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Gamma-ray bursts, the most colossal explosions known in the cosmos, may provide a crucial piece to solving one of the most perplexing puzzles in physics: the genesis of the universe’s heaviest elements. New research suggests that the intense radiation from these bursts could be instrumental in forging these elements within the outer layers of expiring stars, offering a novel perspective on heavy element formation and gamma-ray burst astrophysics.
Unraveling the Origin of Heavy Elements
Prior investigations have indicated that the creation of weighty elements, such as gold, necessitates a significant supply of neutrons. These neutrons are absorbed by atomic nuclei, causing them to expand in mass. Consequently, it was generally “presumed that heavy elements originated solely in environments already abundant with neutrons,” explained Matthew Mumpower, the lead author of the study and a physicist at Los Alamos National Laboratory.
Neutrons are typically confined within atomic nuclei or the structure of neutron stars, incredibly dense celestial bodies. Nuclear reactions, such as fission or fusion, can liberate neutrons for heavy element creation. However, Mumpower noted, “free neutrons generally decay in approximately 15 minutes.” This rapid decay limits the potential scenarios for abundant free neutrons required for heavy element formation to rare events, notably the violent collision of two neutron stars.
“I have dedicated two decades to studying the origins of heavy elements,” Mumpower stated. “The enduring mystery and numerous unknowns make it a captivating and exceptionally challenging problem in physics.”
A Novel Mechanism: Gamma-Ray Bursts as Element Forges
Mumpower and his team now propose a new avenue for the creation of heavy elements. They posit that potent photons, or light particles, emitted from gamma-ray bursts can facilitate neutron generation.
“Energetic photons lead to neutron production, and the presence of neutrons enables the synthesis of heavy elements,” Mumpower clarified.
Stellar Demise and Gamma-Ray Emission
This new hypothesis centers on the dramatic demise of a massive star as it exhausts its nuclear fuel. Lacking the energy to withstand its own immense gravitational force, the star’s core implodes, birthing a black hole. This cataclysmic event can unleash extraordinarily intense bursts of radiation – gamma-ray bursts.
Stars, including our Sun, are in constant rotation. If a black hole formed from a dying star rotates at a sufficient velocity, it can eject a powerful jet, generating high-energy photons deep within. This jet violently impacts the outer shell of the collapsing star, forming a sweltering cocoon of matter. Within this hot cocoon, researchers suggest, the jet’s energetic photons can interact with atomic nuclei, transmuting protons into neutrons with incredible speed – on the scale of nanoseconds. The team also proposes that these photons can shatter atomic nuclei, releasing free neutrons. These liberated neutrons then become available to participate in the creation of heavy elements.
The “Train Through Snow” Analogy
“The inspiration for this particular study arose from conversations with my children,” Mumpower recounted. “We were watching slow-motion videos on YouTube, and a clip of a freight train forcefully pushing through a large snowdrift caught our attention. The snow wasn’t simply eradicated; it was displaced and enveloped the train.”
“I pondered if this train could symbolize an astrophysical jet brimming with high-energy photons, and the snow could represent a star being expelled outward, creating a hot, neutron-generating cocoon,” Mumpower reflected. “This analogy served as my ‘aha’ moment, initiating this line of inquiry.”
Implications and Supporting Evidence
This newly identified mechanism may clarify puzzling prior discoveries, such as the co-occurrence of specific radioactive substances like iron-60 and plutonium-244 in Earth’s deep-sea sediments. Previous studies suggested an extraterrestrial origin for these materials, but neutron star mergers, a primary mechanism for heavy element creation, struggle to account for their presence.
These findings may also shed light on the recent detection of a kilonova – a luminous emission of visible and infrared light – associated with long-duration gamma-ray bursts. Prior research predominantly linked kilonovas to the collision of neutron stars or a neutron star-black hole merger, rather than stellar collapses.
Future Research Directions
Mumpower expresses hope that future observational data will furnish robust evidence to support the team’s novel findings. He suggests that an array of telescopes capable of detecting light, neutrinos, and gravitational waves could monitor the processes by which a collapsing star generates a gamma-ray burst and kilonova. “Such information would offer definitive proof of the proposed physical mechanism,” he concluded.
The researchers published their findings online on March 20 in The Astrophysical Journal.