A distant neutron star merger has triggered one of the strongest short gamma-ray bursts (GRBs) ever seen, according to new observations from ALMA, the Atacama Large Millimeter/submillimeter Array in Chile.
Neutron stars are the super-dense stellar cores left behind after massive stars explode, and when, say, two neutron stars collide, the result is a dramatic explosion, the light of which is called a kilonova. Mergers also release gravitational waves and a brief burst of gamma rays in two narrow jets shooting in opposite directions in space.
On November 6, 2021, a short gamma burst was detected by the European Space Agency’s INTEGRAL X-ray and gamma-ray observatory, which sent an instant alert that triggered NASA Fast satellite, among other things, to ensure follow-up. The burst, cataloged GRB 211106A, lasted less than two seconds, but the kilonova’s afterglow glowed much longer as the jet of particles released by the melt excited the surrounding gas.
“This short gamma-ray burst was the first time we’ve attempted to observe such an event with ALMA,” said Wen-Fai Fong, an astronomer at Northwestern University in Illinois. statement. “Afterglows for short bursts are very hard to come by, so it was spectacular to catch this brilliant so brightly.”
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Detecting the afterglow of fusion in the millimeter-wavelength light that ALMA is tuned to gives astronomers an edge when it comes to understanding these titanic explosions.
“Millimeter wavelengths can tell us about the density of the environment around the GRB,” Genevieve Schroeder, also of Northwestern University, said in the same statement. “And, when combined with X-rays, [the millimeter-wave light] can tell us about the true energy of the explosion.”
As the GRB jets, which are traveling at almost the speed of light, cut through the surrounding gas, the shock waves accelerate electrons. The radiation energy of these electrons peaks at millimeter wavelengths and can therefore tell astronomers the total energy of the explosion.
ALMAMeasurements suggest that GRB 211106A released a total energy of between 2 x 10^50 ergs and 6 x 10^51 ergs, placing it among the strongest short GRBs ever detected. (One erg equals 10^–7 joules; for comparison, the sun only releases 3.8 x 10^33 ergs per second.)
It is particularly impressive that GRB 211106A is so bright, relatively speaking, since the merger occurred between 6.3 and 9.1 billion years ago, and the galaxy in which the merger took place is now at about 20 billion light-years from Earth due to cosmic expansion. At this distance, the gravitational waves released by the merger were too weak to be detected.
Another advantage of observing with ALMA is that the afterglow at millimeter wavelengths lasts longer than in, say, X-rays. This gives astronomers more time to study the GRB jet, which begins as a narrow stream , then gradually widens, like a laser pointer making a larger spot on a wall than the base of the laser.
Fong and Schroeder’s team calculated the jet’s aperture angle to be 16 degrees, which is the widest ever measured for a short GRB. This is important because we only see a GRB when the jet is pointed at us, so the wider the jet the more likely we are to see it.
And probabilities matter: astronomers calculate the fusion rate of neutron stars in the universe based on the number of short GRBs we see and estimates of their jet aperture angles. If shorter GRBs have jets with wider aperture angles, scientists may have overestimated the number of neutron star mergers.
The rate at which neutron stars merge isn’t just an astrophysical curiosity – it has implications for cosmic chemistry. The conditions during neutron star mergers are so intense that some of the heaviest and most valuable items, like gold, platinum and silver, are forged by these collisions. Indeed, scientists have estimated that a single neutron star merger can produce between 3 and 13 landmasses of gold. Hence the cosmic abundance such elements strongly depends on the rate at which neutron star mergers take place.
While the collision is an act of cosmic alchemy, enriching the surrounding region with atomic treasures, the discovery has given astronomers a whole new arena to study short GRBs and their afterglows. “After a decade of observing short GRBs, it is truly amazing to see the power of using these new technologies to unwrap surprise gifts from the universe,” Fong said.
An article describing the results should be published in an upcoming issue of Astrophysical Journal Letters; a preprinted version was released on Monday (August 1).
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