Scientists analyzed an unusually long burst of high-energy radiation known as a gamma-ray burst (GRB) and determined that it was caused by the collision of two ultra-dense neutron stars. And more importantly, this result helped the team observe a flash of light emanating from the same event, confirming that these mergers were places that created elements like gold.
Observations using the James Webb Space Telescope (JWST) and the Hubble Space Telescope allowed scientists to see gold and heavy elements being forged; This could help us better understand how these powerful neutron star merger events create the only turbulent environments in the universe. 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 we’ve never seen before using Hubble and JWST’s powerful eyes,” research team member and University of Rome astrophysicist Eleonora Troja told Space.com. “This is the first time we can confirm that metals heavier than iron and silver have just been made in front of us.”
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GRBs, the most powerful energy explosions in the known universe, have been previously associated with neutron star mergers, but this discovery is different.
These phenomena can be divided into two groups. On one side there are long GRBs lasting more than 2 seconds, and on the other side there are short GRBs lasting less than 2 seconds. While neutron star mergers were associated with short GRBs, long GRBs were believed to result from the collapse of massive stars, not from such collisions.
The extremely bright and long burst, called GRB 230307A and detected by instruments on NASA’s Fermi mission in March 2023, lasted 200 seconds; this was the second most energetic GRB ever seen. It appeared to be associated with a merger of a kilonova called AT2017gfo and a neutron star that occurred about 8.3 million light-years away; This breaks the usual GRB convention and challenges theories of how these high-energy bursts of radiation are initiated.
“It is difficult to imagine that the duration of GRBs resulting from compact binary mergers could extend to tens of seconds,” research team leader and University of Rome postdoctoral astrophysicist Yu-Han Yang told Space.com.
Gamma ray discovery could be a cosmic gold mine
Stars are like stellar furnaces, starting with the nuclear fusion of hydrogen into helium in their cores and continuing with the fusion of helium into heavier elements such as nitrogen, oxygen and carbon, creating the elements in the periodic table.
The most massive stars, about 7 to 8 times more massive than the Sun, can form elements up to iron in their hearts. Once a star’s core is filled with this element, fusion stops. This also cuts the outward energy line that has supported the star against its own gravity for millions, sometimes even billions, of years. The cores of these massive stars then collapse under this crushing gravity and blow off their outer layers in supernova explosions.
This collapse turns the stellar core into a flowing sea of neutrons, overwhelming electrons and protons, particles found in atomic nuclei that very rarely exist “freely”. But in this sea, neutrons are prevented from approaching each other by a quantum principle called neutron degeneracy pressure, which can be overcome by enough mass to form a black hole. However, sometimes there may not be enough mass for a black hole to form.
These dead stellar cores, which lack mass to exceed the degeneracy pressure, remain as 12-mile (20-kilometer) wide bodies with masses between one and two times that of the Sun. But there is a way that neutron stars can add elements heavier than iron to the universe.
Not all neutron stars exist in isolation.
Some wander the universe in neutron star binary systems, which means they have another neutron star in their gravitational grip. As these dead stars orbit each other, they rumble the fabric of space with ripples called gravitational waves that gradually remove angular momentum from the system.
This causes the neutron stars to spiral together, emitting faster gravitational waves as time passes, and “leaking” more angular momentum together. Ultimately, the two collide and merge. This collision creates a gamma-ray burst and sends out a spray of neutron-rich material that helps form the heavier elements of the periodic table.
Other atomic nuclei around these collisions capture free neutrons through the fast neutron capture process, or r-process, and become briefly living superheavy elements called “lanthanides.” These lanthanides then rapidly decay to lighter elements (although the elements are still heavier than lead). This decay causes the emission of radiation from Earth, which is the light we see as “kilonovae.” So tracking the evolution of kilonovae could help track the formation of elements like gold and silver.
“Neutron star mergers could lead to the emergence of an ideal environment for the comprehensive synthesis of heavy elements, which is currently beyond artificial creation,” Yang said. “Studying neutron star mergers helps us rewrite the dark parts of nucleosynthesis.”
Cosmic alchemy in action
Over the course of weeks or even months, kilonovae encompass a wide range of behaviors, Yang explained. These behaviors depend on the composition of the expelled material and the type of residue formed in the center of the coalescence area.
Observations of most kilonovae do not date back this late in their evolution; but AT2017gfo was different. Unfortunately, late observation data collected with the Spitzer Space Telescope for AT2017gfo were limited. They offered only weak signals contaminated by the kilonova’s host galaxy and offered insufficient coverage of different wavelengths of light.
“During the first few days, the behavior of the kilonova is not affected by its chemical composition,” Troja explained. “It takes weeks to uncover what metals were forged in the explosion, and we’ve never had the chance to look at a kilonova for this long.”
These limitations have hindered scientists’ efforts to better understand kilonovas and the processes that create them.
But in the case of AT2017gfo, the sensitivity and multicolor coverage of the JWST and Hubble observations allowed Yang and colleagues to observe the brightness of this kilonova at late times.
“We tracked the evolution of the transient associated with GRB 230307A for up to two months after the explosion and captured the full blue-to-red evolution of this transient, which can be classified as a kilonova,” Yang said. “We discovered the regression of the photospheric radius of late. The declining photospheric radius provides evidence for the recombination of heavy elements, such as lanthanides, occurring during the cooling process. Heavy r-process elements are needed to produce the observed data.”
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This confirmed that neutron star mergers create elements heavier than gold, and that long GRBs can even come from neutron star mergers. It is thought that this particular neutron star merger does not solve the mystery of why it initiated such an unusually long GRB.
“This event proves that a long-lasting GRB resulting from compact binary mergers is not a random event,” Yang said, adding that there are still many questions to be answered about these events. “What insightful insights into nucleosynthesis might recent observations of kilonovas offer?
“We look forward to joint observations of long-duration gamma-ray bursts, kilonovas, and gravitational waves in the future, which will help unlock mysteries about such outliers.”
The team’s research was published in the journal Nature on Wednesday, February 21.