Microscopic Characterization of the Neutron Star Outer Crust in the Era of Multimessenger Physics

Abstract

[EN] In the new era of multimessenger physics, dawned by the photon emission in Supernovae or Gravitational Wave detections, Neutron Stars are playing a crucial role as canonical multimessenger sources. This thesis aims at the characterization of the Equation of State of dense matter, focusing on the outer crust of Neutron Stars, key in some of these events.

After an introduction to the relevant topics and context of NS astrophysics in Chapter 1, in Chapter 2, we explain the theory and technique behind our original Molecular Dynamics code, USAL, developed as part of the work in this thesis. Utilising the efficient Ewald summation technique for long-range electrostatic forces, the code is tailored to simulate the dynamics of ionic layers of the outer crust of the star. We have introduced improvements such as finite size ionic charge distributions that, as embedded in an ultrarelativistic and degenerate electron gas, interact between each other through a screened electrostatic interaction. In Chapter 3, we scrutinize finite-size effects on the crystallization of ionic crust samples, showing how the reduction in binding energy induced by size eventually breaks the crystal lattice produced by electrostatic repulsion. We generalize the definition of the Coulomb melting parameter to include not only the screening parameter but also a new parameter codifying ion finite size. We also report on our ionic Equation of State at finite temperature for the crust, for which we used artificial intelligence techniques to create an efficient method for the community to extract the pressure and energy density information from our simulations. These warm temperatures are studied deeper in Chapter 4, where we analyze finite-size effects on virial coefficients of the Equation of State and the liquid-gas phase transition and study how the system relaxes towards equilibrium at lower temperatures.

In Chapter 5 we report on elastic properties of the astromaterial. We introduce tensile and shear deformations in our simulations and report on the effect of external magnetic field and finite size on them. We compare our obtained mechanical resistance for the outer crust to previous approaches.

In Chapter 6 we explain our efforts to characterize the Equation of State of dense matter in a Neutron Star interior, through the modelling of electromagnetic counterparts associated with Gravitational Wave observations of Binary Neutron Star merger events. These sources are called Kilonovae, and their photometric and spectroscopic properties, are intimately related with the microscopic properties of dense matter. We study correlations of the light curves of Kilonovae with Equation of State and binary parameters, such as compactness or tidal deformability. This synergy between microphysical modelling and observational diagnosis establishes robust constraints on the Equation of State from spectro-photometric Kilonova data. This interdisciplinary work contributes to bridging the gap between nuclear theory and astrophysical observations, providing tools and results relevant for interpreting current data from multimessenger campaigns targeting compact object mergers.

We finalize the manuscript with conclusions and future prospects.