GWX-EOS Working Group

Objectives

Neutron star (NS) is the densest star in the universe. Due to a higher density environment than nuclear density (≃0.17 fm⁻³), various kinds of matter can be considered to appear in NSs, where the equations of state (EOSs) to describe the NS structure and evolution are highly uncertain despite many observational constraints. In particular, the interior of mature NSs with low temperature (to MeV) is believed to include quantum states such as superfluidity (SF) and superconductivity, which are electrically neutral and charged frictionless quantum liquids, respectively. These states include S-wave SF neutrons in the crust, P-wave SF neutrons, and possibly color superconductors of quarks in the core. The description of such high-density exotic states is associated with the nuclear physics of both experiment and theory. Since SF has been actively studied as condensed-matter physics for various terrestrial systems, such as liquid ³He, ⁴He and ultracold atomic gases, the knowledge of SF from these outcomes has also contributed to the current understanding of NS properties. Therefore, it is quite important to understand many fields of physics to uncover the “black box” in the NS, namely EOS.

In practice, we cannot directly see the interior of NSs but can probe it through observations, even though the microscopic physical effects, such as the SF effects, are on a much smaller scale than the NS phenomena. For example, many pulsars exhibit a sudden increase in rotational frequency known as glitch, which is believed to be triggered by the sudden unpinning of SF vortices. The first observation of the glitch in Vela shows a long relaxation time after the glitch, which suggests evidence of a crust SF state. As another example, after the NS is born, it continues to cool down due to neutrino loss, and the observed surface temperature varies drastically depending on the state of the NS interior. In fact, the cooling rate observed in Cassiopeia A may indicate rapid cooling caused by strong SF effects in the core. Thus, understanding the microscopic role helps to elucidate the macroscopic mechanism of NS phenomena.

On the observational side, the development of multi-wavelength detectors for X-rays and gravitational waves (GWs) has significantly advanced the observation of NSs in recent years. For example, the ultra-small X-ray satellite, NinjaSat, has successfully carried out long-term observations of several X-ray transients, revealing features that distinguish it from similar observations. NICER provides a variety of information on NS EOS constraints. In addition, in the near future, several X-ray satellites, such as the eXTP, Athena, NinjaSat2, and Lynx, will be launched to explore the structure and evolution of NS. Thus, more comprehensive theoretical studies on EOSs and astrophysical simulation are demanded.

Regarding the GWs, the 4^th observing (O4) run by the LIGO-Virgo-KAGRA collaboration is currently underway, which may provide new data on GW associated with NSs. Moreover, O5 is also scheduled to begin in late 2027. In these times, there is an urgent need for more accurate theoretical predictions for various GWs associated with NSs. The third-generation detectors, such as Cosmic Explorer in the U.S. and Einstein Telescope in Europe, are also planned to be constructed in 2030s. These detectors will enable us to detect GWs from the post-merger phase of binary NS mergers, which is the only direct probe from the dense matter. The observations of gravitational waves from supernova explosions and neutron star binary mergers may tell us about properties in higher-density regions by identifying them with specific modes.

Given the motivation to understand NS phenomena in microphysics and their relevance to recent and future observations, we need a forum for in-depth discussion among experts in various fields ranging from condensed matter physics and nuclear physics to astrophysics, focusing on the studies of SF properties and their connection with astrophysical phenomena in NSs. Hence, we decided to launch this working group to learn and share the knowledge and information of microphysics and macrophysics related to NSs. We plan to hold several invited seminars and domestic/international workshops as the activity of GWX-EOS, and we believe that these activities will increasingly stimulate discussion and collaboration in iTHEMS more and more.

Facilitators:

Akira Dohi (RIKEN iTHEMS) – Contact: akira.dohi@riken.jp

Yuki Fujimoto (RIKEN iTHEMS/RIKEN-Berkeley Center)

Hajime Sotani (Kouchi Univ. / RIKEN iTHEMS)