Illustration of the Insight-HXMT and GECAM-C observations of the brightest gamma-ray burst (GRB 221009A) [IMAGE: IHEP/HXMT & GECAM TEAM]

At two a.m. on March 29, 2023 (Beijing Time), the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences (CAS), together with some 40 research institutions worldwide, released their latest discoveries on the brightest Gamma-Ray Burst (dubbed GRB 221009A) ever detected. With the unique observations made by two Chinese space telescopes, namely Insight-HXMT and GECAM-C, scientists were able to accurately measure the brightness and level of energy released by this burst, which is the key to understanding it.

For this historic gamma-ray burst, another ground-based facility led by IHEP, namely the Large High Altitude Air Shower Observatory (LHAASO), carried out joint observation with Insight-HXMT and the GECAM-C telescopes. LHAASO made several important first discoveries with its large amount of very high energy observation data, which will be reported later.

As the most energetic explosion phenomenon in the universe, GRBs could be produced by the core collapse of a massive star, which typically lasts longer than two seconds, or the merger of two compact stars, such as neutron stars and black holes, which typically lasts less than two seconds and usually emits gravitational waves. GRB 221009A belongs to the former type of event.

Illustration of the brightest GRB 221009A [IMAGE: IHEP/HXMT & GECAM TEAM]

Using the Insight-HXMT and GECAM-C space telescopes, an IHEP-led international team with researchers from over 30 institutes from China, the USA, Italy, France and Germany, made accurate measurements of the prompt emission and early afterglow of this unprecedented burst in the hard X-ray and soft gamma energy bands.

When this GRB arrived, GECAM-C operated in a special observation mode which is capable of recording extremely bright GRBs such as the brightest part of GRB 221009A, while many other space telescopes would not be able to measure such a bright event.

“Based on the accurate data obtained by GECAM-C, we found that this burst set new records for both the observed brightness and the isotropic-equivalent energy of all detected bursts, making this burst really exceptional,” said Professor Xiong Shaolin, the Principal Investigator of GECAM-C and the leader of this study. “This burst is 50 times brighter than the last record-holder, and the isotropic-equivalent energy of this burst is more than 10^55 erg, which is about the total rest mass energy of eight solar masses. Such a huge energy was released within about one minute in time.”

Insight-HXMT is China’s first X-ray space telescope which is designed to observe the X-ray sources in the Milky Way. However, it was extended to monitor all-sky gamma-rays, especially GRBs. “Insight-HXMT made a clear measurement of GRB 221009A for its precursor and early afterglow, thanks to its very large detection area in the MeV energy range,” said Professor Zhang Shuangnan, the Principal Investigator of Insight-HXMT of IHEP. “Luckily we have Insight-HXMT and GECAM-C complementary to each other, so that we can have a full picture of this precious burst,” he added.

According to the joint observation by Insight-HXMT and GECAM-C, the early afterglow of GRB 221009A appeared to switch from slow decay to rapid decay very early in time, meaning that this burst launched an extremely narrow and luminous jet directly pointing towards us.

“These findings shed new light on the physics of these energetic explosions in the universe,” said Professor Zhang Bing, of the University of Nevada, Las Vegas, who led the theoretical interpretation in this work. “More detailed modeling is needed to understand how GRB engines manage to collimate the jets to produce such a huge isotropic equivalent energy in some cases.”

Source: Institute of High Energy Physics,

Chinese Academy of Sciences