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Noisy neutron stars

Speed read
  • Many neutron star collisions form a black hole that swallows radiation from the merger
  • If the merging stars’ masses are different enough, the result may be noisier
  • This electromagnetic bang could provide a signature to help astronomers detect star mergers

It seems strange to talk about "quiet" versus "noisy" collisions of neutron stars. But many such impacts form a black hole that swallows all but the gravitational evidence. A series of simulations on supercomputers by a Penn State scientist suggested that, when the neutron stars' masses are different enough, the result is far noisier. The model predicts an electromagnetic "bang," which isn't present when the merging stars' masses are similar, that astronomers should be able to detect.

<strong>A neutron star is ripped apart</strong> by tidal forces from its massive companion in an unequal-mass binary neutron star merger.When two objects roughly the mass of the sun and the size of cities slam together, it seems strange to talk about how "quiet" it is. But for many neutron-star collisions, it is quiet, at least in terms of radiation we can detect. A strong surge of gravitational waves emerges from the impact—now being sensed by gravity-wave detectors such as LIGO. But precious little else appears. That's because the incredibly dense collapsed stars combine to form a black hole, which swallows any of the radiation that could have come out of the merger.

But that's not the only way it can play out.

"Recently, LIGO announced the discovery of one [merger] event between two stars with possibly very different masses…The main consequence in this scenario is that we expect this very characteristic electromagnetic counterpart [to the gravitational wave signal]."— David Radice, Penn State

After reporting the first detection of a neutron-star merger in 2017, the LIGO team reported in 2019 the second, which they named GW190425. The first of the two collisions was what astronomers expected, with a total mass of about 2.7 times the mass of our Sun and each of the two neutron stars nearly equal in mass. But GW190425 was heavier, with a combined mass of around 3.5 Solar masses. More importantly, the ratio of the masses of the two participants was more unequal, possibly as high as 2 to 1. 

That may not seem like such a huge difference. But neutron stars can exist only in a narrow range of masses between about 1.2 and 3 times the mass of our Sun. Lighter stellar remnants don't collapse to form neutron stars and form white dwarfs instead. Heavier objects collapse directly to form black holes.

<strong>Unequal partners.</strong> Most of the smaller star’s mass falls onto the more massive star, causing it to collapse and form a black hole.When the difference between the merging stars gets as large as in GW190425, scientists suspected that the merger could be messier—and louder in electromagnetic radiation. Astronomers had detected no such signal from GW190425's location. But coverage of that area of the sky by conventional telescopes that day wasn't good enough to rule it out.

David Radice of Penn State, working as member of CoRe, the Computational Relativity International Collaboration, wanted to better understand the phenomenon of unequal neutron stars colliding and to predict signatures of such collisions that astronomers could look for. To do so, he turned to simulations on a number of supercomputers.

Simulations don't disappoint

To run his simulations, Radice needed an unusual combination of computing speed, large memory, and flexibility in moving data between memory and computation. That's partly because scientists know so little about these mergers for certain. To test their ideas required running about 20 simulations, each of which needed 500 compute cores for several weeks.

<strong>A flash of electromagnetic radiation.</strong> Some of the material from the merged stars is ejected into space. The rest falls back to form a massive accretion disk around the black hole. Radice employed a number of high-performance computing systems for this work, including XSEDE-allocated Stampede2 at the Texas Advanced Computing Center and Blue Waters at the National Center for Supercomputing Applications. But the most useful ones he found for these particular simulations were Comet at the San Diego Supercomputer Center and Bridges at the Pittsburgh Supercomputing Center.

The computations did not disappoint the scientists' expectations of an electromagnetic bang.

As the two simulated neutron stars spiraled in toward each other, the gravity of the larger star tore its partner apart. That meant that the smaller neutron star didn't hit its more massive companion all at once. The initial dump of the smaller star's matter turned the larger into a black hole. But the rest of its matter was too far away for the black hole to capture immediately. Instead, the slower rain of matter into the black hole created a flash of electromagnetic radiation.

"Without the availability of supercomputing power outside of private computing centers, our groups would not be able to tackle important problems in multimessenger astronomy and contribute to the field."—David Radice, Penn State

The group reported their results in the Monthly Notices of the Royal Astronomical Society in June 2020. Their hope is that the simulated signature they found can be used by astronomers using a combination of gravity-wave and conventional telescopes to detect the paired signals that would herald the breakup of a smaller neutron star merging with a larger one.

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