Like the simulation depicting how a neutron star cracks, this one also predicts the characteristics of the resulting flares astronomers might see through telescopes. In the fleeting moments when monster shock waves rip outward and a black hole pulsar forms, telescopes may be able to catch outbursts of radio waves or a combination of X-rays and gamma rays. In short, the simulations performed by Most and colleagues provide a deeper understanding of the physics driving some of the most energetic events in the universe.
Undulating Space and Time
When two black holes collide, they generate not only shock waves and flares of light but also another type of radiation known as gravitational waves. These ripples in the fabric of space and time itself were first predicted more than 100 years ago by Albert Einstein. The Caltech- and MIT-led LIGO (Laser Interferometer Gravitational-wave Observatory), which is funded by the National Science Foundation (NSF), famously made the first direct detection of gravitational waves, generated from the coalescence of two black holes, in 2015. The achievement would later earn three of the collaboration's leading teammates the 2017 Nobel Prize in Physics.
In 2017, LIGO and Virgo, its European sister observatory, observed a different kind of collision: that between two neutron stars. The fiery explosion, called a kilonova, unleashed a spray of metals, including the element gold. That event emitted both gravitational waves and light. LIGO–Virgo first caught the blast in gravitational waves and then notified astronomers around the world who followed up with telescopes in space and on the ground to detect a broad range of electromagnetic, or light, wavelengths, ranging from high-energy gamma rays to low-energy radio waves.
Whether a neutron star–black hole collision would also produce a similar light show is not clear, but so far none have been seen. Still, it is possible that the neutron star–black hole mergers, even if they fail to produce a cloud of glowing material, might flash with brief radio and/or other electromagnetic signals right before and during the collisions. Simulations like those from Most and his colleagues help astronomers know which electromagnetic signals to look for.
To aid in the hunt for these precursor signals, the LIGO team is working to detect mergers up to a minute before they occur, which would give astronomers more time to point their telescopes at the blasts and search for tell-tale signs of an impending crash.
"LIGO can detect mergers before they happen because the pair of colliding objects emit gravitational waves in the frequency band that LIGO detects as they spiral closer and closer together," says Chatziioannou, who is part of the LIGO team. "Currently, we can detect the collisions just seconds before they occur, and we are working up to a full minute. The gravitational waves are one piece of the puzzle while the electromagnetic radiation is another. We want to put the puzzle pieces together."
The Most Advanced Computers
A major factor in the success of the team's recent neutron star–black hole simulations is the use of supercomputers containing GPUs (graphics processing units). For these recent studies, the team used the Perlmutter supercomputer located at the Lawrence Berkeley National Laboratory in Berkeley (named after astronomer Saul Perlmutter, who won the 2011 Nobel Prize in Physics with two other scientists for discovering that the universe is accelerating). GPUs provide processing power for video games and AI programs like ChatGPT; in this case, the massive parallel computing power of GPUs allowed the Perlmutter supercomputer to handle the codes needed to simulate the intricate interactions between a converging neutron star and black hole.
"When you simulate two black holes merging," Most says, "you need the equations of general relativity to describe the gravitational waves. But when you have a neutron star, there's a lot more physics taking place including the complex nuclear physics of the star and plasma dynamics around it."
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