Discovery of deep space: the Oddball gamma-ray burst forces a revision of the theoretical framework

The mysteries of the cosmos continue to amaze astronomers, and each new sighting is a chance to deepen – or shake up – our understanding of the universe.

In the December 7 issue of the journal Nature, an international team of astrophysicists reports the discovery of a unique cosmological gamma-ray burst (GRB) that challenges mainstream theories about the formation of violent cosmic explosions. This “quirky” burst led the team to propose a new model, or source, for certain types of GRB.

Gamma-ray bursts are the brightest and most violent explosions in the universe. They signify the death of stars or collisions of stellar remnants. Observed GRBs are generally classified into two categories: short-lived and long-lived GRBs. Long GRBs arise from the death of massive stars and are usually associated with bright optical transients called supernovae. Short GRBs have a duration of less than two seconds and arise from the collisions of two neutron stars or a neutron star and a black hole, and are usually associated with fainter optical transients called kilonovae.

For decades, GRBs have snuggled up nicely in these cozy categories. So far.

On December 11, 2021, a GRB triggered several gamma ray detectors in space, including NASA’s Fermi Gamma Ray Telescope and the Neil Gehrels Swift Observatory. This burst, lasting nearly 70 seconds, would generally be considered a normal long GRB. That is, until several teams from the United States and Europe made follow-up observations and discovered a startling signature.

“This GRB consists of two parts: a 13-second hard peak and a softer 55-second sustained emission,” said Bin-Bin Zhang, a former UNLV student and corresponding author of the study, who is currently working on the study. Chinese University of Nanjing. “The hard peak duration of 13 seconds should have completely excluded this burst from the short GRB category.”

In other words, instead of showing a much brighter supernova, as expected, the sighting was consistent with a kilonova which is more commonly associated with a short GRB.

“Such a peculiar GRB was the first of its kind ever detected,” said Bing Zhang, professor of astrophysics at UNLV, co-corresponding author of the study. Nature paper. “This finding has not only challenged our understanding of the origins of GRBs, but also compels us to consider a new model for how certain GRBs form.”

The research team believe this unique GRB, known as GRB 211211A, likely formed by a collision between a neutron star and a white dwarf, known as a WD-NS merger.

White dwarfs are Earth-sized objects that form as a result of the death of low-mass stars, those with a mass less than that of about eight of our Suns. Neutron stars form when more massive stars, those with masses between about 8 and 20 suns, die. When even larger stars die, they directly form black holes.

Low-density massive stars produce long-lived GRBs, while high-density stars, including neutron stars, produce short-lived GRBs. According to UNLV’s Zhang, white dwarfs have intermediate densities, making them ideal origins for the type of GRB discovered in 2021 because it displays a moderately long duration without involving a massive star.

“Despite the relatively large number of GRBs observed each year, the unique signature of GRB 211211A pushed the boundaries of our current categorical systems and required a new way of thinking,” Zhang said. “After careful consideration, the only merger scenario that made sense was a white dwarf and a neutron star.”

Shunke Ai, a PhD student at UNLV, and a student from Nanjing University collaborated to develop a detailed model to interpret the peculiar kilonova signature observed by GRB 211211A. Ai discovered that if a WD-NS merger leaves behind a rapidly rotating neutron star, known as a magnetar, the injection of additional energy from the magnetar combined with the nuclear reaction energy of the material ejected during the burst may explain the kilonova emission observed for GRB 211211A.

The study, “A long-lasting gamma-ray burst with a particular origin”, appeared on December 7 in the journal Nature. The article includes 10 co-authors from 4 institutions, with UNLV and Nanjing University being the lead institutions. Published in the same issue, three parallel articles report the detection of the kilonova. This article focuses on the particular gamma ray emission itself and proposes the WD-NS fusion model to interpret the data.