1 Astrophysical Constraints on Dense Matter in Neutron Stars
37
Fig. 1.5 Gravitational wave frequency at the point of tidal disruption as a function of neutron
star mass. The solid and dotted lines refer, respectively, to the hard and soft equations of state from
[100]. In both cases we assume that the companion is a neutron star of equal mass. Tidal effects will
affect the phasing of the inspiral below f GW,disrupt , but the effects fall off rapidly with decreasing
frequency. Good sensitivity at high frequencies is essential for these effects to be detected
but this is a promising avenue to pursue. It has also been noted that the precision of
constraints will be improved significantly by combining the analyses of >10 bursts,
if systematic errors are under control [71], and that observations with the planned
third-generation gravitational wave detector the Einstein Telescope will improve
precision by an order of magnitude [219].
In summary, observations of gravitational waves from compact object coalescences will yield promising new constraints on the properties of neutron stars. This
will occur because of new mass measurements and radius constraints. Most of the
information exists at high frequencies, so in order to maximize this information it
may be necessary to explore techniques beyond the second generation of detectors,
such as squeezing of light [1].
1.7 Summary
Significant progress has been made in the last decade on mass estimates of neutron
stars. The maximum mass is > ∼ 2 M , which along with the lack of evidence of
rapid cooling suggests that non-nucleonic degrees of freedom appear unnecessary
to explain current data. There is, however, considerable freedom that would allow
exotic phases. The main limitation of existing observations is that radius estimates
are shrouded in systematic errors. No current method is both precise and reliable
enough to pose significant constraints on the structure of neutron stars. This is
unfortunate, because good radii would do more than any other single measurement
to inform us about the matter in the cores of neutron stars [130].
37
Fig. 1.5 Gravitational wave frequency at the point of tidal disruption as a function of neutron
star mass. The solid and dotted lines refer, respectively, to the hard and soft equations of state from
[100]. In both cases we assume that the companion is a neutron star of equal mass. Tidal effects will
affect the phasing of the inspiral below f GW,disrupt , but the effects fall off rapidly with decreasing
frequency. Good sensitivity at high frequencies is essential for these effects to be detected
but this is a promising avenue to pursue. It has also been noted that the precision of
constraints will be improved significantly by combining the analyses of >10 bursts,
if systematic errors are under control [71], and that observations with the planned
third-generation gravitational wave detector the Einstein Telescope will improve
precision by an order of magnitude [219].
In summary, observations of gravitational waves from compact object coalescences will yield promising new constraints on the properties of neutron stars. This
will occur because of new mass measurements and radius constraints. Most of the
information exists at high frequencies, so in order to maximize this information it
may be necessary to explore techniques beyond the second generation of detectors,
such as squeezing of light [1].
1.7 Summary
Significant progress has been made in the last decade on mass estimates of neutron
stars. The maximum mass is > ∼ 2 M , which along with the lack of evidence of
rapid cooling suggests that non-nucleonic degrees of freedom appear unnecessary
to explain current data. There is, however, considerable freedom that would allow
exotic phases. The main limitation of existing observations is that radius estimates
are shrouded in systematic errors. No current method is both precise and reliable
enough to pose significant constraints on the structure of neutron stars. This is
unfortunate, because good radii would do more than any other single measurement
to inform us about the matter in the cores of neutron stars [130].