Neutron stars are among the most extraordinarily dense objects in the Universe, with an unparalleled capacity to compress vast amounts of matter within a compact space. These stars can contain several solar masses within a radius of merely 20 kilometers. Upon collision, two neutron stars emit a colossal quantity of energy in the form of a kilonova.
This release of energy is sufficiently intense to dismantle atoms, generating a plasma of free electrons and atomic nuclei—an environment reminiscent of the primordial state of the Universe shortly after the Big Bang.
Despite the high energy output of kilonovae, observing and analyzing them poses significant challenges due to their transient nature and rapid fade. The first confirmed observation of a kilonova event occurred in 2017, designated as AT2017gfo. In this nomenclature, “AT” represents Astronomical Transient, followed by the observation year and a unique three-letter identifier.
Recent research on AT2017gfo has yielded further insights into the mechanisms underlying this remarkable and 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 initiates the expansion of a spherical plasma, resembling the high-energy conditions present in the early Universe following the Big Bang. This plasma, composed of ions and electrons, remains in an atomic-free state due to the extreme temperatures that inhibit atomic bonding.
As the plasma begins to cool, atoms emerge through nucleosynthesis, a process of profound interest to scientists. There are three primary nucleosynthesis pathways: slow neutron capture (s-process), proton capture (p-process), and rapid neutron capture (r-process). In kilonovae, the r-process dominates, facilitating the formation of heavy elements such as gold, platinum, and uranium. Among the atoms produced, some are radioactive, undergoing decay shortly after formation, which in turn releases the energy responsible for the characteristic luminosity of a kilonova.
This study marks the first observation of atom formation within a kilonova, offering a unique insight into nucleosynthesis in these highly energetic stellar events.
“For the first time we see the creation of atoms.” Rasmus Damgaard, co-author, PhD student at Cosmic DAWN Center
Events in a kilonova unfold rapidly, and no single terrestrial telescope can continuously observe the phenomenon, as Earth’s rotation periodically moves it out of 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 relied on coordinated observations from multiple ground-based telescopes, each taking turns to monitor the kilonova as Earth rotated. Additionally, the Hubble Space Telescope contributed critical data from its vantage point 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 begin to form, mirroring a similar process that occurred in the early Universe after the Big Bang. During this epoch, as the Universe expanded and cooled, the formation of atoms allowed light to propagate freely, no longer hindered by free electrons. The kilonova event AT2017gfo exhibited a comparable process.
This research analyzed spectral data collected from 0.5 to 9.4 days following the merger, focusing on optical and near-infrared (NIR) wavelengths. In the initial days after the merger, the ejecta remains opaque to shorter wavelengths, such as X-rays and ultraviolet (UV) light, rendering these wavelengths ineffective for observation. In contrast, optical and NIR wavelengths act as “open windows” into the ejecta, allowing scientists to observe the detailed spectra of newly-formed elements, which are essential in understanding the composition and nature of kilonovae.
The P Cygni spectral line is a key diagnostic feature in this research, signifying the presence of an expanding shell of gas around the kilonova. This line consists of both emission and absorption components, providing a rich source of information on several key parameters: velocity, density, temperature, ionization state, and flow direction.
Strontium plays a prominent role in kilonovae and this study, producing distinct emission and absorption features within optical and near-infrared wavelengths. These spectral signatures not only confirm the presence of strontium but also help identify other newly formed elements. The spectral lines, in conjunction with the P Cygni profile, enable researchers to determine the ejecta’s velocity, the internal velocity structures, as well as temperature and ionization conditions within the expanding material.
The spectra of AT2017gfo are intricate and challenging to interpret. However, within this data, researchers have detected synthesized elements, including tellurium, lanthanum, cesium, and yttrium. This discovery provides invaluable insight into the nucleosynthetic processes occurring within kilonovae.
“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 approximately 16,000 Earth masses of heavy elements, including around 10 Earth masses of gold and platinum.
Neutron star mergers often result in black hole formation, and the AT2017gfo event is believed to have created the smallest black hole ever observed, although this conclusion remains tentative. The gravitational wave event GW170817, detected by LIGO in August 2017, is associated with this kilonova and marked the first instance of a gravitational wave detection being observed alongside an electromagnetic counterpart. Combined, the gravitational wave data and accompanying observations suggest that a black hole may have formed; however, uncertainty remains. Some researchers propose that a magnetar could have been involved instead.
- See also: NASA’s Hubble Sees a Stellar Volcano
Kilonovae are intricate and dynamic astrophysical phenomena, serving as natural laboratories for investigating extreme nuclear physics. They play a crucial role in the cosmic production of heavy elements, making them essential to our understanding of nucleosynthesis. Researchers are particularly interested in modeling the processes within kilonovae to gain deeper insight into the mechanisms through which elements are synthesized in such extreme environments.
