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Three powerful high-performance computers were used by a research team led by David Radice at Pennsylvania State University to create novel neutron star merger simulations. Expanse at the San Diego Supercomputer Center at UC San Diego, Bridges-2 at Pittsburgh Supercomputing Center and Stampede2 at Texas Advanced Computing Center were used by Radice and an international team from the University of Trento in Italy and the University of Jena in Germany to analyze the binary neutron star merger known as GW190425. As the name indicates, it was discovered on April 25, 2019, with the Laser Interferometer Gravitational-Wave Observatory (LIGO).

The massive GW190425 collision occurred when two neutron stars were trapped in one another's gravitational fields and spiraled inward toward each other. During these types of collisions, the results are typically either a black hole or a massive neutron star – with byproducts of heavy elements such as gold, as well as the electromagnetic radiation released from their formation. GW190425, however, was different as illustrated by Radice’s simulations, published this month in the Monthly Notices of the Royal Astronomical Society.

The group performed general relativistic simulations of the merger of neutron star binaries within the range of masses inferred for GW190425 to understand whether or not an electromagnetic signal (optical and infrared) could have accompanied this event. None was found despite a large number of additional studies by worldwide research teams.

When the LIGO/Virgo scientific collaboration announced GW190425, we were surprised by the fact that this binary is so different from the binary neutron star systems in our galaxy,” said Radice, assistant professor at PSU’s Departments of Physics and Astronomy & Astrophysics. “We did not know what to expect for binary neutron star systems from such binaries and were motivated to perform targeted simulations.”

According to Radice, the  study showed that GW190425 must have been very dim in the electromagnetic spectrum. This explains why no explosion was found in optical searches following the gravitational wave signal. “Given the masses of the stars, their collision should have been indicated electromagnetically, as other binary systems with lower mass had been electromagnetically bright; however, it turned out that with these higher mass systems only a small amount of material was able to escape the black hole formed in the collision,” he said.

The researchers’ results showed that, even if current telescopes had been pointed in the right direction, they would not have been able to find the electromagnetic counterpart because it was too dim. Their results will now help the team better understand similar phenomena that occur closer to Earth as well as their repercussions.

“Ours was the first study targeting GW190425 and included a sophisticated treatment of the nuclear forces in the stars and of neutrinos, needed to make quantitative predictions of the electromagnetic signals from the merger,” Radice said. “In the future, we hope to be ready to analyze the next discovery by LIGO, Virgo, or KAGRA, and to be able to explain the presence or absence of electromagnetic radiation.”

This research was funded by the European Union (ERC Starting Grant No. BinGraSp-714626); the Deutsche Forschungsgemeinschaft (DFG) project (MEMI No. BE 6301/2-1.); the U.S. Department of Energy, Office of Science, Division of Nuclear Physics (Award No. DE-SC0021177); the National Science Foundation (Grant Nos. PHY-2011725, PHY-2020275, PHY-2116686, and AST-2108467); and the Fondazione CARITRO, program Bando post-doc 2021 (Project No. 11745).