The lightest elements, hydrogen and helium, were formed mainly in the Big Bang that gave birth to the Universe. Somewhat heavier elements, such as oxygen and iron, are forged inside the hot cores of ordinary stars like the Sun and expelled into space when they die in supernova explosions. Rare elements much heavier than iron, such as gold and platinum, however, are only created in far more extreme conditions than those found in ordinary stars. For decades, nuclear astrophysicists have been working to identify the events in nature that can synthesize these heavy elements.
Now, a multi-institutional group of researchers led by Professor Brian Metzger and doctoral candidate Anirudh Patel have a fresh answer to this question, which challenges existing ideas about where heavy elements are created. In a new paper, they demonstrate that elements much heavier than iron were created in a famous cosmic event from over 20 years ago, which released more energy in half a second than our Sun produces in a quarter of a million years. The finding from that single event offers important insight into how these elements are synthesized in general.
"Comparing our theoretical models to observed data, we found evidence that one of the brightest explosions ever observed in our Galaxy-a powerful burst of gamma-ray radiation in 2004-produced a huge amount of heavy elements exceeding in mass the planet Mars," said Patel. "It was an incredible feeling to see our prediction confirmed by existing data and to realize the implications this discovery has for the history of some of the matter making up our planet."
On December 27, 2004, several satellites, including the European Space Agency's INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) space telescope, detected an extremely powerful burst of gamma-ray radiation from a magnetar in our Galaxy.
Magnetars are a class of neutron stars harboring the strongest magnetic fields in the Universe, over 10 trillion times stronger than the typical refrigerator magnet. Neutron stars are the compact bodies left over when massive stars collapse and explode in supernovae. The immense magnetic energy of magnetars powers extreme outbursts similar to but much more energetic than flares of particles that our own Sun produces.
Although the magnetar, SGR 1806-20, lies roughly 30,000 light-years away, 2004's "giant flare" was bright enough to affect the upper layers of the Earth's atmosphere. After the initial gamma-ray burst, the INTEGRAL space telescope also detected a dimmer but longer gamma-ray signal from the source lasting for several hours. Although this "afterglow" was first reported by a team of researchers in 2005, no compelling physical explanation was offered by scientists at the time.
Now, Metzger, Patel, and their collaborators have shown that this previously unexplained signal from the famous 2004 magnetar giant flare can be attributed to gamma-ray emission from the radioactive decay of heavy elements-elements that were freshly synthesized by a series of nuclear reactions in the crust of the neutron star as it was expelled into space during the giant flare. The researchers estimate that up to 10% or more of the precious metals on Earth can be produced by magnetars. Although many potential phenomena that create these elements have been proposed by scientists over the years, this represents only the second confirmed event in which the heaviest elements in our Universe can be synthesized; the first was a merger of neutron stars predicted by Metzger in 2010, and observationally confirmed in 2017.
The researchers began their discovery late last year when Patel was calculating what elements are created in magnetar flares. Just two weeks before the 20th anniversary of the 2004 giant flare, Patel made preliminary calculations of the gamma-ray radiation expected from these elements. "The peak brightness and time-scale of the predicted emission matched perfectly the unexplained observation from 2004. At this moment we realized we might have just made a discovery," said Patel.
"This single giant flare was so prodigious in creating these heavy elements that the accumulation of similar events over our Galaxy's history could contribute a significant fraction of all of these elements on Earth," Metzger said. "It's humbling to realize that the platinum in my wedding band may have been forged in such a cataclysmic event. This discovery opens a whole series of new questions related to the role that magnetars may play in seeding elements throughout the universe."
Research Report:Direct Evidence for r-process Nucleosynthesis in Delayed MeV Emission from the SGR 1806-20 Magnetar Giant Flare
Related Links
European Space Agency's INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL)
Stellar Chemistry, The Universe And All Within It
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