Gravitational Wave and Equation of State Working Group (February 1st 2024)


Owing to direct detections of gravitational waves (GWs) from compact binary mergers, GWs now become important information to obtain the astronomical properties together with electromagnetic waves and neutrinos, i.e., multimessenger observations. Next to compact binary mergers, a supernova explosion, which happens at the end of the massive star’s life, is a promising candidate. On the other hand, since the density inside a neutron star, which is produced via a supernova, significantly exceeds the standard nuclear density, the equation of state (EOS) for neutron star matter is still unfixed from the terrestrial experiments. GWs and EOS are subject not only in astrophysics but also strongly associated with nuclear physics and computational science. Thus, interdisciplinary collaborations are absolutely necessary, where GWs and EOS, including neutron star physics, must be one of the significant subjects to discuss as iTHEMS working group. 

The GW detector network with Laser Interferometer GW Observatory (LIGO), Virgo, and the Japanese detector, KAGRA, is now in operation as the fourth observing run (O4 run) since May 2023. In addition to the current operating detectors, discussion for the third-generation GW detectors, such as the Einstein Telescope and Cosmic Explorer, has already been started. With these detectors, it is expected that the number of GW events from compact binary mergers will drastically increase more and more. Furthermore, the GWs from core-collapse supernova explosions might be detected, using higher-performance detectors. 

Due to the nature of the high permeability of GWs, one expects to get raw information on source objects through GW observations. That is, via GW observations, one may be able to see (or hear) “invisible” interior properties. This is one of the significances in the GW detections. In order to discuss such properties, we especially focus on the following points in this working group:

  1. Understanding the GW spectrum from the core-collapse supernova
  2. Finding the universal relation in GW frequencies
  3. Discussion and exploration of the newly obtained GW data.

In addition to these topics, we are also considering to discuss a neutron star itself from the microscopic point of view, because it must be helpful to understand the above topics. The neutron star is the densest in the universe. Due to low temperature (≲ MeV) and higher density environment than nuclear density (≃ 0.16 fm-3), the interior of a mature neutron star is believed to include the superfluid and superconductive states, which are inviscid quantum liquids electrically neutral and charged, respectively. Even so, we cannot directly see the interior of neutron stars but can probe it through only observations, e.g., glitches, neutron star cooling, or maybe GWs. The superfluid effect, which is a quantum effect, is on a much smaller scale, but it plays an important role in neutron star phenomena. Thus, understanding the microscopic role of superfluid properties helps us clarify the macroscopic mechanism of neutron star physics. So, in addition to the above topics, we are also discussing the following topics:

  1. Understanding the superfluidity in the neutron stars and its effects.

This working group has been already launched since Feb. 2020, where we have discussed many subjects associated with GWs and neutron star physics. To advance our discussion further, this time, we propose the extension of the period of this working group.

Hajime Sotani (RIKEN CPR/RIKEN iTHEMS) *Contact at
Tomoya Takiwaki (NAOJ)
Hajime Togashi (Daegu University)