## Gravitational Waves from Neutron Star Mergers and Research at UTK

Note: We're not a part of the LIGO/VIRGO collaboration and they have done excellent work which you should read about on the LIGO or VIRGO collaboration pages.

## What Happened?

130 million years ago, two neutron stars merged into each other eventually forming what is likely to be a black hole around three times the mass of the sun. The cataclysmic event generated $10^{52}$ ergs of energy, most of it emitted in gravitational waves. Those gravitational waves traveled outwards from the merger in all directions at the speed of light, and eventually were detected on Earth on August 17th, 2017 by three gravitational wave detectors. This event also generated gamma, X-ray, ultraviolet, optical, infrared, and radio photons which traveled to Earth and were observed for the weeks following August 17th.

## Why Is This Interesting?

• Mergers like this are likely to be the source of many of the heaviest elements here on earth.

Iodine, platinum, gold, uranium, plutonium and several other elements may be created in neutron star mergers just like the one observed. That is, the nuclei of all gold atoms on Earth have been unchanged since Earth was originally formed. These gold nuclei were created in neutron star mergers before the solar system was formed, the mergers ejected these gold nuclei into the surrouding space and eventually became part of the material that formed our solar system.

Iodine, in turn, is important for the human body. (Think of "iodized salt".) Your body is partially made of ashes from a previous neutron star merger (and also from the ashes from previous supernovae and also leftovers from the big bang).

• Neutron star mergers will eventually teach us more about nuclei here on earth.

It turns out that the size of a nucleus is extremely difficult to measure. We can measure the "charge radius", the size of the proton sphere relatively well. However, heavy nuclei have more neutrons than protons, thus the actual size of the nucleus is determined by how far out the neutrons are from the center, i.e. the "neutron radius" [1]. (The neutron radius of nuclei is being measured by the PREX II and CREX experiments at Jefferson Lab.) The neutron radius in heavy nuclei may be correlated with the radius of all neutron stars in the universe [2]. In turn, neutron star radii affect neutron star mergers. Using this chain backwards we may be able to use neutron star mergers to learn about heavy nuclei.

Nuclei, in turn, make up all atoms on Earth, so understanding them is an important goal in nuclear physics.

• The electromagnetic signal is an impressive confirmation of a recent prediction.

The fact that neutron stars merge is now somewhat unsurprising. Hulse and Taylor got a Nobel Prize in 1993 for showing that the two neutron stars B1913+16 are spiraling in towards each other. While this particular pair of neutron stars won't merge for another 5 billion years, there are many neutron star pairs like this.

What was much less well understood until recently was the electromagnetic signal (the flash of light) which would accompany a neutron star merger. Astronomers believe they have detected these mergers in events called "short gamma-ray bursts".

Probability distributions (arbitrary normalization) for the tidal deformability (here called $\lambda$ ) of a 1.4 solar mass neutron star from our paper published in January 2015

## How is this Connected to Research at UTK?

• At UTK, we have generated predictions for the neutron star tidal deformability (you can think of this kind of like "squishiness").

Two large nearly-spherical objects generate tidal forces when they are brought close to each other. The ocean tides are generated principally by the proximity of the earth to the moon. These tidal forces cause the oceans to change, but they also cause the solid part of the earth to deform slightly. The earth becomes a bit smaller in one direction and a bit larger in the other direction because of the gravitational pull of the moon. The extent to which the earth deforms because of the moon is called the earth's "tidal deformability". An interesting discussion of this effect is available here.

Neutron stars, when they get close enough to each other, also create tidal forces and deform according to their tidal deformability. This "squishiness" affects the gravitational wave signal in a merger. LIGO was not yet able to obtain a measurement of the tidal deformability, but they were able to show that it has an upper limit. According to the LIGO paper published in Physical Review Letters, this upper limit is inside the upper end of our predicted range for the tidal deformability (see the second paragraph in the right-hand column of page 6 of the LIGO paper).

UTK predictions for the gravitational wave signal from a core-collapse supernova from our from new paper.
• At UTK, we have generated predictions for the gravitational wave generated in a core-collapse supernova

There is a tremendous potential for LIGO to provide information about the end times of massive stars. Stars larger than about eight times the mass of the sun end their lives in a cataclysmic explosion referred to as a core-collapse supernova. The best example is "Supernova 1987A" the death of a star in the Large Magellenic Cloud observed in January of 1987. The observation of the light and neutrinos from this explosion was a landmark in modern astronomy.

These stellar explosions also generate gravitational waves. Such a detection would be important because it would provide new information about how core-collapse supernovae work. Researchers Konstantin Yakunin, Tony Mezzacappa, and others at UTK have been generating predictions for what LIGO will detect when they observe their first core-collapse supernovae. They have been working closely with members of the LIGO Scientific Collaboration and have led an effort to organize the worldwide core collapse supernova modeling community to provide needed theoretical input to this and the European-counterpart, Virgo Scientific Collaboration. LIGO observations may answer the fundamental question about supernovae which has lingered for over 30 years, "How do stars explode?".

Finally, UTK has just obtained a \$1.2 million grant from the Department of Energy to simulate neutron star mergers and core-collapse supernovae.

As described here, UTK is a part of the TEAMS collaboration, led by Raph Hix at Oak Ridge National Laboratory, which is building a new set of computer simulations of mergers and supernovae which will be important for understanding not only this current neutron star merger event but also LIGO detections in the future.

## Notes

• Note 1: This is a simplification, since nuclei are only properly described using quantum mechanics. What we mean by "neutron radius" here is really the radial extent of the wave functions for all of the neutrons. The first people to demonstrate that this correlation is present were Charles Horowitz and Jorge Piekarewicz in an article published in 2001.
• Note 2: This correlation may be weak if neutron stars contain significant amounts of exotic matter other than neutrons or protons, such as hyperons or deconfined quarks.