The Aftermath of a Neutron Star Collision Resembles the Conditions in the Early Universe

Neutron stars are extraordinarily dense objects, the densest in the Universe. They pack a lot of matter into a small space and can squeeze several solar masses into a radius of 20 km. When two neutron stars collide, they release an enormous amount of energy as a kilonova.

That energy tears atoms apart into a plasma of detached electrons and atomic nuclei, reminiscent of the early Universe after the Big Bang.

Even though kilonova are extraordinarily energetic, they’re difficult to observe and study because they’re transient and fade quickly. The first conclusive kilonova observation was in 2017, and the event is named AT2017gfo. AT stands for Astronomical Transient, followed by the year it was observed, followed by a sequence of three letters that are assigned to uniquely identify the event.

New research into AT2017gfo has uncovered more details of this energetic event. The research is “Emergence hour-by-hour of r-process features in the kilonova AT2017gfo.” It’s published in the journal Astronomy and Astrophysics, and the lead author is Albert Sneppen from the Cosmic Dawn Center (DAWN) and the Niels Bohr Institute, both in Copenhagen, Denmark.

A kilonova explosion creates a spherical ball of plasma that expands outward, similar to the conditions shortly after the Big Bang. Plasma is made up of ions and electrons, and the intense heat prevents them from combining into atoms.

However, as the plasma cools, atoms form via nucleosynthesis, and scientists are intensely interested in this process. There are three types of nucleosynthesis: slow neutron capture (s-process), proton process (p-process), and rapid neutron capture (r-process). Kilonovae form atoms through the r-process and are known for forming heavier elements, including gold, platinum, and uranium. Some of the atoms they form are radioactive and begin to decay immediately, and this releases the energy that makes a kilonova so luminous.

This study represents the first time astronomers have watched atoms being created in a kilonova.

“For the first time we see the creation of atoms.”

Rasmus Damgaard, co-author, PhD student at Cosmic DAWN Center

Things happen rapidly in a kilonova, and no single telescope on Earth can watch as it plays out because the Earth’s rotation removes it from view.

“This astrophysical explosion develops dramatically hour by hour, so no single telescope can follow its entire story. The viewing angle of the individual telescopes to the event is blocked by the rotation of the Earth,” explained lead author Sneppen.

This research is based on multiple ground telescopes that each took their turn watching the kilonova as Earth rotated. The Hubble also contributed observations from its perch in low-Earth orbit.

“But by combining the existing measurements from Australia, South Africa and The Hubble Space Telescope, we can follow its development in great detail,” Sneppen said. “We show that the whole shows more than the sum of the individual sets of data.”

As the plasma cools, atoms start to form. This is the same thing that happened in the Universe after the Big Bang. As the Universe expanded and cooled and atoms formed, light was able to travel freely because there were no free electrons to stop it. AT2017gfo produced

The research is based on spectra collected from 0.5 to 9.4 days after the merger. The observations focused on optical and near-infrared (NIR) wavelengths because, in the first few days after the merger, the ejecta is opaque to shorter wavelengths like X-rays and UV. Optical and NIR are like open windows into the ejecta. They can observe the rich spectra of newly-formed elements, which are a critical part of kilonovae.

The P Cygni spectral line is also important in this research. It indicates that a star, or in this case, a kilonova, has an expanding shell of gas around it. It’s both an emission line and an absorption line and has powerful diagnostic capabilities. Together, they reveal velocity, density, temperature, ionization, and direction of flow.

Strontium plays a strong role in this research and in kilonovae. It produces strong emission and absorption features in Optical/NIR wavelengths, which also reveal the presence of other newly formed elements. These spectral lines do more than reveal the presence of different elements. Along with P Cygni, they’re used to determine the velocity of the ejecta, the velocity structures in the ejecta, and the temperature conditions and ionization states.

The spectra from AT2017gfo are complex and anything but straightforward. However, in all that light data, the researchers say they’ve identified elements being synthesized, including Tellurium, Lanthanum, Cesium, and Yttrium.

“We can now see the moment where atomic nuclei and electrons are uniting in the afterglow. For the first time we see the creation of atoms, we can measure the temperature of the matter and see the micro physics in this remote explosion. It is like admiring the cosmic background radiation surrounding us from all sides, but here, we get to see everything from the outside. We see before, during and after the moment of birth of the atoms,” says Rasmus Damgaard, PhD student at Cosmic DAWN Center and co-author of the study.

“The matter expands so fast and gains in size so rapidly, to the extent where it takes hours for the light to travel across the explosion. This is why, just by observing the remote end of the fireball, we can see further back in the history of the explosion,” said Kasper Heintz, co-author and assistant professor at the Niels Bohr Institute.

The kilonova produced about 16,000 Earth masses of heavy elements, including 10 Earth masses of the elements gold and platinum.

Neutron star mergers also create black holes, and AT2017gfo created the smallest one ever observed, though there’s some doubt. The gravitational wave GW170817 is associated with the kilonova and was detected by LIGO in August 2017. It was the first time a GW event was seen in conjunction with its electromagnetic counterpart. Taken together, the GW data and other observations suggest that a black hole was created, but overall, there’s uncertainty. Some researchers think a magnetar may be involved.

Kilonovae are complex objects. They’re like mini-laboratories where scientists can study extreme nuclear physics. Kilonovae are important contributors of heavy elements in the Universe, and researchers are keen to model and understand how elements are created in these environments.