A distant merger of neutron stars unleashed one of the most powerful short-term gamma-ray bursts (GRB) ever, according to new observations from ALMA, the Atacama Large Millimeter/submillimeter Array in Chile.
Neutron stars are the super-dense stellar nuclei that remain after massive stars explode, and when, say, two neutron stars collide, the result is a dramatic explosion, the light of which is referred to as a kilonova. The mergers are also letting go gravitational waves and a brief blast of gamma rays in two tight jets shooting into space in opposite directions.
On November 6, 2021 a short gamma ray burst was detected by the European Space Agency’s INTEGRAL X-ray and Gamma-Ray Observatory, which sent out an instant warning that NASA’s Fast eg satellite to follow up. The eruption, cataloged as GRB 211106A, lasted less than two seconds, but the afterglow of the kilonova appeared much longer as the jet of particles released from the merger ignited the surrounding gas.
“This brief gamma-ray burst was the first time we attempted to observe such an event with ALMA,” Wen-Fai Fong, an astronomer at Northwestern University in Illinois, said in a statement. pronunciation. “Afterglows for short bursts are very hard to come by, so it was spectacular to see this shining so brightly.”
Detecting the afterglow of the fusion in the millimeter-wavelength light ALMA is tuned to gives astronomers an edge when it comes to understanding these giant 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, combined with the X-rays, [the millimeter-wave light] can tell us about the true energy of the explosion.”
As the GRB’s jets, moving at nearly the speed of light, break through the surrounding gas, the shock waves accelerate electrons. The energy of the radiation from those electrons peaks at millimeter wavelengths and can therefore tell astronomers about the total energy of the explosion.
ALMA‘s measurements suggest that GRB 211106A released a total energy between 2 x 10^50 ergs and 6 x 10^51 ergs, making it one of the most powerful short GRBs ever detected. (One erg equals 10^–7 joules; for comparison: the sun releases only 3.8 x 10^33 ergs per second.)
It’s particularly impressive that GRB 211106A was so bright relatively speaking, as the merger occurred sometime between 6.3 and 9.1 billion years ago, and the galaxy in which the merger occurred is now about 20 billion light-years from Earth as a result. of cosmic expansion. At this distance, the gravitational waves released by the merger were too weak to detect.
Another advantage of observing with ALMA is that the afterglow at millimeter wavelengths lasts longer than with, for example, X-rays. This gives astronomers more time to study the GRB beam, which begins as a narrow stream, then gradually opens up, like a laser pointer making a larger spot on a wall than the base of the laser.
Fong and Schroeder’s team calculated that the jet’s opening angle was 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 the odds matter: Astronomers calculate the rate of neutron star merger in the universe based on the number of short GRBs we see and estimates of their jet’s opening angles. If more short GRBs have jets with wider apertures, scientists may have overestimated how many neutron star mergers take place.
The rate at which neutron stars are merging isn’t just an astrophysical curiosity — it has implications for cosmic chemistry. Conditions during the merger of neutron stars are so intense that some of the heaviest and most most valuable elements, such as gold, platinum and silver, are forged by these collisions. Indeed, scientists have estimated that a single merger of neutron stars can produce between 3 and 13 Earth masses’ worth of gold. hence the cosmic abundance of such elements is highly dependent on the rate at which neutron star mergers occur.
While the collision is an act of cosmic alchemy, enriching the surrounding region with atomic treasures, the discovery has opened up a whole new arena for astronomers to study short GRBs and their afterglow. “After a decade of observing short GRBs, it’s really amazing to witness the power of using these new technologies to unwrap surprise gifts from the universe,” Fong said.
A paper detailing the findings will be published in an upcoming issue of Astrophysical Journal Letters; a preprinted version was posted on Monday (1 Aug.).