Seminar report:
The multi-messenger picture
of merging neutron stars

Contents:

• The multi-messenger picture of merging neutron starsby Tim Dietrich
• Seminar scheduleSeminar schedule

How to cure a boring day:

Even if our instrument are very advanced today we can't measure everything but we can do computer simulations of extreme environments. This is what this seminar are about and how they simulate neutron star mergers.

Some useful links, Neutron star merger:

• https://en.wikipedia.org/ wiki/ Neutron star merger (Neutron star merger, Wikipedia)

The multi-messenger picture of merging neutron stars:

By Dr. Tim Dietrich

You can read his abstract here:
"With the detection of the binary neutron star merger GW170817, a new era of multi-messenger astronomy started. GW170817 demonstrated that neutron star mergers are ideal laboratories for constraining the equation of state of cold supranuclear matter, to study the central engines of short GRBs, to understand the origin and production of heavy elements, and to measure the expansion rate of the Universe. We discuss how the last milliseconds before and after the merger can be investigated with full 3D numerical relativity simulations. We present results from state-of-the-art numerical relativity simulations covering large areas of the binary neutron star parameter space and we explain how these simulations help to develop gravitational wave and electromagnetic models. These models allow us to constrain the unknown equation of state of cold supranuclear matter and in the future might even be helpful to determine the dark matter content in our Universe. I will conclude with a short review of the first months of the ongoing observing run of the LIGO and Virgo gravitational wave detectors with respect to binary neutron star candidates."

The last 1½ years Tim has been at Nikhef in Amsterdam, before that he was at Max Planck Institute. The seminar was held at Stockholm University 2019.

Tim gives some background about neutron stars. They have a mass from 1.0 to 2.2 of our Sun's mass. The radius is only 10 to 15 km thus very high density, up to hundred million tons per cm³, thus, there are so extreme (and interesting) says Tim. They are possible the end state of a massive star.

Tim explain the reason to study neutron stars.

Some of the reasons:

• Neutron stars provide new nuclear physics insights
• Gravitational Wave Astronomy
• High-Energy astrophysics
• Test of General Relativity, Cosmology, ...

The signals we get from these objects create a multi-messenger picture:

• Gravitational Waves
• Electromagnetic Waves
• Neutrinos

We don't get a complete image of the neutron stars from our instruments, especially when they merge. But it can be simulated with help of super computers. The field equations that are used is from Albert Einstein's General relativity.

Some useful links, General relativity:

• https://en.wikipedia.org/ wiki/ General relativity (General relativity, Wikipedia)

Lars:
I remember these equations from my study of cosmology, not easy to handle or understand.

Tim tells:
We can do measurement along the yellow line, just before it collapse when the neutron stars merge. After that the signal is too weak, but we can do simulation.

Lars:
BNS stands for Binary Neutron Star, the object GW 170817 is that. On my question how they handle how the neutron star loss their energy and merge in the simulation Tim answer: The orbit shrinks and the stars merge because of the radiation of gravitational waves. In the simulations, we really solve Einstein's field equations and therefore the emission of gravitational waves is included, which then drives the stars to merger.

Tim tells:
As seen from the image, different initial parameters give very different gravitational wave signals. Examples of: equal mass, high mass ratio, high mass, processing, viscous hydrodynamics and eccentric. Normally, numerical relativity simulations take months even with the best existing algorithms running on big supercomputers. The time scale of the simulation range up to 250 milliseconds.

Tim tells:
All these data are collected in a database, it's free to use.

• http://www.computational-relativity.org/ (CoRe Computational Relativity)

Tim tells:
Postmerger signal at high frequencies are hard to detect with todays instrument.

Lars:
Signals from LIGO and Virgo gravitational waves instrument.

Tim explains that merging neutron stars will also create electromagnetic signals that can be detected. This was the case for GW170817 and its counterpart AT2017gfo.

Can we role out another origin for GW 170817 ?

Tim explains the LIGO / Virgo source classification and say: The classification contains also binary black holes and black hole - neutron stars systems etc. and there have also been triggers that fall inside the green region.

If you want to know more about Tim's research I have found a couple of links:

• https://www.aei.mpg.de/ 2033702/ thesis-prize-for-tim-dietrich (Simulation of mergers, Max Planck Institute)
• https://www.gauss-centre.eu/ results/ astrophysics/ article/ binary-neutron-star-merger-simulations/ (Binary Neutron Star Merger Simulations, GCS)
• https://www.db-thueringen.de/ receive/ dbt mods 00029293 (Binary Neutron Star Merger Simulations, db thuringen)
• https://www.nikhef.nl/ ~diettim/ (Tim Dietrich, Nikhef)
• https://www.youtube.com/ watch?v= FGC_DM7ZgAk (Simulation of the binary black-hole coalescence GW170104, Youtube)

These seminars are a good cure to my boring days. Do you have boring days too? Join me next time!

• https://www.nordita.org/ events/ seminars/ schedule/ (Theory seminar schedule, Nordita)

If you don't live in Stockholm in Sweden as I do I'm sure that you can find something similar to visit at your place. Or you do as all other do, search the internet, TED Talks is good.

• https://www.ted.com/ (TED Talks)

No more boring days!