Astronomers catch a super-energetic collision of dead stars

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

Neutron stars are the super-dense stellar cores left 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 also release gravitational waves and a brief blast of gamma-ray radiation in two tight jets shooting opposite directions into space.

On Nov. 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 alert that triggered NASA's Swift satellite, among others, to follow up. The burst, cataloged as GRB 211106A, lasted less than two seconds, but the afterglow from the kilonova shone for far longer as the jet of particles released by the merger excited the surrounding gas. 

"This short gamma-ray burst was the first time we tried to observe such an event with ALMA," Wen-Fai Fong, an astronomer at Northwestern University in Illinois, said in a statement. "Afterglows for short bursts are very difficult to come by, so it was spectacular to catch this shining so brightly."

Related: Gamma-ray bursts might be much rarer than we thought, study suggests

Detecting the afterglow from the merger in the millimeter-wavelength light that ALMA is tuned to gives astronomers an advantage 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 the X-rays, [the millimeter-wave light] can tell us about the true energy of the explosion."

As the GRB's jets, which move at nearly the speed of light, smash through the surrounding gas, the shockwaves accelerate electrons. The energy of the radiation from those electrons peaks at millimeter wavelengths, and therefore can 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, which places it among the most powerful short GRBs ever detected. (One erg is equal to 10^–7 joules; for comparison, the sun releases just 3.8 x 10^33 ergs per second.)  

It's particularly impressive that GRB 211106A was so bright, relatively speaking, since the merger happened sometime between 6.3 and 9.1 billion years ago, and the galaxy in which the merger took place is now approximately 20 billion light-years from Earth due to cosmic expansion. At this distance, the gravitational waves released by the merger were too feeble to detect.

Another advantage to come from 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 opens out, like a laser pointer that makes a larger spot on a wall than the laser's base.

Fong and Schroeder's team calculated the opening angle of the jet 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 toward us, so the wider the jet, the higher chance we have of seeing it.

And the odds matter: Astronomers calculate the rate of neutron-star mergers in the universe based on how many short GRBs we see and estimates of their jet's opening angles. If more short GRBs have jets with wider opening angles, scientists may have overestimated how many neutron-star mergers are taking place.

The rate at which neutron stars merge isn't just an astrophysical curiosity — it has repercussions for cosmic chemistry. The conditions during neutron-star mergers are so intense that some of the universe's heaviest and most precious elements, such as 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 Earth masses worth of gold. Hence the cosmic abundance of such elements is heavily dependent upon the rate at which neutron-star mergers take place. 

While the collision is an act of cosmic alchemy, enriching the surrounding region with atomic treasure, the discovery has offered astronomers a whole new arena for studying short GRBs and their afterglows. "After a decade of observing short GRBs, it is truly amazing to witness the power of using these new technologies to unwrap surprise gifts from the universe," Fong said.

A paper describing the findings is set to be published in a forthcoming issue of Astrophysical Journal Letters; a preprint version was posted on Monday (Aug. 1).

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