursts. However, the data obtained by GWAC showed that the stellar wind around the progenitor was quite small, even at a very close distance from the burst. This suggests that the progenitor may have had a smaller stellar mass than previously thought.
After the launch of SVOM, simultaneous observations by GWAC and SVOM space-based instruments will have the potential to provide essential data for GRB studies, and finally a large sample with prompt optical observations will be built during SVOM mission.
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