1 Astrophysical Constraints on Dense Matter in Neutron Stars
33
if the other components were oversubtracted this would lead to an underestimate of
the cooling temperature.
The final caveat relates to the age. If the star was born with a much more rapid
spin than it has now and it has slowed down exclusively by magnetic dipole radiation
then its current age is P /(2 ˙
P ), where P is the current spin period and ˙
P is the
current spin derivative. In some cases this age estimate can be checked using a
kinematic age, either from a supernova remnant or from the angular distance above
the Galactic plane (where massive stars are born) divided by the angular proper
motion away from the plane. Unfortunately there is often a discrepancy between
these estimates of a factor of three or more, so all of these numbers must be treated
with caution.
With the preceding in mind, Figure 8 in [173] shows the comparison between
the minimal cooling model (with light or heavy element envelopes) and the data.
Within the significant uncertainties we note that all of the data are consistent with
the minimal cooling model, although there is some evidence that variation in the
envelope composition is needed to explain the data. As [170] note, however, there is
a significant selection effect at work: if there are stars that have cooled rapidly, they
are obviously more difficult to see. It is thus possible that using our current satellites
we can only observe comparatively hot stars.
Recently it was suggested that the neutron star in the supernova remnant Cas A
has cooled very rapidly over the past decade or so [104]. Further observational analyses, especially taking into account the complexity of the surrounding supernova
remnant emission [76] and the possibility of changes in the calibration, emitting
region size, or absorbing column [180] have made it much less clear that there
actually is anomalously fast cooling. If the evidence strengthens for such cooling,
it has potentially exciting implications for the physics of the interior of this neutron
star, with the leading idea being Cooper pair creation in the superfluid [174, 202].
One-pion exchange and polarization effects could also play a role [35].
1.5.5 Current Status and Future Prospects
Currently there is no evidence that exotic components are necessary, although given
the large uncertainties in data they could certainly be accommodated. The possible
lack of exotic components is consistent with the tentative evidence presented in
Sect. 1.4 that some neutron stars have masses > ∼ 2.0 M , and could mean that
nucleonic degrees of freedom dominate the internal structure of neutron stars.
However, the data are not clear.
To improve the observational situation it will be necessary to have much largerarea future X-ray observatories, such as the approved mission Athena+ [161]. The
resulting high-signal observations would play two important roles: (1) they would
reduce the bias against rapidly cooled stars, and would thus possibly reveal evidence
for exotic components, and (2) they would allow us to distinguish empirically
between different candidate atmospheric spectra (nonmagnetic hydrogen or helium,
33
if the other components were oversubtracted this would lead to an underestimate of
the cooling temperature.
The final caveat relates to the age. If the star was born with a much more rapid
spin than it has now and it has slowed down exclusively by magnetic dipole radiation
then its current age is P /(2 ˙
P ), where P is the current spin period and ˙
P is the
current spin derivative. In some cases this age estimate can be checked using a
kinematic age, either from a supernova remnant or from the angular distance above
the Galactic plane (where massive stars are born) divided by the angular proper
motion away from the plane. Unfortunately there is often a discrepancy between
these estimates of a factor of three or more, so all of these numbers must be treated
with caution.
With the preceding in mind, Figure 8 in [173] shows the comparison between
the minimal cooling model (with light or heavy element envelopes) and the data.
Within the significant uncertainties we note that all of the data are consistent with
the minimal cooling model, although there is some evidence that variation in the
envelope composition is needed to explain the data. As [170] note, however, there is
a significant selection effect at work: if there are stars that have cooled rapidly, they
are obviously more difficult to see. It is thus possible that using our current satellites
we can only observe comparatively hot stars.
Recently it was suggested that the neutron star in the supernova remnant Cas A
has cooled very rapidly over the past decade or so [104]. Further observational analyses, especially taking into account the complexity of the surrounding supernova
remnant emission [76] and the possibility of changes in the calibration, emitting
region size, or absorbing column [180] have made it much less clear that there
actually is anomalously fast cooling. If the evidence strengthens for such cooling,
it has potentially exciting implications for the physics of the interior of this neutron
star, with the leading idea being Cooper pair creation in the superfluid [174, 202].
One-pion exchange and polarization effects could also play a role [35].
1.5.5 Current Status and Future Prospects
Currently there is no evidence that exotic components are necessary, although given
the large uncertainties in data they could certainly be accommodated. The possible
lack of exotic components is consistent with the tentative evidence presented in
Sect. 1.4 that some neutron stars have masses > ∼ 2.0 M , and could mean that
nucleonic degrees of freedom dominate the internal structure of neutron stars.
However, the data are not clear.
To improve the observational situation it will be necessary to have much largerarea future X-ray observatories, such as the approved mission Athena+ [161]. The
resulting high-signal observations would play two important roles: (1) they would
reduce the bias against rapidly cooled stars, and would thus possibly reveal evidence
for exotic components, and (2) they would allow us to distinguish empirically
between different candidate atmospheric spectra (nonmagnetic hydrogen or helium,