Bizarre long gamma-ray burst came from merging stellar corpses

Scientists have tracked a high-energy blast of radiation back to the collision of two neutron stars and the kilonova explosion that resulted from the violent merger.

The discovery could change theories regarding the origins of the universe's most powerful explosions.

"This event represents an exciting paradigm shift for gamma-ray-burst astronomy," Jillian Rastinejad, a doctoral candidate in the Northwestern University Department of Physics and Astronomy who led the research, said in a statement.

Related: Possible massive 'kilonova' explosion creates an epic afterglow

An artist's depiction of merging neutron stars producing a kilonova. (Image credit: Dreamstime)

The powerful, 50-second blast of high-energy radiation called a long gamma-ray burst (GRB) was detected in December 2021 from a source 1.1 billion light-years away, prompting astrophysicists to search for its low-energy afterglow.

The incredibly luminous but rapidly fading burst of light of a long-GRB afterglow often portends a supernova,  a powerful explosion triggered when a massive star dies. But in the case of this GRB, named GRB 211211A, the team found that the afterglow was followed by a kilonova, a rare cosmic explosion that was thought to happen only when a neutron star, the dense remnants of an exploded star, merges with another neutron star or a black hole.

The discovery of this chain of events could overturn the theory that long GRBs are created solely by the collapse of massive stars at the ends of their nuclear-fuel-burning lives. 

In addition, because the merger of neutron stars is suspected to forge the universe's heavier elements, such as gold, the discovery could help reveal how and where heavy metals are forged.

"This event looks unlike anything else we have seen before from a long gamma-ray burst," Rastinejad said. "Its gamma rays resemble those of bursts produced by the collapse of massive stars. Given that all other confirmed neutron star mergers we have observed have been accompanied by bursts lasting less than two seconds, we had every reason to expect this 50-second GRB was created by the collapse of a massive star."

But that turned out not to be the cause this time.

"Instead, what we found was very different," study senior author Wen-fai Fong, an assistant professor in the Northwestern University Department of Physics and Astronomy, said in the statement. "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."

An illustration of a kilonova and a gamma-ray burst, with blue representing squeezed material and red indicating material ejected by the two neutron stars swirling around the merged object they created.   (Image credit: Aaron M. Geller/Northwestern/CIERA and IT Research Computing Services)

Long gamma-ray burst points to short kilonova 

Considered the universe's brightest and most energetic explosions, GRBs are traditionally divided into two classes: Those that last less than two seconds are considered short GRBs, and those that last longer are classified as long GRBs. 

Short GRBs have previously been associated with neutron star mergers, but these mergers had been ruled out as the origin of long GRBs, simply because these densely packed stellar remnants, which have masses around that of the sun or slightly greater, were considered to possess too little material to power such bursts.

Thus, scientists believed that these bursts of energy on each side of the two-second dividing line must have separate origins.

The collapse of giant stars was posited as a cause of long GRBs, as these huge stars can have masses equivalent to tens, or even hundreds, of suns. As these stars exhaust fuel for nuclear fusion, the outward pressure balancing against the inward pressure of gravity ceases. This causes a vast amount of this material to rush inward to create and feed a newborn black hole and the violent event marked by a supernova.

The remaining material is grabbed by the magnetic field of this black hole and is launched out into space at near light speed, thus powering a long GRB.

"When you put two neutron stars together, there's not really much mass there," Fong said. "A little bit of mass accretes and then powers a very short-duration burst. In the case of massive star collapses, which traditionally power longer gamma-ray bursts, there is a longer feeding time."

At first, researchers hadn't suspected anything unusual about the 50-second GRB 211211A or anything that could change these origin theories. At 1.1 billion light-years from Earth, the long GRB was relatively close for such an event, allowing the team to study it with a range of telescopes and across various wavelengths of light. 

As they did this, they found a faint object that faded quickly in near-infrared images. As supernovas don't fade rapidly and are much brighter than this object, the astronomers realized they had spotted something unexpected.

"There are a lot of objects in our night sky that fade quickly," Fong said. "We image a source in different filters to obtain color information, which helps us determine the source's identity."

In this case, the red 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," Fong said. 

Additional research published in a second paper used modeling to analyze the event and concurred that the signal matched a kilonova, according to a statement.

The wrong galaxy 

The fact that the long GRB seems to have been triggered by a kilonova from a neutron star merger isn't the only unusual thing about GRB 211211A; prior knowledge of such events suggests it's in the wrong type of galaxy. 

The high-energy blast was traced to a young and relatively small star-forming galaxy named SDSS J140910.47+275320.8. The properties of this galaxy are almost the complete opposite of the characteristics of the only other known neutron star merger-hosting galaxy in the local universe: NGC4993, a massive "red-and-dead" host galaxy.

"This galaxy is fairly young, actively star-forming, and not actually that massive," study co-author Anya Nugent, a graduate student at Northwestern, said of SDSS J140910.47+275320.8. "In fact, it looks more similar to short GRB hosts seen deeper in the universe."

Now that astronomers have a better idea of what to look for, Nugent thinks the findings should change the types of galaxies astronomers watch when searching for nearby kilonovas. 

"Kilonovae are powered by the radioactive decay of some of the heaviest elements in the universe," Rastinejad added. "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 discovery also could change how astronomers hunt for heavy elements, like platinum and gold, for which clear signs of creation are currently elusive. Modeling work suggested that an event like this one would have produced vast quantities of heavy elements.

"We found that this one event produced about 1,000 times the mass of the Earth in very heavy elements," Matt Nicholl, an astrophysicist at the University of Birmingham in the U.K. said in a statement. "This supports the idea that these kilonovae are the main factories of gold in the universe."

The James Webb Space Telescope (Webb or JWST), which started beaming images back to Earth in July, could assist in the hunt to find such signatures in the explosion's aftermath.

JWST can capture the spectra of distant astronomical objects containing the fingerprints of different elements. As such, astronomers using the space telescope could conclusively identify the creation sites of heavy elements — a task that has proved too challenging for even the most sophisticated Earth-based telescopes. 

"Unfortunately, even the best ground-based telescopes are not sensitive enough to perform spectroscopy," Rastinejad said. "With the JWST, we could have obtained a spectrum of the kilonova. Those spectral lines provide direct evidence that you have detected the heaviest elements."

The research is described in two papers published Wednesday (Dec. 7) in the journal Nature.

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Robert Lea
Senior Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.