Across the cosmos, many stars can be found in pairs, gracefully circling one another. Yet one of the most dramatic pairings occurs between two orbiting black holes, formed after their massive progenitor stars exploded in supernova blasts. If these black holes lie close enough together, they will ultimately collide and form an even more massive black hole. Sometimes a black hole is orbited by a neutron star-the dense corpse of a star also formed from a supernova explosion but which contains less mass than a black hole. When these two bodies finally merge, the black hole will typically swallow the neutron star whole.
To better understand the extreme physics underlying such a grizzly demise, researchers at Caltech are using supercomputers to simulate black hole-neutron star collisions. In one study appearing in The Astrophysical Journal Letters , the team, led by Elias Most , a Caltech assistant professor of theoretical astrophysics, developed the most detailed simulation yet of the violent quakes that rupture a neutron star's surface roughly a second before the black hole consumes it.
"The neutron star's crust will crack open just like the ground in an earthquake," Most says. "The black hole's gravity first shears the surface, causing quakes in the star and the opening of rifts."
While cracks in the crust of a neutron star had been predicted before, the simulation is the first to demonstrate what kinds of light flares astronomers might see in the future when pointing telescopes in space and on the ground at such an event.
"This goes beyond educated models for the phenomenon-it is an actual simulation that includes all the relevant physics taking place when the neutron star breaks like an egg," says co-author Katerina Chatziioannou , assistant professor of physics at Caltech and a William H. Hurt Scholar.
Zoom In to Image These three panels are taken from a supercomputer simulation of a merger between a black hole (large black circle) and a neutron star (colored blob). The images, which move forward in time from left to right, show how the intense gravity of the black hole stretches the neutron star, before the black hole ultimately consumes it. Credit: Elias Most/Caltech
In a second, more recent paper in The Astrophysical Journal Letters , published March 31 of this year, the team used a supercomputer to simulate what happens after the neutron star fractures-a brief milliseconds-long window when monster shock waves, the most powerful predicted shock waves in the universe, shoot outward from the star. These monster shock waves had only recently been predicted by co-author Andrei Beloborodov of Columbia University. Now, the simulation, along with another from a different study published by the team last year, are the first to show how they form.
What is more, the most recent simulation does not stop when the monster shock waves form-it proceeds to show the neutron star being swallowed, which then triggers the creation of an exotic object called a "black hole pulsar."
A classic pulsar is a highly magnetized neutron star that emits beams of radiation, which sweep around like a lighthouse beacon as the star spins on its axis. A black hole pulsar is a hypothetical object in which a black hole launches magnetic winds that would also sweep around it as it spins, mimicking the appearance of a pulsar. While black hole pulsars had been previously conjectured, the simulation is the first to show how such a rare object could actually form in nature from the collision of a neutron star and black hole.
"When the neutron star plunges into the black hole, the monster shock waves are launched," says Yoonsoo Kim (MS '24), a Caltech graduate student working with Most, and lead author of the study on monster shock waves and black hole pulsars. "After the star is sucked in, whipping winds are formed, creating the black hole pulsar. But the black hole cannot sustain its winds and will become quiet again within seconds."
