Astronomers used NASA's JWST and other telescopes to detect bright gamma-ray bursts produced by collisions of neutron stars, allowing the first direct observation of heavy metals such as tellurium in space. This discovery sheds light on the origin of heavy elements in the universe.

An unusual burst of high-energy light in the sky points astronomers to a pair of metal-forged neutron stars 900 million light-years away from Earth. Using multiple observatories, astronomers have directly detected tellurium in two merging neutron stars.

An international team of astronomers, including MIT scientists, report in a recent study published in the journal Nature that they have detected an extremely bright gamma-ray burst (GRB), the most powerful type of explosion known in the universe. This particular gamma-ray burst was the second brightest ever detected, and astronomers later traced its origin to two merging neutron stars. Neutron stars are the ultra-dense cores of collapsed massive stars and are thought to be the birthplace of many heavy metals in the universe.

Evidence of heavy metals in space

The team found that when these stars orbit each other and eventually merge, they release huge amounts of energy in the form of GRBs. Moreover, for the first time, astronomers have directly detected signs of heavy metals in the remains of stars. Specifically, they captured a clear signal of tellurium, a heavy metal that is slightly toxic and rarer than platinum on Earth but thought to be abundant throughout the universe.

In this artist's impression, two neutron stars begin to merge, ejecting jets of high-speed particles and creating clouds of debris. Source: A. Simonnet (Sonoma State University) and Goddard Space Flight Center

Astronomers estimate that the merger released enough tellurium to equal the mass of 300 Earths. If tellurium is present, the merger must have also stirred up other closely related elements, such as iodine, a mineral nutrient essential for most life on Earth.

global astronomy effort

The discovery was the result of a collaborative effort by astronomers around the world, using NASA's James Webb Space Telescope (JWST) and other ground-based and space-based telescopes, including NASA's TESS satellite (an MIT-led mission) and Chile's Very Large Telescope (VLT).

"This discovery is an important step forward in our understanding of where heavy elements formed in the universe, and it demonstrates the power of combining observations at different wavelengths to reveal new insights into these extremely energetic explosions," said study co-author Benjamin Schneider, a postdoctoral fellow at MIT's Kavli Institute for Astrophysics and Space Studies.

The research was co-led by Andrew Levan of Radboud University in the Netherlands and the University of Warwick in the UK.

On March 7, 2023, NASA's Fermi Gamma-ray Space Telescope detected the initial burst and determined that it was an unusually bright gamma-ray burst, which astronomers named GRB230307A.

Michael Fasnow was a research scientist at MIT at the time and is now an assistant professor at Texas Tech University. "In gamma-ray astronomy, you usually count individual photons. But with so many photons coming in, the detector can't resolve individual photons, and it's a bit like maxing out the meter."

A star field image of GRB230307A taken by JWST/NIRCam showing the associated kilonova and its host galaxy. Image source: NASA, ESA, CSA, STScI, AndrewLevan (IMAPP, Warw)

This super-bright burst was also particularly long, lasting 200 seconds, while neutron star mergers typically produce short GRBs that flash for less than two seconds. This bright and long-lasting flare immediately sparked worldwide interest, and astronomers trained scores of other telescopes on the burst. This time, the brightness of the gamma-ray burst worked in scientists' favor, as the gamma-ray flare was detected by satellites across the solar system. By triangulating these observations, astronomers pinpointed the location of the gamma-ray burst in the southern constellation Montenegro.

At MIT, Schneider and Fasnow joined a multi-pronged search. Shortly after Fermi first detected GRB230307A, Fausnaugh checked whether the outburst was present in data taken by the TESS satellite, which happened to be pointed at the same patch of sky where GRB230307A was first detected. Fausnaugh went back and looked at that portion of the TESS data, discovered the burst, and then tracked its activity from beginning to end.

"We can see everything at the same time," Fausnaugh said. "We saw a very bright flash, followed by a small bump, or afterglow. It's a very unique light curve. Without TESS, it would have been almost impossible to observe early optical flashes that occur at the same time as gamma rays."

Meanwhile, Schneider studied the burst using another ground-based telescope: the Very Large Telescope (VLT) in Chile. As a member of the large gamma-ray burst observation project operating on this telescope, Schneider happened to be on duty shortly after Fermi's first observation and pointed the telescope at the burst.

The VLT observations matched the TESS data and revealed an equally peculiar pattern: GRB emission appeared to transition rapidly from blue to red wavelengths. This pattern is characteristic of a kilonova - a big explosion that usually occurs when two neutron stars collide. The MIT team's analysis, combined with other observations from around the world, helped determine that this GRB is likely the product of the merger of two neutron stars.

Tracking neutron star mergers

Where did the merger itself come from? To do this, astronomers turn to JWST's deep field observations, because JWST can see farther than any other telescope. Astronomers used JWST to observe GRB230307A, hoping to find the host galaxy where the neutron star originated. Telescope images show that, strangely, the GRB does not appear to be connected to any host galaxy. But there does appear to be a galaxy nearby, about 120,000 light-years away.

Telescopic observations indicate that these neutron stars were kicked out of nearby galaxies. They most likely formed as a pair of massive stars in a binary system. Eventually, both stars collapsed into neutron stars, and in a powerful event the pair were "kicked" out of their home galaxy, causing them to escape to a new location, where they slowly circled each other before merging hundreds of millions of years later.

JWST also detected a clear signal of tellurium in the high-energy radiation produced by the merger. While most stars can produce light elements below iron, it is thought that all other heavier elements in the universe were formed in more extreme environments such as neutron star mergers. JWST's detection of tellurium further confirms that the original gamma-ray burst was produced by the merger of neutron stars.

"This is just the beginning for JWST, and it's already making a huge difference," Schneider said. "In the coming years, many more neutron star mergers will be detected. The combination of JWST and other powerful observatories will be critical to revealing the nature of these extreme explosions."