When two neutron stars collide, sometimes the result is a black hole that swallows all but the gravitational evidence of the collision. In a series of simulations, an international team of researchers, including a scientist from Penn State, found that these normally quiet collisions can sometimes be far louder, at least in terms of the radiation we can detect on Earth.
“When two incredibly dense collapsed neutron stars combine to form a strong black hole Gravitational waves arise from impact, ”said David Radice, assistant professor of physics and astronomy and astrophysics at Penn State and a member of the research team. “We can now record these waves with detectors such as LIGO in the United States and Virgo in Italy. A black hole usually swallows any other radiation that might come from the fusion that we could detect on Earth. However, through our simulations, we have found that this may not always be the case. ”
The research team found that if the masses of the two colliding neutron stars are different enough, the larger companion will tear the smaller one apart. This leads to a slower fusion in which an electromagnetic “bang” can escape. Astronomers should be able to capture this electromagnetic signal, and the simulations provide signatures for those noisy collisions that astronomers could look for from Earth.
The research team, made up of members of the international CoRe (Computational Relativity) collaboration, describes its results in an online article in the Monthly releases from the Royal Astronomical Society.
Through a series of simulations, an international team of researchers has found that some fusions of neutron stars generate radiation that should be detectable from Earth. When neutron stars of unequal mass merge, tidal forces tear the smaller star apart from its massive companion (left). Most of the smaller partner’s mass falls onto the massive star, causing it to collapse and form a black hole (center). But some of the material is ejected into space; The rest falls back and forms a massive accretion disk around the black hole (right). Photo credits: Adapted from Figure 4 in “Accretion-induced immediate formation of black holes in asymmetrical neutron star fusions, dynamic ejecta and kilonova signals”. Bernuzzi et al., Monthly Bulletin of the Royal Astronomical Society.
“LIGO recently announced the discovery of a fusion event where the two stars may have very different masses,” said Radice. “The main consequence in this scenario is that we expect this very characteristic electromagnetic counterpart to the gravitational wave signal.”
After the LIGO team reported the first discovery of a neutron star fusion in 2017 in 2017, it reported the second, which they named GW190425. The result of the 2017 collision was roughly what astronomers expected, with a total mass about 2.7 times the mass of our Sun and each of the two neutron stars being roughly the same size. GW190425 was much heavier, however, with a combined mass of about 3.5 solar masses and a more unequal ratio of the two participants – possibly even 2 to 1.
“While a 2 to 1 mass difference doesn’t seem like a big difference, only a small mass range is possible for neutron stars,” said Radice.
Neutron stars can only exist in a narrow mass range between 1.2 and 3 times the mass of our sun. Lighter star remnants do not collapse into neutron stars and instead form white dwarfs, while heavier objects collapse directly into black holes. When the difference between the merging stars becomes as big as it did in GW190425, scientists suspected that the merger might be more chaotic – and louder with electromagnetic radiation. Astronomers had not detected such a signal from the GW190425’s location, but conventional telescope coverage of this area of the sky that day wasn’t good enough to rule it out.
To understand the phenomenon of disparate neutron stars colliding and to predict signatures of such collisions that astronomers might look for, the research team ran a series of simulations using the Bridges platform at the Pittsburgh Supercomputing Center and the Comet platform at the San Diego Supercomputer Center – both in the National Science Foundation’s XSEDE network of supercomputing centers and computers as well as other supercomputers.
The researchers found that the gravity of the larger star tore its partner apart as the two simulated neutron stars moved towards each other. That meant the smaller ones Neutron star did not meet his more massive companion at once. The initial deposition of matter from the smaller star turned the larger one into a black hole. But the rest of his business was too far for the black hole to catch on immediately. Instead, the slower rain of matter into the black hole created a flash of electromagnetic radiation.
The research team hopes that the simulated signature they find can help astronomers use a combination of gravitational wave detectors and conventional telescopes to identify the paired signals that would herald the breakup of a smaller neutron star merging with a larger one.
The simulations required an unusual combination of computing speed, enormous amounts of memory and flexibility in moving data between memory and computation. The team used around 500 cores that ran across around 20 separate instances for weeks. The many physical quantities that had to be taken into account in every calculation required around 100 times as much memory as a typical astrophysical simulation.
“There is a lot of uncertainty about the properties of neutron stars,” said Radice. “To understand them, we have to simulate many possible models to see which ones are compatible with astronomical observations. A single simulation of a model wouldn’t tell us much; We have to do a large number of rather computationally intensive simulations. We need a combination of high capacity and high capacity that only machines like bridges can offer. This work would not have been possible without access to such national supercomputing resources. ”
Reference: “Accretion-induced immediate formation of black holes in asymmetrical neutron star fusions, dynamic ejecta and kilonova signals” by Sebastiano Bernuzzi, Matteo Breschi, Boris Daszuta, Andrea Endrizzi, Domenico Logoteta, Wsewolod Nedora, Albino Perego, David Radice, Federico Schianchi, Francesco Zappa, Ignazio Bombaci and Nestor Ortiz, June 27, 2020, Monthly releases from the Royal Astronomical Society.
DOI: 10.1093 / mnras / staa1860
