Neutron stars are the remnants of massive stars that explode as supernovae at the end of their fusion lives. They’re super-dense cores where all of the protons and electrons are crushed into neutrons by the overpowering gravity of the dead star. They’re the smallest and densest stellar objects, except for black holes, and possibly other arcane, hypothetical objects like quark stars.
When two neutron stars merge, we can detect the resulting gravitational waves. But some aspects of these mergers are poorly-understood. One question surrounds short-lived gamma-ray bursts from these mergers. Previous studies have shown that these bursts may come from the decay of heavy elements produced in a neutron star merger.
A new study strengthens our understanding of these complex mergers and introduces a model that explains the gamma rays.
The new research is titled “A Magnetar Engine for Short GRBs and Kilonovae.” The lead author is Philipp Mösta from the University of Amsterdam. It’s published in The Astrophysical Journal Letters.
When two neutron stars merge it creates a kilonova. Astrophysicists think that kilonovae explosions emit short gamma radiation bursts. The merger also produces heavy elements, which are sources of powerful electromagnetic energy as they decay.
Artist’s impression of neutron stars merging, producing gravitational waves, and resulting in a kilonova. Image Credit: By University of Warwick/Mark Garlick, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=63436916
Some neutron stars have extremely powerful magnetic fields. Those stars are called magnetars. The magnetic field around a magnetar can be a trillion times more powerful than Earth’s. Magnetars also rotate more slowly than other neutron stars. Their powerful magnetic fields decay after about 10,000 years, and researchers think that about one in ten supernovae result in magnetars.
Neutron star mergers can also create magnetars, and that’s what this new study is focused on.
Neutron star mergers are a relatively new field of study. While long-theorized, it wasn’t until 2017 that the first one was observed. Thanks to observations of these mergers, astrophysicists have confirmed things that were once only theorized.
In the introduction to their paper, the researchers outline some of what’s already known about kilonovae. “Radioactive material ejected during and after the merger powers a kilonova transient and creates the heaviest elements in the universe. Jetted outflows from the merger remnant can launch a short gamma-ray burst.”
They also point out two key questions they hoped to address: How do these mergers generate fast enough outflows to explain the observed blue kilonova component in the 2017 kilonova event. The blue component refers to an optical afterglow detected in the 2017 kilonova that is absent from other observed short Gamma-Ray bursts. And can magnetars launch short Gamma-Ray Burst jets?
This animation is based on a series of spectra from the 2017 kilonova observed by the X-shooter instrument on ESO’s Very Large Telescope in Chile. They cover a period of 12 days after the initial explosion on 17 August 2017. The kilonova is very blue initially but then brightens in the red and fades.
Credit:ESO/E. Pian et al./S. Smartt & ePESSTO/L. Calçada
It’s a star’s job to synthesize heavier elements. And even though neutron stars have left fusion behind, they still have one final encore of elemental synthesis. When they merge, they create elements like Strontium and Gold. How do these stars do it?
The team of researchers created a more detailed model of neutron star mergers than ever before. They included variables like the theory of relativity, magnetic fields, neutrino effects, gas laws, and nuclear physics. They ran their simulations on two supercomputers: the Frontera supercomputer at the University of Texas, Austin, and the Blue Waters supercomputer at the University of Illinois.
Their simulations revealed some new details in the mergers. The merger creates a ring around the stars, and gamma radiation travels up and down that ring in thin strands. The merged stars create an intense magnetic field, and eventually the gamma radiation travels away along the field lines.
There’s also an hourglass-shaped cone that moves up and down. Inside that hourglass, heavier elements like the strontium and gold are created. But the gamma rays are the more surprising result of the simulations.
The research team ran four different simulations in their work, each one with slightly different variables. In three of them especially, the magnetar created an hourglass shape. Heavy elements like strontium and gold are created inside the hourglass. Gamma radiation travels away from the magnetar along the magnetic field lines, which are twisted into a toroidal shape. Image Credit: Mösta et al., 2020.
“The gamma radiation is really new for these kinds of simulations. That radiation had not appeared in the old simulations,” lead author Mösta said in a press release. “The production of heavy elements, such as gold, had already been simulated.”
“However, our simulation shows that these heavy elements move much faster than previously predicted. Our simulation is therefore more in line with what astronomers observed in the merging neutron stars in 2017.”
Their simulations also shed some light on short gamma-ray bursts (sGRB). Previously, researchers thought that the gamma-rays came from the decay of heavy elements synthesized in the kilonova. But this study shows that the magnetar’s intense magnetic fields can amplify the jets of material coming from the magnetar. Those jets reach near-relativistic speeds, releasing sGRBs. “…our simulations indicate that magnetars formed in NS mergers are a promising sGRB engine.
Relativistic jets come not only from neutron star merger remnants. They also come from black holes. This illustration shows relativistic jets of material streaming out of a supermassive black hole. Credit: NASA/Dana Berry, SkyWorks Digital
This research and the powerful new simulations the team created have offered explanations for some of the neutron star mergers and kilonovae observations. But the team of researchers isn’t done yet. The authors plan to expand their simulation to include supernovae explosions and the merger of neutron stars with black holes.