Scientists have analyzed an unusually long explosion of high-energy radiation known as a gamma-ray burst (GRB) and determined that it came from the collision of two ultra-dense neutron stars. More importantly, this result helped the team observe a flash of light arising from the same event, confirming that these mergers are the locations that create elements like gold.
The observations, made using the James Webb Space Telescope (JWST) and the Hubble Space Telescope, allowed scientists to see gold and heavy elements being forged, which could help us better understand how these powerful neutron star mergers create the only environments in the create universes that are turbulent. enough to create elements heavier than iron, such as silver and gold, resulting in a flash of light called a kilonova.
“It was exciting to study a kilonova like we had never seen before using the powerful eyes of Hubble and JWST,” research team member and University of Rome astrophysicist Eleonora Troja told Space.com. “This is the first time we have been able to verify that metals heavier than iron and silver are freshly made for us,” he says.
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GRBs, the most powerful energy explosions in the known universe, have previously been linked to neutron star mergers – but this discovery is different.
These phenomena can be divided into two groups. On the one hand there are the long GRBs that last longer than 2 seconds and on the other hand there are short GRBs that last less than 2 seconds. Although neutron star mergers have been associated with short GRBs, long GRBs were thought to form due to the collapse of massive stars and not as a result of such collisions.
The extremely bright and long burst, designated GRB 230307A, and detected by devices aboard NASA’s Fermi mission in March 2023 lasted 200 seconds; this was the second most energetic GRB ever seen. It appeared to be related to a kilonova, named AT2017gfo, and a neutron star merger that occurred some 8.3 million light-years away, breaking the usual GRB convention and putting to rest theories about how these blasts of high-energy radiation are produced launched, are called into question.
“It is challenging to imagine that the duration of GRBs from compact binary mergers can reach tens of seconds,” Yu-Han Yang, research team leader and postdoctoral astrophysicist from the University of Rome, told Space.com.
The discovery of gamma rays could be a cosmic goldmine
Stars are like stellar furnaces that forge the elements in the periodic table, starting with the fusion of hydrogen into helium in their cores and continuing with the fusion of helium into heavier elements such as nitrogen, oxygen and carbon.
The heaviest stars, about seven to eight times as massive as the Sun, can forge elements right to their core. Once a stellar core is filled with this element, fusion ceases. That also cuts off the outward energy line that has supported the star against its own gravity for millions, or sometimes billions, of years. The cores of these massive stars then collapse under this crushing gravity, blowing away their outer layers in supernova explosions.
This collapse transforms the star’s core, crushing electrons and protons into a sea of streaming neutrons, particles found in atomic nuclei that very rarely exist ‘freely’. Yet in this sea, the neutrons are prevented from coming close to each other by a quantum principle called the neutron degeneracy pressure, which can be overcome with enough mass to create a black hole. But sometimes there isn’t enough mass for a black hole to form.
Those dead stellar cores without the mass to overcome the degeneracy pressure are left behind as 20-kilometer-wide bosies with a mass between one and two times that of the Sun. However, there is a way in which neutron stars can contribute heavier elements than iron to the universe.
Not all neutron stars exist alone.
Some traverse the cosmos in binary neutron star systems, meaning they have another neutron star in their gravitational coupling. As these dead stars orbit each other, they ripple the fabric of space with ripples called gravitational waves that gradually remove angular momentum from the system.
This causes neutron stars to spiral toward each other, emitting gravitational waves faster as time passes while “leaking” more angular momentum. Ultimately, the two collide and merge. This collision causes a gamma-ray burst and emits a beam of neutron-rich material that helps create the heavier elements of the periodic table.
Other atomic nuclei around these collisions grab the free neutrons via the fast neutron capture process, or r-process, and become short-lived superheavy elements called “lanthanides.” Those lanthanides then quickly decay into lighter elements (although elements are still heavier than lead). This decay causes the emission of radiation, light that we see from Earth as a ‘kilonova’. So tracking the evolution of kilonovas can help track the creation of elements like gold and silver.
“The merger of neutron stars could give rise to an ideal environment to synthesize heavy elements on a large scale, which is currently beyond artificial creation,” Yang said. “Studying neutron star mergers helps us rewrite the obscure chapters of nucleosynthesis.”
Cosmic alchemy in action
Over the course of weeks to months, Yang explained that kilonovas encompass a wide range of behaviors. This behavior depends on the composition of the ejected material and the type of remnant formed at the center of the merger site.
Observations of most kilonovas don’t extend to such late times in their evolution – but AT2017gfo was different. Unfortunately, however, late observation data for AT2017gfo collected with the Spitzer Space Telescope was limited. They offered only weak signals contaminated by the kilonova’s host galaxy and did not provide sufficient coverage at different wavelengths of light.
“During the first few days, the behavior of a kilonova is unaffected by its chemical composition,” Troja explains. “It will take weeks to reveal which metals were created in the explosion, and we have never been able to stare at a kilonova for so long.”
These limitations had hindered scientists from better understanding kilonovas and the processes that create them.
In the case of AT2017gfo, however, the sensitivity and multicolor coverage of the JWST and Hubble observations allowed Yang and his colleagues to observe the brightness of this kilonova at later times.
“We monitored the evolution of the transient event associated with GRB 230307A for up to two months after the outburst and recorded the complete blue-to-red evolution of this transient event, which can be classified as a kilonova,” Yang said. “We discovered the recession of the photospheric jet at a late time. The receding photospheric jet provides evidence for the recombination of heavy elements, such as lanthanides, that occur during the cooling process. Heavy r-process elements are needed to convert the observed data produce.”
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This confirmed that neutron star mergers create elements heavier than gold, and even confirms that long-GRBs can come from neutron star mergers. It did not, it was thought, solve the mystery of why this particular neutron star merger launched such an unusually long GRB.
“This event proves that a long-lasting GRB arising from compact binary mergers is not a random event,” Yang said, adding that there are many questions that remain to be answered about these events. ‘What illuminating revelations can late observations of kilonovae provide about nucleosynthesis?
“We look forward to joint observations of long-duration gamma-ray bursts, kilonovas and gravitational waves in the future, which will help unlock the mysteries about such outliers.”
The team’s research was published Wednesday (Feb. 21) in the journal Nature.