A novel model of the origin of gamma-ray bursts has been proposed by scientists at NJU. The results appeared in the journal Nature on December 7, 2022.
GRBs, also known as gamma-ray bursts, are bursts of high-energy emission that occur in distant galaxies. A gamma-ray burst releases the same amount of energy in one second as the sun releases in approximately five billion years. Nearly 10,000 gamma-ray bursts have been observed by astronomers since 1967, and their durations range from milliseconds to thousands of seconds. Gamma-ray bursts have generally been divided into two categories over the last few decades: short bursts (lasting less than two seconds) and long bursts (lasting two seconds or longer). Long-term research has focused on the physical origins of short and long bursts. Now, the widely accepted GRB origin model holds that the long bursts originate from the collapse of massive stars associated with supernovae, while the short bursts originate from the merger of binary compact stars associated with kilonovae (i.e., neutron star-neutron star or neutron star-black hole merger).
Recently, an oddball GRB prompted a revision of the theoretical framework described above. The GRB, known as GRB 211211A, was observed on December 11, 2021, coming from a galaxy located one billion light-years away from the Milky Way. This bright gamma-ray burst lasted for more than a minute. Professor Bin-Bin Zhang at the School of Astronomy and Space Science of Nanjing University led a research team that conducted a detailed analysis of the data of the gamma-ray burst GRB 211211A.
“This GRB includes two parts: a 13-second long hard spike and a 55-second soft extended emission,” said NJU professor and study corresponding author Bin-Bin Zhang. “The duration of the 13-second sharp spike should have completely excluded this burst from the short GRB category”. However, the other properties of this GRB are all inconsistent with the characteristics of typical long GRBs. Furthermore, the team found evidence of the existence of kilonovae through multi-band optical observation data. This is the first time a kilonova has been discovered in a long GRB. This confirms that the long GRB has a physical origin of binary compact star mergers. This is difficult to explain with existing GRB origin models.
“Such a peculiar GRB was the first of its kind ever detected,” said UNLV astrophysics professor Bing Zhang, co-corresponding author of the Nature paper. “This discovery not only challenges our understanding of GRB origins, but it also requires us to consider a novel model for how some GRBs form.”
The team further proposed that one of the double compact stars that produced this gamma-ray burst should be a white dwarf and a neutron star, thus forming a "white dwarf-neutron star". Massive, low-density stars make long-duration GRBs whereas high-density stars, including neutron stars, make short-duration GRBs. According to the study, white dwarfs have intermediate densities, which make them ideal origins for the type of GRB discovered in 2021 as it displays an intermediately long duration without involving a massive star.
Artist illustration of GRB 211211A , Credit : Hanyu Lei & Jing Chen, School of Arts, Nanjing University.
About the study
The study, “A long-duration gamma-ray burst with a peculiar origin”, appeared on Dec. 7 in the journal Nature. The paper includes 10 co-authors from 4 institutions, with Nanjing University and UNLV being the lead institutions. Published in the same issue are three parallel papers that report the detection of a kilonova. This paper focuses on the peculiar gamma-ray emission and proposes the WD-NS merger model to interpret the data.
Related paper: https://www.nature.com/articles/s41586-022-05403-8