Astronomers discover heavy elements after bright gamma-ray burst from neutron star merger

The massive explosion unleashed a gamma-ray burst, GRB230307A, the second brightest in 50 years of observations and about 1,000 times brighter than a typical gamma-ray burst. GRB230307A was first detected by NASA's Fermi Gamma-Ray Space Telescope on March 7, 2023.

Using multiple space- and ground-based telescopes, including NASA's James Webb Space Telescope, the largest and most powerful telescope ever launched into space, scientists were able to pinpoint the source of the gamma-ray burst in the sky and track how its brightness changed.

With the information gathered, the researchers determined the burst was the result of two neutron stars that merged in a galaxy 1 billion light-years from Earth to form a kilonova. The researchers observed evidence of tellurium, one of the rarest elements on Earth.

The breakthrough discovery puts astronomers one step closer to solving the mystery of the origin of elements that are heavier than iron.

"I'm a high energy astrophysicist. I like explosions. I like the gamma rays that come from them. But I'm also an astronomer who really cares about fundamental questions like how did heavy elements form," Hartmann said.

Gamma-ray bursts (GRBs) are bursts of gamma-ray light -- the most energetic form of light -- that last anywhere from seconds to minutes. The first GRBs were detected in the 1960s by satellites built to monitor nuclear testing.

GRBs have different causes.

Long duration GRBs are caused by supernovas, the point when a massive star reaches the end of its life and explodes into a burst of light. Short duration GRBs are caused by the merger of two neutron stars, known as a kilonova, or the merger of a neutron star and a black hole.

Although GRB230307A lasted for 200 seconds, scientists saw the afterglow color change from blue to red, a signature of kilonova.

"The burst itself actually indicated a long duration event, and it should have been a normal supernova-type situation. But it had unusual features. It didn't quite fit the patterns of long bursts," Hartmann said. "It turns out that this radioactive cloud, that kilonova afterglow, which had all these nuclear synthetic fingerprints in it, is the signature of a binary merger. The excitement comes from using the Webb to identify a chemical fingerprint that we had expected for short bursts and seeing it inside a long burst."

Hartmann said the Big Bang produced hydrogen and helium. All other elements were made by stars and processes in the interstellar medium.

"Some of them are massive enough to explode and they return that material to their gaseous environments which later make new stars. So, there's a cycle in the universe that makes us more enriched in carbon, nitrogen, oxygen, all the things we need," he said. "We call stars the cauldrons of the universe."

Thermonuclear reactions, or fusion, make stars shine. That leads successively to the production of more heavy elements, Hartmann said. But when it gets to iron, there isn't much energy left to squeeze out, he said.

So, where do all the heavy elements such as gold and uranium come from?

"The heavy elements have special origins. There are two processes that dominate. One is called rapid; the other is called slow. We believe the r-process happens in those neutron star mergers," Hartmann said.

Theoretical modeling suggested kilonovas should produce tellurium, but the detection of a spectral line by the James Webb Space Telescope provided experimental evidence. A spectral line is a dark or bright line within a continuous spectrum. It is produced by transitions within atoms or ions.

"We think it's a pretty secure identification, but it's not beyond a reasonable doubt like they would say in court," Hartmann said.

In addition to Hartmann, researchers from several universities in the United States as well as scientists from the Netherlands, the United Kingdom, Italy, Japan, Denmark, Spain, Sweden, Australia, Ireland, France, New Zealand, Canada, Israel, Iceland, Czech Republic and Germany were involved.