Session C12: Neutron Stars and Stellar Objects

Pulse profile modeling of thermonuclear burst oscillations

Pulse profile modeling (PPM) is a sophisticated relativistic ray-tracing technique used to deduce key parameters such as the mass, radius, and surface characteristics of neutron stars. It is the technique being used by the Neutron Star Interior Composition Explorer (NICER) collaboration to constrain the properties of rotation-powered millisecond pulsars (RMPs).PPM can also be applied to thermonuclear burst oscillation (TBO) sources, accreting neutron stars that develop surface hotspots during thermonuclear explosions (Type I X-ray bursts), and this is a major science goal for future large-area X-ray spectral-timing telescopes. TBO sources have higher spin rates than most RMPs, promising more precise constraints on the mass and radius. The abundance of sources enhances the prospects of sampling a wider spectrum of masses and radii, thereby reducing statistical uncertainties. And multiple opportunities exist for independent cross-verification through complementary constraints, mitigating potential systematic errors. However, TBO sources also present a significant challenge due to their inherent and dramatic short-time-scale variability. I will outline analysis methods that we have developed to accurately determine the properties of TBO sources using PPM and present results for the TBO source XTE J1814−338.   

Long term resistive GRMHD simulations of self-consistent rotating neutron stars with mixed magnetic fields

Highly magnetized neutron stars are promising candidates for peculiar astronomical phenomena, including fast radio bursts, soft gamma-ray repeaters and X-ray pulsars.

Detailed investigation of the dynamics of these systems is extremely challenging yet significant.

Despite much amount effort has been made in magnetohydrodynamic in the context of neutron stars, their quasi-equilibrium configurations are still largely unknown.

In this talk, we present the first long term full 3D resistive GRMHD simulations of self-consistent rotating neutron stars with mixed poloidal and toroidal fields.

In particular, we will discuss the dynamical instabilities took place at the beginning of the simulation, and discuss the possible quasi-equilibrium configurations obtained from the end of the simulations.

Moreover, we will also discuss the role played by resistivity in the dynamical evolution of neutron stars, and hence their effects onto the neutron star quasi-equilibrium configurations.   

An Examination of Distinct Spheroidal Deformations of Highly Magnetized Neutron Stars

Traditional models of non-rotating neutron stars in the framework of general relativity assume perfect spherical symmetry. However, for classes of neutron stars where high magnetic fields are present, these objects can exhibit anistropies and break from spherical symmetry deforming them into oblate or prolate spheroids. Recent work on the global stellar structure of these deformed compact objects show that stellar properties such as mass, radii, and gravitational redshift can be significantly different from spherical models due to the deformation. Additionally, the mass distribution of these oblate and prolate objects is inhomogeneous and thus they are expected to have a non-zero gravitational quadrupole moment. In this work, we calculate the gravitational quadrupole moment of these objects for varying degrees of deformation. The inhomogeneous distribution of mass is also investigated via statistical methods and visualizations which suggest a distinction between the two spheroid types.   

Highly-accurate Neutron Star Solutions in the Hartle-Thorne Approximation

The physics and astrophysics inferences that we will draw from future X-ray observations with NICER and LOFT and gravitational-wave observations with current and next-generation ground-based detectors will require accurate models of neutron stars. In this talk, I will describe the construction of highly-accurate solutions for slowly-rotating neutron stars, extending the Hartle-Thorne approximation to seventh-order in the slow-rotation expansion. This approximation allows us to calculate second, fourth and sixth order spin corrections to the observed mass and the moment of inertia, the second and fourth-order spin correction to the quadrupole and octopole moments, the second-order in spin correction to the hexadecapole and dotricontapole moment, and the leading-order-in-spin expressions for the hexacontatetrapole and the hectoicosaoctapole, while simultaneously providing accurate error estimates. These accurate solutions enable the calculation of similarly accurate X-ray pulse profile models, I-Love-Q relations, and three-hair relations for slowly-rotating neutron stars that be able useful to break parameter degeneracies in future observations.   

Why is the average stiffness inside neutron stars approximately universal?

The accurate observations of neutron stars have deepened our knowledge of both general relativity and the properties of nuclear physics at large densities and low temperatures. Relating these observations to the microphysics that governs matter at these densities can sometimes be aided by approximate universal relations. One such relation connects the central pressure to the central energy density and the compactness of the star, and it has been found to be insensitive to the equation of state to a 10% level. In this talk, I will show that this relation can be interpreted as the average of the speed of sound squared in the interior of a star of a given compactness. Furthermore, I will show that for a class of equations of state, the universality is rooted in the behavior of the equation of state at energy densities below nuclear saturation, using post-Minkowskian expansions and geometric techniques.    

Non-equilibrium effects on stability of hybrid stars with first-order phase transitions

Constraining the dense matter equation of state (EOS), including the nature of its phase transitions, is a fundamental goal of nuclear physics and neutron star astrophysics. To this end, we consider the effects of out-of-chemical-equilibrium physics at first-order phase transitions on the radial modes and hence stability of compact stars. For barotropic EOS, this is done by allowing the adiabatic sound speed to differ from the equilibrium sound speed. We show that doing so extends the stable branches of stellar models, allowing stars with rapid phase transitions to support stable higher-order stellar multiplets similarly to stars with multiple slow phase transitions. For non-barotropic EOS, we derive a new junction condition to impose on the oscillation modes at the phase transitions. This "reactive condition" is consistent with the generalized junction conditions between two phases and has the common rapid and slow conditions as limiting cases. We apply this condition to hybrid stellar models and show that like in the slow limiting case, some stars that are unstable according to the standard stability criterion are stabilized by a finite chemical reaction speed.   

Stability of dark matter admixed neutron stars

Dark matter is an essential ingredient in our understanding of the Universe. Although most studies have focused on investigating dark matter at large scales, recently there has been an exploration of the effect that dark matter can have on neutron stars, mostly referring to their instability. However, these studies have used a stability criterion that makes a number of, until now, unproven assumptions about the normal modes of the system. In this talk, I will prove the first few of these assumptions (finding a canonical energy and a Lagrangian) and I will discuss the remaining necessary steps to fully prove the validity of the stability criterion.