Future Probes of the Neutron Star Equation of State Using X-ray Bursts
9
Citation
2
Reference
10
Related Paper
Citation Trend
Abstract:
Observations with NASA’s Rossi X‐ray Timing Explorer (RXTE) have resulted in the discovery of fast (200 – 600 Hz), coherent X‐ray intensity oscillations (hereafter, “burst oscillations”) during thermonuclear X‐ray bursts from 12 low mass X‐ray binaries (LMXBs). Although many of their detailed properties remain to be fully understood, it is now beyond doubt that these oscillations result from spin modulation of the thermonuclear burst flux from the neutron star surface. Among the new timing phenomena revealed by RXTE the burst oscillations are perhaps the best understood, in the sense that many of their properties can be explained in the framework of this relatively simple model. Because of this, detailed modelling of burst oscillations can be an extremely powerful probe of neutron star structure, and thus the equation of state (EOS) of supranuclear density matter. Both the compactness parameter β = GM/c2R, and the surface velocity, vrot = ΩspinR, are encoded in the energy‐dependent amplitude and shape of the modulation pulses. The new discoveries have spurred much new theoretical work on thermonuclear burning and propagation on neutron stars, so that in the near future it is not unreasonable to think that detailed physical models of the time dependent flux from burning neutron stars will be available for comparison with the observed pulse profiles from a future, large collecting area X‐ray timing observatory. In addition, recent high resolution burst spectroscopy with XMM/Newton suggests the presence of redshifted absorption lines from the neutron star surface during bursts. This leads to the possibility of using large area, high spectral resolution measurements of X‐ray bursts as a precise probe of neutron star structure. In this work I will explore the precision with which constraints on neutron star structure, and hence the dense matter EOS, can be made with the implementation of such programs.Keywords:
Thermonuclear Fusion
X-ray binary
Compact star
Cite
Citations (1)
Neutron stars are some of the most compact objects found in the Universe reaching densities inaccessible to Earth-based experiments. They can contain as much as two times the mass of the Sun in a region comparable to a small city. The extreme densities reached by neutron stars allow us to probe the elusive neutron-star equation of state: the equation that determines the behavior of matter at supranuclear densities . In this thesis, we present new methods to constrain the neutron-star equation of state using gravitational-wave observations of binary neutron star mergers.
Compact star
Cite
Citations (0)
The structure of neutron stars is determined by the equation of state of dense matter in their interiors. Brief review of the equation of state from neutron star surface to its center is presented. Recent discovery of two two-solar-mass pulsars puts interesting constraints on the poorly known equation of state of neutron-star cores for densities greater than normal nuclear matter density. Namely, this equation of state has to be stiff enough to yield maximum allowable mass of neutron stars greater than two solar masses. There are many models of neutron stars cores involving exclusively nucleons that satisfy this constraint. However, for neutron-star models based on recent realistic baryon interaction, and allowing for the presence of hyperons, the hyperon softening of the equation of state yields maximum masses significantly lower than two solar masses. Proposed ways out from this "hyperon puzzle" are presented. They require a very fine tuning of parameters of dense hadronic matter and quark matter models. Consequences for the mass-radius relation for neutron stars are illustrated. A summary of the present situation and possible perspectives/challenges, as well as possible observational tests, are given.
Solar mass
Cite
Citations (1)
The equation of state (EOS) of dense matter has been a long-sought goal of nuclear physics. Equations of state generate unique mass versus radius (M-R) relations for neutron stars, the ultra-dense remnants of stellar evolution. In this work, we determine the neutron star mass-radius relation and, based on recent observations of both transiently accreting and bursting sources, we show that the radius of a 1.4 solar mass neutron star lies between 10.4 and 12.9 km, independent of assumptions about the composition of the core. We show, for the first time, that these constraints remain valid upon removal from our sample of the most extreme transient sources or of the entire set of bursting sources; our constraints also apply even if deconfined quark matter exists in the neutron star core. Our results significantly constrain the dense matter EOS and are, furthermore, consistent with constraints from both heavy-ion collisions and theoretical studies of neutron matter. We predict a relatively weak dependence of the symmetry energy on the density and a value for the neutron skin thickness of lead which is less than 0.20 fm, results that are testable in forthcoming experiments.
Dense matter
Cite
Citations (406)
Strange quark
Cite
Citations (4)
Determining the equation of state of matter at nuclear density and hence the structure of neutron stars has been a riddle for decades. We show how the imminent detection of gravitational waves from merging neutron star binaries can be used to solve this riddle. Using a large number of accurate numerical-relativity simulations of binaries with nuclear equations of state, we have found that the postmerger emission is characterized by two distinct and robust spectral features. While the high-frequency peak has already been associated with the oscillations of the hypermassive neutron star produced by the merger and depends on the equation of state, a new correlation emerges between the low-frequency peak, related to the merger process, and the compactness of the progenitor stars. More importantly, such a correlation is essentially universal, thus providing a powerful tool to set tight constraints on the equation of state. If the mass of the binary is known from the inspiral signal, the combined use of the two frequency peaks sets four simultaneous constraints to be satisfied. Ideally, even a single detection would be sufficient to select one equation of state over the others. We have tested our approach with simulated data and verified it works well for all the equations of state considered.
Cite
Citations (9)
I discuss why state-of-the art perturbative QCD calculations of the equation of state at large chemical potential that are reliable at asymptotically high densities constrain the same equation of state at neutron-star densities. I describe how these theoretical calculations affect the EOS at lower density. I argue that the ab-initio calculations in QCD offer significant information about the equation of state of the neutron-star matter, which is complementary to the current astrophysical observations.
Perturbative QCD
Dense matter
Star (game theory)
Cite
Citations (0)
We present two recent parametrizations of the equation of state (FSU2R and FSU2H models) that reproduce the properties of nuclear matter and finite nuclei, fulfill constraints on high-density matter stemming from heavy-ion collisions, produce 2$M_{\odot}$ neutron stars, and generate neutron star radii below 13 km. Making use of these equations of state, cooling simulations for isolated neutron stars are performed. We find that two of the models studied, FSU2R (with nucleons) and, in particular, FSU2H (with nucleons and hyperons), show very good agreement with cooling observations, even without including nucleon pairing. This indicates that cooling observations are compatible with an equation of state that produces a soft nuclear symmetry energy and, thus, generates small neutron star radii. Nevertheless, both schemes produce cold isolated neutron stars with masses above $1.8 M_{\odot}$.
Cite
Citations (1)
r-process
Dense matter
Cite
Citations (28)
We study the properties of neutron stars using the complete relativistic equation of state based on the relativistic mean field theory. The complete relativistic equation of state covers a wide density range from 10-7 to 1.2 fm-3 for the use of describing both the interior region and the crusts of neutron stars. We include the Λ, Σ, and Ξ hyperons which may appear as new degrees of freedom at high densities. We also examine the sensibility of the results to the choice of hyperon couplings.
Mean field theory
Relativistic quantum chemistry
Cite
Citations (4)