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An unusual bright blast of light detected by multiple telescopes in December 2021 was the result of a rare cosmic explosion that created a wealth of heavy elements such as gold and platinum.
The gamma-ray burst, called GRB 211211A, lasted about a minute. Gamma-ray bursts are considered among the strongest and brightest explosions in the universe, and they can range from a few milliseconds to several hours in length.
The duration of the burst hinted it was caused by the explosion of a massive star as it died in a supernova. But the aftermath of the gamma-ray burst was faint and faded more quickly than those created by supernovas, and astronomers analyzing the event also spied an excess of infrared light.
“There are a lot of objects in our night sky that fade quickly,” said Wen-fai Fong, assistant professor of physics and astronomy at Northwestern University’s Weinberg College of Arts and Sciences and senior author and coauthor of one of four studies published about the event Wednesday in the journal Nature Astronomy.
“We image a source in different filters to obtain color information, which helps us determine the source’s identity. In this case, red color prevailed, and bluer colors faded more quickly. This color evolution is a telltale signature of a kilonova, and kilonovae can only come from neutron star mergers.”
Kilonovas are rare, massive explosions caused by the catastrophic collisions between neutron stars, which are the incredibly dense remnants of exploded stars, or collisions between neutron stars and black holes.
After determining that a kilonova created the infrared light, astronomers grew even more puzzled by the gamma-ray burst’s duration. Gamma-ray bursts caused by these rare explosions have only ever been observed to last less than two seconds, but this signal lasted for at least one minute.
“When we followed this long gamma-ray burst, we expected it would lead to evidence of a massive star collapse,” Fong said. “Instead, what we found was very different. When I entered the field 15 years ago, it was set in stone that long gamma-ray bursts come from massive star collapses. This unexpected finding not only represents a major shift in our understanding, but also excitingly opens up a new window for discovery.”
Neutron stars are compact cosmic objects, so researchers never expected them to contain enough material to create a gamma-ray burst that could last nearly a minute.
The explosion occurred in a galaxy about 1 billion light-years away from Earth. Since the event happened relatively close, astronomically speaking, astronomers used multiple telescopes to glean unprecedented detail.
“We found that this one event produced about 1,000 times the mass of the Earth in very heavy elements. This supports the idea that these kilonovae are the main factories of gold in the Universe,” said Dr. Matt Nicholl, an associate professor at the University of Birmingham in the UK and coauthor of one of the Nature Astronomy studies, in a statement.
The newly observed characteristics of this event are changing the way astronomers understand gamma-ray bursts, or GRBs.
“Such a peculiar GRB was the first of its kind ever detected,” said Bing Zhang, an astrophysics professor at the University of Nevada, Las Vegas, and coauthor of one of the Nature Astronomy studies, in a statement. “This discovery not only challenged our understanding of GRB origins, (but) it also requires us to consider a new model for how some GRBs form.”
Zhang’s team believes that the unique nature of the burst could have resulted from a likely collision between a neutron star and a white dwarf, or the Earth-size remnant that emerges when low-mass stars die.
The event has also helped answer some questions around the creation of the heaviest elements in the universe.
“Kilonovae are powered by the radioactive decay of some of the heaviest elements in the universe,” said Jillian Rastinejad, a doctoral student in astronomy at Northwestern and first author of one of the Nature Astronomy studies. “But kilonovae are very hard to observe and fade very quickly. Now, we know we can also use some long gamma-ray bursts to look for more kilonovae.”
The James Webb Space Telescope will enable astronomers to search for the emissions released by kilonovae using spectroscopy, or measuring different wavelengths of light.
“Unfortunately, even the best ground-based telescopes are not sensitive enough to perform spectroscopy,” Rastinejad said. “With the (Webb telescope), we could have obtained a spectrum of the kilonova. Those spectral lines provide direct evidence that you have detected the heaviest elements.”