Neutron star observations: Prognosis for equation of state constraints

Abstract

We investigate how current and proposed observations of neutron stars can lead to an understanding of the state of their interiors and the key unknowns: the typical neutron star radius and the neutron star maximum mass. We consider observations made not only with photons, ranging from radio waves to X-rays, but also those involving neutrinos and gravity waves. We detail how precision determinations of structural properties would lead to significant restrictions on the poorly understood equation of state near and beyond the equilibrium density of nuclear matter.To begin, a theoretical analysis of neutron star structure, including general relativistic limits to mass, compactness, and spin rates is made. A review is the made of recent observations such as pulsar timing (which leads to mass, spin period, glitch and moment of inertia estimates), optical and X-ray observations of cooling neutron stars (which lead to estimates of core temperatures and ages and inferences about the internal composition), and X-ray observations of accreting and bursting sources (which shed light on both the crustal properties and internal composition). Next, we discuss neutrino emission from proto-neutron stars and how neutrino observations of a supernova, from both current and planned detectors, might impact our knowledge of the interiors, mass and radii of neutron stars. We also explore the question of how superstrong magnetic fields could affect the equation of state and neutron star structure. This is followed by a look at binary mergers involving neutron stars and how the detection of gravity waves could unambiguously distinguish normal neutron stars from self-bound strange quark matter stars.

Soft nuclear equation-of-state from heavy-ion data and implications for compact stars

“…With their immense interest in nuclear physics [22], neutron star radii are a focus of major attention [23][24][25][26][27][28][29][30]. Limits on radii are obtained from X-ray bursters and lowmass X-ray binaries (see [24,25,[30][31][32][33][34][35][36] and references therein) whereas such studies could be substantially refined in the future with the Large Observatory For X-ray Timing (LOFT) [37].…”

Observational Constraints on the Surface Gravitational Redshift of a Neutron Star

“…The best-fit spin frequency, 0.8±0.2 kHz, is lower than the break-up spin frequency, 1-2 kHz, of a NS [1] and the error region overlaps with the spin frequencies measured for other burst sources, 11-620 Hz [6]. The emergence of the spectral feature may be naturally explained by the change of the ionization degree of the burst products from fully ionized state to H/He-like state.…”

“…There are various methods to probe the interior of a NS (see e.g., [1] for reviews). Among them, measurement of its mass and radius, or even the mass-radius ratio alone, provides indispensable information to constrain the equation of state of ultra-dense matter.…”

The missing compact star of SN1987A: a solid quark star?

“…The newly confirmed mass of PSR J1614-2230 of 2M⊙ (Demorest et al 2010) rules out almost all currently proposed hyperon or boson condensate EoSs (Lattimer & Prakash 2007) and traditional soft EoS of SS (Chan et al 2009). Lai & Xu (2009) argued that the solid quark-cluster stars (SQS), a special kind of SS, could have the maximum mass to be larger than 2 M⊙ because of the very stiff EoS.…”