New simulations are predicting the characteristics of flares resulting from neutron star and black hole mergers, which astronomers may soon observe using telescopes. The models suggest that telescopes could detect radio wave outbursts,or combinations of X-rays and gamma rays,during the brief moments when shock waves erupt and a black hole pulsar takes shape. These simulations, led by Most and colleagues, offer a more profound grasp of the physics powering some of the universe’s most energetic phenomena.

Undulating Space and Time

The collision of two black holes generates shock waves, light flares, and gravitational waves, which are disturbances in space-time predicted by Albert Einstein over a century ago. The Laser Interferometer Gravitational-wave Observatory (LIGO), a Caltech- and MIT-led project funded by the National Science Foundation (NSF), achieved the first direct detection of gravitational waves from colliding black holes in 2015, an accomplishment that earned three team members the 2017 Nobel Prize in Physics.

In 2017, LIGO and Virgo, its European counterpart, detected a collision between two neutron stars. This explosion, known as a kilonova, released a spray of metals, including gold, and emitted both gravitational waves and light. LIGO-Virgo initially detected the event via gravitational waves and alerted astronomers globally, who then used space and ground-based telescopes to observe a wide spectrum of electromagnetic wavelengths, from gamma rays to radio waves.

It remains uncertain whether a neutron star-black hole collision woudl produce a similar light show, as none have been observed to date.However, neutron star-black hole mergers might emit brief radio or electromagnetic signals just before and during the collisions, even if they don’t produce glowing material.Simulations like those from Most and his team are helping astronomers identify which electromagnetic signals to seek.

To enhance the detection of these precursor signals, the LIGO team aims to detect mergers up to a minute in advance, providing astronomers more time to aim their telescopes and search for signs of an impending collision.

“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, a member 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.”

“Before this simulation, people thought you could crack a neutron star like an egg, but they never asked if you could hear the cracking,” Most says. “Our work predicts that, yes, you could hear or detect it as a radio signal.”

The Most Advanced Computers

The use of supercomputers with graphics processing units (GPUs) has been crucial to the success of recent neutron star-black hole simulations. The team utilized the perlmutter supercomputer at the Lawrence Berkeley National Laboratory in Berkeley, named after astronomer Saul Perlmutter (who shared the 2011 Nobel Prize in Physics for discovering the accelerating expansion of the universe). The Perlmutter supercomputer’s GPUs, which provide processing power for video games and AI programs like ChatGPT, enabled it to manage the complex interactions between a 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.”

The simulations take approximately four to five hours to complete.Most and his team had been working on similar simulations for about two years using supercomputers without GPUs before using Perlmutter. “That’s what unlocked the problem,” Most says. “With GPUs, suddenly, everything worked and matched our expectations. We just did not have enough computing power before to numerically model these highly complex physical systems in a sufficient detail.”

Simulation Secrets

The initial cracking simulation illustrates the events as a neutron star approaches a black hole. The black hole’s gravitational forces shear the neutron star’s surface,causing it to fracture.Neutron stars possess strong magnetic fields, and when their surfaces break due to tidal forces, these magnetic fields fluctuate, creating magnetic ripples known as Alfvén waves, named after Hannes Alfvén, who won the 1970 Nobel Prize in Physics for his work on magnetohydro