2
M. C. Miller
Abstract Ever since the discovery of neutron stars it has been realized that they
serve as probes of a physical regime that cannot be accessed in laboratories:
strongly degenerate matter at several times nuclear saturation density. Existing
nuclear theories diverge widely in their predictions about such matter. It could be
that the matter is primarily nucleons, but it is also possible that exotic species such
as hyperons, free quarks, condensates, or strange matter may dominate this regime.
Astronomical observations of cold high-density matter are necessarily indirect,
which means that we must rely on measurements of quantities such as the masses
and radii of neutron stars and their surface effective temperatures as a function of
age. Here we review the current status of constraints from various methods and the
prospects for future improvements.
1.1 Introduction
The nature of the matter in the cores of neutron stars is of great interest to nuclear
physicists and astrophysicists alike, but its properties are difficult to establish in
terrestrial laboratories. This is because neutron star cores reach a few times the
density of matter in terrestrial nuclei and yet they are strongly degenerate and they
have far more neutrons than protons. The core matter thus occupies a different phase
than is accessible in laboratories. Within current theoretical uncertainties there are
many possibilities for the state of this matter: it could be primarily nucleonic, or
dominated by deconfined quark matter, or mainly hyperons, or even mostly in a
condensate.
Only astrophysical observations of neutron stars can constrain the properties of
the cold supranuclear matter in their cores. Because we cannot sample the matter
directly, we need to infer its state by measurements of neutron star masses, radii,
and cooling rates. For the last two of these, the method of measurement is highly
indirect and thus subject to systematic errors. Note, to be precise, that throughout
this review we mean by mass the gravitational mass (which would be measured
by using Kepler’s laws for a satellite in a distant orbit around the star) rather than
the baryonic mass (which is the sum of the rest masses of the individual particles
in the star); for a neutron star, the gravitational mass is typically less than the
baryonic mass by ∼20%. We also mean by radius the circumferential radius, i.e.,
the circumference at the equator divided by 2π, rather than other measures such as
the proper distance between the stellar center and a point on the surface. Again, for
objects as compact as neutron stars, the difference can amount to tens of percent.
In this review we discuss current attempts to measure the relevant stellar
properties. We also discuss future prospects for constraints including those that will
come from analysis of gravitational waves. For each of the constraint methods, we
discuss the current uncertainties and assess the prospects for lowering systematic
errors in the future. In Sect. 1.2 we set the stage by discussing current expectations
from nuclear theory and laboratory measurements. In Sect. 1.3 we examine mass
measurements in binaries. In Sect. 1.4 we discuss current attempts to measure
M. C. Miller
Abstract Ever since the discovery of neutron stars it has been realized that they
serve as probes of a physical regime that cannot be accessed in laboratories:
strongly degenerate matter at several times nuclear saturation density. Existing
nuclear theories diverge widely in their predictions about such matter. It could be
that the matter is primarily nucleons, but it is also possible that exotic species such
as hyperons, free quarks, condensates, or strange matter may dominate this regime.
Astronomical observations of cold high-density matter are necessarily indirect,
which means that we must rely on measurements of quantities such as the masses
and radii of neutron stars and their surface effective temperatures as a function of
age. Here we review the current status of constraints from various methods and the
prospects for future improvements.
1.1 Introduction
The nature of the matter in the cores of neutron stars is of great interest to nuclear
physicists and astrophysicists alike, but its properties are difficult to establish in
terrestrial laboratories. This is because neutron star cores reach a few times the
density of matter in terrestrial nuclei and yet they are strongly degenerate and they
have far more neutrons than protons. The core matter thus occupies a different phase
than is accessible in laboratories. Within current theoretical uncertainties there are
many possibilities for the state of this matter: it could be primarily nucleonic, or
dominated by deconfined quark matter, or mainly hyperons, or even mostly in a
condensate.
Only astrophysical observations of neutron stars can constrain the properties of
the cold supranuclear matter in their cores. Because we cannot sample the matter
directly, we need to infer its state by measurements of neutron star masses, radii,
and cooling rates. For the last two of these, the method of measurement is highly
indirect and thus subject to systematic errors. Note, to be precise, that throughout
this review we mean by mass the gravitational mass (which would be measured
by using Kepler’s laws for a satellite in a distant orbit around the star) rather than
the baryonic mass (which is the sum of the rest masses of the individual particles
in the star); for a neutron star, the gravitational mass is typically less than the
baryonic mass by ∼20%. We also mean by radius the circumferential radius, i.e.,
the circumference at the equator divided by 2π, rather than other measures such as
the proper distance between the stellar center and a point on the surface. Again, for
objects as compact as neutron stars, the difference can amount to tens of percent.
In this review we discuss current attempts to measure the relevant stellar
properties. We also discuss future prospects for constraints including those that will
come from analysis of gravitational waves. For each of the constraint methods, we
discuss the current uncertainties and assess the prospects for lowering systematic
errors in the future. In Sect. 1.2 we set the stage by discussing current expectations
from nuclear theory and laboratory measurements. In Sect. 1.3 we examine mass
measurements in binaries. In Sect. 1.4 we discuss current attempts to measure