Cosmic Alchemy: Neutron Star Mergers Create Nurseries for Precious Metal Atoms?

Neutron star mergers: a nursery for precious metal atoms

The process of stellar destruction creates brief gamma-ray bursts that can provide important context for understanding similar explosions.

A distant neutron star merger released one of the most powerful short gamma-ray bursts (GRBs) ever recorded, according to new observations from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.

Neutron stars are the ultra-dense stellar cores left behind after a massive star explodes, and when two neutron stars collide, the result is a dramatic explosion that produces a burst of light known as a "kilonova." The merger also releases gravitational waves and a brief burst of gamma-ray radiation, with two dense jets shooting off into space in opposite directions.

On November 6, 2021, the space agency's INTEGRAL X-ray and Gamma-ray Observatory detected a brief gamma-ray burst, which issued an immediate alert, triggering NASA's Swift satellite and others to track it. The burst, numbered GRB 211106A, lasted less than two seconds, but the afterglow from the kilonova shone for much longer as the particle jets released by the merger excited the surrounding gas.

“This brief gamma-ray burst was our first attempt to observe such an event with ALMA,” Wen-Fai Fong, an astronomer at Northwestern University in Illinois, said in a statement. “The afterglow of a brief burst is difficult to obtain, so it is spectacular to capture such a bright flash.”

Artist's impression of a neutron star merger (left), producing a relativistic particle jet that interacts with the surrounding gas to produce the afterglow. (Image credit: ALMA (ESO/NAOJ/NRAO), M. Weiss (NRAO/AUI/NSF))

ALMA's tuned millimeter-wavelength observations, while detecting the afterglow from the merger, will enable astronomers to better understand the explosions of these massive stars.

"Millimeter waves can tell us about the density of the environment surrounding a GRB," Genevieve Schroeder of Northwestern University said in the same statement. "And, when combined with X-rays, millimeter waves can tell us the true energy of the burst."

As a GRB's jet moves at nearly the speed of light, the shock wave accelerates electrons. The energy of the radiation from these electrons peaks at millimeter wavelengths, so that can tell astronomers the total energy of the explosion.

ALMA's measurements indicate that the total energy released by GRB 211106A is between 2 x 10^50 ergs and 6 x 10^51 ergs, making it one of the most powerful short GRBs ever monitored. (An erg is equal to 10^-7 joules; by comparison, the Sun releases only 3.8 x 10^33 ergs per second.)

Artist's impression of two neutron stars before they collide. (Image credit: NASA/Goddard Space Flight Center)

It's particularly impressive that GRB 211106A is so bright, relatively speaking, because the merger occurred sometime between 630 million and 910 million years ago, and the galaxy in which the merger took place is now about 20 billion light-years from Earth due to the expansion of the universe. At this distance, the gravitational waves released by the merger are too faint to be detected.

Another benefit of observing with ALMA is that the millimeter-wavelength afterglow lasts longer than the X-ray afterglow. This gives astronomers more time to study the GRB jet, which starts out as a narrow stream and then gradually opens up, like a laser pointer forming a dot on a wall that is larger than the laser base.

Fong and Schroeder's team calculated that the jet is open at an angle of 16 degrees, the widest angle ever measured for a short GRB. This is important because we can only see a GRB if the jet is pointed toward us, so the wider the jet, the better our chances of seeing it.

This conclusion is important: Astronomers calculate the rate of neutron star mergers in the universe based on the number of short gamma-ray bursts we see and estimates of the angles at which the jets open. If shorter gamma-ray bursts have jets with wider opening angles, scientists may overestimate the number of neutron star mergers that are occurring.

Observing neutron star mergers isn't just a curiosity for astrophysicists — it has implications for cosmic chemistry. The energy generated by neutron star mergers is so intense that some of the heaviest and most precious elements in the universe, such as gold, platinum, and silver, are forged from these collisions. In fact, scientists estimate that a single neutron star merger can produce between three and 13 Earth masses of gold. Therefore, the cosmic abundance of these elements depends largely on how quickly neutron star mergers occur.

While this collision can be seen as cosmic alchemy, creating precious metal atoms scattered around, the discovery provides astronomers with a whole new arena for studying short gamma-ray bursts and their afterglow. "After decades of observation, what's really exciting is seeing these new techniques unlock the surprise gifts from the universe," Fong said.

A paper describing the findings will appear in an upcoming issue of The Astrophysical Journal Letters.

BY:Keith Cooper

FY: Qing

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