Now that scientists can detect the wiggly distortions in space-time created by the merger of massive black holes, they are setting their sights on the dynamics and aftermath of other cosmic duos that unify in catastrophic collisions.
Working with an international team, scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed new computer models to explore what happens when a black hole joins with a neutron star – the superdense remnant of an exploded star.
Using supercomputers to rip open neutron stars
The simulations, carried out in part at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC), are intended to help detectors home in on the gravitational-wave signals. Telescopes, too, can search for the brilliant bursts of gamma-rays and the glow of the radioactive matter that these exotic events can spew into surrounding space.
In separate papers published in a special edition of the scientific journal Classical and Quantum Gravity, Berkeley Lab and other researchers present the results of detailed simulations.
One of the studies models the first milliseconds (thousandths of a second) in the merger of a black hole and neutron star, and the other details separate simulations that model the formation of a disk of material formed within seconds of the merger, and of the evolution of matter that is ejected in the merger.
That ejected matter likely includes gold and platinum and a range of radioactive elements that are heavier than iron.
Any new information scientists can gather about how neutron stars rip apart in these mergers can help to unlock their secrets, as their inner structure and their likely role in seeding the universe with heavy elements are still shrouded in mystery.
“We are steadily adding more realistic physics to the simulations,” said – Foucart, who served as a lead author for one of the studies as a postdoctoral researcher in Berkeley Lab’s Nuclear Science Division.
“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.”
Finding signs of a black hole–neutron star merger
Foucart, who will soon be an assistant professor at the University of New Hampshire, added, “We are trying to move more toward actually making models of the gravitational-wave signals produced by these mergers,” which create a rippling in space-time that researchers hope can be detected with improvements in the sensitivity of experiments including Advanced LIGO, the Laser Interferometer Gravitational-Wave Observatory.
In February 2016, LIGO scientists confirmed the first detection of a gravitational wave, believed to be generated by the merger of two black holes, each with masses about 30 times larger than the sun.
The signals of a neutron star merging with black holes or another neutron star are expected to generate gravitational waves that are slightly weaker but similar to those of black hole–black hole mergers, Foucart said.
Radioactive ‘waste’ in space
Daniel Kasen, a scientist in the Nuclear Science Division at Berkeley Lab and associate professor of physics and astronomy at UC Berkeley who participated in the research, said that inside neutron stars “there may be exotic states of matter unlike anything realized anywhere else in the universe.”
In some computer simulations the neutron stars were swallowed whole by the black hole, while in others there was a fraction of matter coughed up into space. This ejected matter is estimated to range up to about one-tenth of the mass of the sun.
While much of the matter gets sucked into the larger black hole that forms from the merger, “the material that gets flung out eventually turns into a kind of radioactive ‘waste,’” he said. “You can see the radioactive glow of that material for a period of days or weeks, from more than a hundred million light years away.” Scientists refer to this observable radioactive glow as a “kilonova.”
The simulations use different sets of calculations to help scientists visualize how matter escapes from these mergers. By modeling the speed, trajectory, amount and type of matter, and even the color of the light it gives off, astrophysicists can learn how to track down actual events.