by Scientists have analyzed an unusually long burst of high-energy radiation, known as a gamma-ray burst (GRB), and determined that it resulted from the collision of two ultradense neutron stars. And, most importantly, this result helped the team observe a flash of light emanating from the same event that confirms these compounds are the sites that create gold-like elements.
The observations, made using the James Webb Space Telescope (JWST) and the Hubble Space Telescope, allowed scientists to see gold and heavy elements forged, which could help us better understand how these powerful neutron star merger events generate the only environments in the turbulent universe. enough to create elements heavier than iron, such as silver and gold resulting in a flash of light called a kilonova.
“It was really exciting to study a kilonova because we’ve never seen it 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 succeeded in verifying that metals heavier than iron and silver were formed before us,”
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GRBs, which are the most powerful bursts of energy in the known universe, have been associated with neutron star mergers before – but this discovery is different.
These phenomena can be divided into two groups. On the one hand, there are the long GRBs lasting more than 2 seconds and, on the other, short GRBs lasting less than 2 seconds. Although neutron star mergers have been associated with short GRBs, long GRBs were believed to occur as a result of the collapse of massive stars and not due to such collisions.
The extremely bright and long burst, named GRB 230307A, and detected by instruments on board NASA’s Fermi mission in March 2023, lasted 200 seconds; this was the second most energetic GRB ever observed. It appeared to be related to a kilonova, named AT2017gfo, and a neutron star merger that occurred about 8.3 million light-years away, breaking the usual GRB convention and challenging theories about how these bursts of high-energy radiation are sent.
“It is challenging to think that the duration of GRBs resulting from a compact binary merger can extend to thousands of seconds,” Yu-Han Yang, research team leader and postdoctoral astrophysicist of the University of Rome, told Space.com.
Gamma-ray discovery could be a cosmic gold mine
Stars are like stellar furnaces that forge the elements in the periodic table, starting with the nuclear fusion of hydrogen into helium in their cores and continuing with the fusion of helium with heavier elements like nitrogen, oxygen and carbon.
The most massive stars, about 7 to 8 times as massive as the sun, can forge elements all the way up to iron in their cores. Once a stellar core is filled with this element, fusion ceases. This also reduces the outward energy line that has been supporting 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 stellar core, pushing electrons and protons into a sea of streaming neutrons, particles found in atomic nuclei that are rarely “free.” But, in this sea, the neutrons are prevented from squeezing close together by a quantum principle called neutron decay 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 that do not have the mass to overcome decay pressure are left as 12-mile (20-kilometer) wide galaxies with masses one to two times that of the sun. However, there is a way that neutron stars can add elements heavier than iron to the universe.
Not every neutron star exists on its own.
Some traverse the cosmos in binary neutron star systems, meaning that another neutron star is in its gravitational claws. As these dead stars orbit each other, they set up the fabric of the ring space with ripples called gravitational waves that gradually carry angular momentum out of the system.
This causes neutron stars to spiral together, emitting faster gravitational waves as time passes and collectively “leaking” more angular momentum. Ultimately, the two collide and merge. This collision creates a gamma-ray burst and sends out a spray of neutron-rich material that helps create the heaviest elements of the periodic table.
Other atomic nuclei around these collisions capture the free neutrons through the fast-neutron capture process, or the r-process, and become short-lived, heavy elements called “lanthanides.” The lanthanides then rapidly decay into lighter elements (although there are still elements heavier than lead.) This decay causes the emission of radiation, a light we see from Earth as a “kilonova.” Thus, tracking the evolution of kilonovas can help follow the creation of elements such as gold and silver.
“Neutron star mergers may provide an ideal environment for the widespread synthesis of heavy elements, 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 a period of weeks to months, Yang explained that kilonovas encompass a wide range of behaviors. These behaviors depend on the composition of the emitted material and the type of residue formed in the center of the fusion site.
Observations of most kilonovas do not extend to late times in their evolution – but AT2017gfo was different. Unfortunately, however, the late observational data for AT2017gfo, collected with the Spitzer Space Telescope, was limited. They offered only weak signals that were contaminated by the host galaxy of the kilonova and presented insufficient coverage in different wavelengths of light.
“During the first few days, the behavior of a kilonova does not affect its chemical composition,” explained Troja. “It takes weeks to reveal what metals are forged in the explosion, and we’ve never had the chance to look at a kilonova for that long.”
These limitations have hindered scientists aiming to better understand kilonovas and the processes that create them.
In the case of AT2017gfo, however, the sensitivity and multi-color coverage of the JWST and Hubble observations allowed Yang and his colleagues to observe the luminosity of this kilonova at late times.
“We tracked the evolution of the transient event associated with GRB 230307A up to two months after the burst and captured the entire blue-to-red evolution of this transient, which can be classified as a kilonova,” Yang said. “We detected the retrograde of the photosphere at late times. The retrograde photosphere provides evidence for the recombination of heavy elements, such as lanthanides, which occurs in the cooling process. Heavy e-process elements are required to produce the observed data. “
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This confirmed that neutron star mergers forge elements heavier than gold, and even confirms that long GRBs can come from neutron star mergers. It didn’t, I think, solve the mystery of why this particular neutron star merger sent off an unusually long GRB.
“This event proves that a long-term GRB coming from a close binary merger is not a fortuitous occurrence,” Yang said, adding that many questions remain to be answered about these events. “What illuminating revelations can late observations of kilonovas offer on nucleosynthesis?
“We look forward to future joint observations of long-period gamma-ray bursts, chileannova and gravitational waves, which will help unravel the mysteries of these dreamers.”
The team’s research was published on Wednesday (February 21) in the journal Nature.
