Prompt optical emission of a gamma-ray burst detected by an array of ground wide angle cameras.

Researchers led by Dr. Shen Liping of the Space Variable Object Observation (SVOM) Research Team and National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) detected the instantaneous optical emission and its early stage transmission. Afterglow of a gamma-ray burst (GRB 201223A), using the Ground-based Wide-Angle Camera (GWAC) array at NAOC’s Xinglong Observatory. The study has been published in natural astronomy On April 10th.

Gamma ray bursts (GRBs) are caused by the collapse of massive stars or the merger of binary neutron stars. They are accompanied by intense relativistic jets that emit huge amounts of energy within a few seconds of bursts. This phenomenon includes the immediate emission caused by the shock in the jet and the afterglow caused by the interaction between the jet and the external medium.

A typical high-energy emission lasts from a few milliseconds to tens of seconds, and is difficult to follow in real time when ground-based optical telescopes receive alerts triggered by high-powered space instruments. To date, only a few cases of optical emission have been detected before the end of instantaneous high-energy emission. These GRBs have a longer duration of high-energy emission (>30 s). Moreover, all of these measurements were contaminated by a reverse shock, making it difficult to review the transition from immediate emission to afterglow.

GWAC, proposed and led by Professor Wei Jianyan, principal investigator of the SVOM mission, is one of the main ground-based telescopes for the SVOM project. It can cover a very large sky area with a time resolution of 15 seconds and a detection power of 16. Its scientific purpose is to systematically research the instantaneous optical emission of GRBs detected by the SVOM mission.

In this study, the GWAC recorded the entire process—before, during, and after the time the burst was fired. The duration of the high energy emission was 29 seconds. Optical emission and gamma-ray emission were detected simultaneously.

“The instantaneous optical emission is about four orders of magnitude brighter than would be expected if only gamma-ray emissions were analysed, which requires a special physical interpretation of these measurements,” said Dr Shen.

According to a joint analysis using follow-up observations by F60A, an optical telescope operated jointly by NAOC and Guangxi University, the full transition from immediate optical emission to afterglow was achieved without any contamination from the reverse shock.

The very early unique data provided by the GWAC place a subtle constraint on progenitor characteristics. Scientists expect strong stellar winds around a massive star, which is thought to be the perfect precursor to a gamma-ray burst. However, the stellar winds are very small for this event, even very close to the explosion, indicating that the progenitor had a small stellar mass.

After the SVOM is launched, simultaneous observations by the space-based GWAC and SVOM instruments will have the potential to provide baseline data for GRB studies, and finally a large sample build with real-time optical feedback will be done during the SVOM mission.