Researchers now better understand what happens when a black hole merges with a neutron star, the dense remnant of a supernova. The event is one of the most extreme in the universe and scientists think there’s a lot to learn from these events.
Collisions between black holes and neutron stars are believed to be responsible for the formation of the heaviest chemical elements in the universe. The researchers of the new study, published in Classical and Quantum Gravity, developed simulations to explore the instants following a merger to provide clues for what astronomers should look for in the sky to spot the aftermath.
"We are steadily adding more realistic physics to the simulations," lead author Dr Francois Foucart, from the Lawrence Berkeley National Laboratory, said in a statement. "But we still don't know what's happening inside neutron stars. The complicated physics that we need to model make the simulations very computationally intensive."
These mergers are also one of the strongest sources of gravitational waves that can be detected with current observatories, although we haven’t detected them yet. The researchers are actually using the simulation to make better models of what these signals look like.
Ideally, the LIGO detectors would see the signal and astronomers would point telescopes towards the region of the sky where it was emitted from in order to see powerful gamma-ray bursts or bright short-lived explosions called kilonovae. In reality, LIGO still can’t precisely pinpoint the location of a signal in the sky.
Still, a gravitational wave observation could help scientists understand the internal processes of a neutron star. These objects are as big as a city, but weigh more than the Sun. They are made of degenerate matter, neutrons in an extreme state, and they have an incredible density.
The team's simulation shows that when a neutron star collides with another extremely dense object, some of its material is propelled outwards by up to 100,000 kilometers (60,000 miles) per second. The material spins around, stretching into a bright long tail.
"This would be strange material that is loaded with neutrons," co-author Professor Daniel Kasen said. "As that expanding material cools and decompresses, the particles may be able to combine to build up into the heaviest elements." Elements like Uranium or Platinum are believed to only form in these kind of events.
“If we can follow up LIGO detections with telescopes and catch a radioactive glow, we may finally witness the birthplace of the heaviest elements in the universe,” Kasen added.
The kilonova might shine for a few weeks, so the follow-up needs to be prompt. Hopefully, we will soon see these events and work out how close our simulations are to reality.
