Space

Here’s how a collision of star remnants launches a gleaming jet

An enlarged magnetic field plays a crucial role

After two neutron stars of unequal mass merge and form a black hole (center, not visible), an enlarged, twisted magnetic field (pink) causes the celestial body to shoot a bidirectional jet of high energy matter (green arrows), a new simulation shows.

K. Hayashi/Max Planck Institute for Gravitational Physics (Albert Einstein Institute)

By McKenzie Prillaman

23 hours ago

When the remnants of two stars collide, their union can launch a dazzling jet of high energy matter. A new computer simulation reveals how the merger, which forms a black hole, emits that bright beam, researchers report in the May 30 Physical Review Letters.

Scientists found in 2020 that gravitational waves detected a year earlier came from two neutron stars — dense leftovers of exploded stars — paired up in a system 3.4 times the mass of the sun. The duo was “quite [a] heavy one, which we have never seen before,” says astrophysicist Kota Hayashi of the Max Planck Institute for Gravitational Physics in Potsdam, Germany.

Upon colliding, the stars probably collapsed into a black hole, after which electromagnetic signals from a jet should have been produced. But none were observed. Such signals are challenging to spot, and the matter from a jet may not have escaped the black hole. So Hayashi and colleagues modeled a similar merger to investigate.

This computer simulation shows two neutron stars of unequal mass spiraling toward one another. When they merge, they collapse into a black hole. The lighter star gets ripped apart, and its guts form a disk of matter that surrounds the celestial body. The disk and black hole’s rotation results in a huge magnetic field that launches a bidirectional jet of high energy matter along the rotational axis.

The team used a supercomputer to simulate the fusion of two neutron stars 1.25 times and 1.65 times the mass of the sun, up to about 1.5 seconds after they merge. The stars spiral together, losing orbital energy emitted as gravitational waves. When they combine, the system immediately collapses into a black hole. It swallows the heavier star, while the lighter one gets ripped apart into a disk around the black hole, Hayashi says.

Despite the star’s shredded state, its magnetic field survives the merger. Within the disk, matter rotates faster at the center than at the edge, a motion that causes the magnetic field to get “stretched around this black hole,” Hayashi says.

That field accumulates at the poles, perpendicular to the disk. Once it penetrates the celestial body along the rotational axis, the black hole’s spin further amplifies it.

Eventually, a tornado-like magnetic field extends thousands of kilometers from both poles, and the system spits a jet of disk matter. The bidirectional jet travels at almost the speed of light, Hayashi says, and “it will eventually shine as what is called a gamma ray burst, the brightest emission known in the current universe.”

The lighter star’s magnetic field started at just 10 kilometers long, he notes, and the work shows that drastically scaling up the magnetic field is a crucial part of jet launching.