23 research outputs found
Statistical theory of thermal evolution of neutron stars
Thermal evolution of neutron stars is known to depend on the properties of
superdense matter in neutron star cores. We suggest a statistical analysis of
isolated cooling middle-aged neutron stars and old transiently accreting
quasi-stationary neutron stars warmed up by deep crustal heating in low-mass
X-ray binaries. The method is based on simulations of the evolution of stars of
different masses and on averaging the results over respective mass
distributions. This gives theoretical distributions of isolated neutron stars
in the surface temperature--age plane and of accreting stars in the photon
thermal luminosity--mean mass accretion rate plane to be compared with
observations. This approach permits to explore not only superdense matter but
also the mass distributions of isolated and accreting neutron stars. We show
that the observations of these stars can be reasonably well explained by
assuming the presence of the powerful direct Urca process of neutrino emission
in the inner cores of massive stars, introducing a slight broadening of the
direct Urca threshold (for instance, by proton superfluidity), and by tuning
mass distributions of isolated and accreted neutron stars.Comment: 13 pages, 20 figure
NS 1987A in SN 1987A
The possible detection of a compact object in the remnant of SN 1987A
presents an unprecedented opportunity to follow its early evolution. The
suspected detection stems from an excess of infrared emission from a dust blob
near the compact object's predicted position. The infrared excess could be due
to the decay of isotopes like 44Ti, accretion luminosity from a neutron star or
black hole, magnetospheric emission or a wind originating from the spindown of
a pulsar, or thermal emission from an embedded, cooling neutron star (NS
1987A). It is shown that the last possibility is the most plausible as the
other explanations are disfavored by other observations and/or require
fine-tuning of parameters. Not only are there indications the dust blob
overlaps the predicted location of a kicked compact remnant, but its excess
luminosity also matches the expected thermal power of a 30 year old neutron
star. Furthermore, models of cooling neutron stars within the Minimal Cooling
paradigm readily fit both NS 1987A and Cas A, the next-youngest known neutron
star. If correct, a long heat transport timescale in the crust and a large
effective stellar temperature are favored, implying relatively limited crustal
n-1S0 superfluidity and an envelope with a thick layer of light elements,
respectively. If the locations don't overlap, then pulsar spindown or accretion
might be more likely, but the pulsar's period and magnetic field or the
accretion rate must be rather finely tuned. In this case, NS 1987A may have
enhanced cooling and/or a heavy-element envelope.Comment: 21 pages, 6 figures, to be published in Ap
Cooling of the Cassiopeia A neutron star and the effect of diffusive nuclear burning
The study of how neutron stars cool over time can provide invaluable insights
into fundamental physics such as the nuclear equation of state and
superconductivity and superfluidity. A critical relation in neutron star
cooling is the one between observed surface temperature and interior
temperature. This relation is determined by the composition of the neutron star
envelope and can be influenced by the process of diffusive nuclear burning
(DNB). We calculate models of envelopes that include DNB and find that DNB can
lead to a rapidly changing envelope composition which can be relevant for
understanding the long-term cooling behavior of neutron stars. We also report
on analysis of the latest temperature measurements of the young neutron star in
the Cassiopeia A supernova remnant. The 13 Chandra observations over 18 years
show that the neutron star's temperature is decreasing at a rate of 2-3 percent
per decade, and this rapid cooling can be explained by the presence of a proton
superconductor and neutron superfluid in the core of the star.Comment: 7 pages, 7 figures; to appear in the AIP Conference Proceedings of
the Xiamen-CUSTIPEN Workshop on the EOS of Dense Neutron-Rich Matter in the
Era of Gravitational Wave Astronomy (January 3-7, 2019, Xiamen, China
Constraining the properties of dense neutron star cores:the case of the low-mass X-ray binary HETE J1900.1-2455
Measuring the time evolution of the effective surface temperature of neutron
stars can provide invaluable information on the properties of their dense
cores. Here, we report on a new Chandra observation of the transient neutron
star low-mass X-ray binary HETE J1900.1-2455, which was obtained ~2.5 yr after
the end of its ~10-yr long accretion outburst. The source is barely detected
during the observation, collecting only six net photons, all below 2 keV.
Assuming that the spectrum is shaped as a neutron star atmosphere model we
perform a statistical analysis to determine a 1-sigma confidence upper range
for the neutron star temperature of ~30-39 eV (for an observer at infinity),
depending on its mass, radius and distance. Given the heat injected into the
neutron star during the accretion outburst, estimated from data provided by
all-sky monitors, the inferred very low temperature suggests that either the
core has a very high heat capacity or undergoes very rapid neutrino cooling.
While the present data do not allow us to disentangle these two possibilities,
both suggest that a significant fraction of the dense core is not
superfluid/superconductor. Our modeling of the thermal evolution of the neutron
star predicts that it may still cool further, down to a temperature of ~15 eV.
Measuring such a low temperature with a future observation may provide
constraints on the fraction of baryons that is paired in the stellar core.Comment: 14 pages (12 main, 2 appendix), 10 figures, published in MNRA