38
M. C. Miller
Where, then, do we stand? In the near future it is plausible that our understanding
of thermonuclear X-ray bursts, accreting neutron stars, or the emission from cooling
neutron stars may evolve to the point that the complex phenomena associated
with them can be interpreted with confidence, and reliable radii and masses
will emerge. In principle the needed data have already been collected, but our
evolving understanding that, e.g., bursts do not settle quickly to a constant area
of emission and that cooling neutron stars display nonthermal emission has led
to caution. At the same time, numerical simulations are improving rapidly in
sophistication. It is possible, although far from guaranteed, that within a few years
magnetohydrodynamic simulations of accretion disks or bursting atmospheres will
yield results that are close enough to what is observed that our understanding will
solidify and radius measurements will become a powerful tool for constraining the
state of matter in the cores of neutron stars.
If ground-based gravitational wave facilities make detections as expected within
5 years, then as we discussed in the previous section this will open up a new way to
measure masses and radii, with systematic errors that are at a minimum different
from those that bedevil current attempts, and more optimistically might be less
significant than statistical uncertainties. However, most of the information resides
at high frequencies where, at least for standard configurations, second-generation
detectors will be insensitive enough that it may require a rare high-signal event
to derive restrictive constraints. A third-generation detector such as the Einstein
Telescope should be able to obtain all the required information, and even before
such detectors exist it may be possible to use configurations optimized for high
frequencies or 2.5 generation technology such as squeezed light to obtain the
information.
Electromagnetic observatories are also improving, and some of the old reliable
methods may improve our understanding substantially. For example, the Jansky
Very Large Array or (in roughly a decade) the Square Kilometer Array might
have enough sensitivity, bandwidth, and computer power to detect a much larger
population of double neutron star binaries, among which we might by chance have
some with stars of M > 2.0 M . Even without such serendipitous discoveries, the
mere accumulation of time and data on NS-WD binaries in globular clusters seems
likely to yield high-mass objects that will provide firm lower limits to the maximum
mass that are much stronger than currently exist. As we have discussed, many other
improvements are expected using the large area and excellent spectral resolution of
Athena+ [161], and the high area and timing resolution of NICER [90] and LOFT
[78]. This is especially true of radius estimates from fits to X-ray waveforms.
Overall, although we expect that eventually radius measurements will play a
major role, in the next several years it appears that mass measurements of neutron
stars in binaries will continue to dominate the discussion of the cold high-density
equation of state. The expected improvements in data and models will allow us
to provide nuclear physicists with more certain constraints, and we await the
theoretical developments that result.
M. C. Miller
Where, then, do we stand? In the near future it is plausible that our understanding
of thermonuclear X-ray bursts, accreting neutron stars, or the emission from cooling
neutron stars may evolve to the point that the complex phenomena associated
with them can be interpreted with confidence, and reliable radii and masses
will emerge. In principle the needed data have already been collected, but our
evolving understanding that, e.g., bursts do not settle quickly to a constant area
of emission and that cooling neutron stars display nonthermal emission has led
to caution. At the same time, numerical simulations are improving rapidly in
sophistication. It is possible, although far from guaranteed, that within a few years
magnetohydrodynamic simulations of accretion disks or bursting atmospheres will
yield results that are close enough to what is observed that our understanding will
solidify and radius measurements will become a powerful tool for constraining the
state of matter in the cores of neutron stars.
If ground-based gravitational wave facilities make detections as expected within
5 years, then as we discussed in the previous section this will open up a new way to
measure masses and radii, with systematic errors that are at a minimum different
from those that bedevil current attempts, and more optimistically might be less
significant than statistical uncertainties. However, most of the information resides
at high frequencies where, at least for standard configurations, second-generation
detectors will be insensitive enough that it may require a rare high-signal event
to derive restrictive constraints. A third-generation detector such as the Einstein
Telescope should be able to obtain all the required information, and even before
such detectors exist it may be possible to use configurations optimized for high
frequencies or 2.5 generation technology such as squeezed light to obtain the
information.
Electromagnetic observatories are also improving, and some of the old reliable
methods may improve our understanding substantially. For example, the Jansky
Very Large Array or (in roughly a decade) the Square Kilometer Array might
have enough sensitivity, bandwidth, and computer power to detect a much larger
population of double neutron star binaries, among which we might by chance have
some with stars of M > 2.0 M . Even without such serendipitous discoveries, the
mere accumulation of time and data on NS-WD binaries in globular clusters seems
likely to yield high-mass objects that will provide firm lower limits to the maximum
mass that are much stronger than currently exist. As we have discussed, many other
improvements are expected using the large area and excellent spectral resolution of
Athena+ [161], and the high area and timing resolution of NICER [90] and LOFT
[78]. This is especially true of radius estimates from fits to X-ray waveforms.
Overall, although we expect that eventually radius measurements will play a
major role, in the next several years it appears that mass measurements of neutron
stars in binaries will continue to dominate the discussion of the cold high-density
equation of state. The expected improvements in data and models will allow us
to provide nuclear physicists with more certain constraints, and we await the
theoretical developments that result.