240 research outputs found
K-theoretic duality for shifts of finite type
C*-algebras generalizing Cuntz-Krieger algebras can be associated to
hyperbolic homeomorphisms of compact metric spaces. They satisfy a
non-commutative form of Spanier-Whitehead duality with respect to K-theory. We
prove this for the case of subshifts of finite type. The special feature of the
present situation is that the constructions are all done on the full Fock space
and are very explicit, while the general theorem requires much more abstract
machinery.Comment: 23 pages, Latex fil
Neutrino-pair bremsstrahlung by electrons in neutron star crusts
Neutrino-pair bremsstrahlung by relativistic degenerate electrons in a
neutron-star crust at densities (10^9 - 1.5x10^{14}) g/cm^3 is analyzed. The
processes taken into account are neutrino emission due to Coulomb scattering of
electrons by atomic nuclei in a Coulomb liquid, and electron-phonon scattering
and Bragg diffraction (the static-lattice contribution) in a Coulomb crystal.
The static-lattice contribution is calculated including the electron
band-structure effects for cubic Coulomb crystals of different types and also
for the liquid crystal phases composed of rod- and plate-like nuclei in the
neutron-star mantle (at 10^{14} - 1.5x10^{14} g/cm^3). The phonon contribution
is evaluated with proper treatment of the multi-phonon processes which removes
a jump in the neutrino bremsstrahlung emissivity at the melting point obtained
in previous works. Below 10^{13} g/cm^3, the results are rather insensitive to
the nuclear form factor, but results for the solid state near the melting point
are affected significantly by the Debye-Waller factor and multi-phonon
processes. At higher densities, the nuclear form factor becomes more
significant. A comparison of the various neutrino generation mechanisms in
neutron star crusts shows that electron bremsstrahlung is among the most
important ones.Comment: 17 pages, 13 figures, LaTeX using aa.cls and epsf.sty. A&A, in pres
The strong Novikov conjecture for low degree cohomology
We show that for each discrete group G, the rational assembly map
K_*(BG) \otimes Q \to K_*(C*_{max} G) \otimes \Q is injective on classes dual
to the subring generated by cohomology classes of degree at most 2 (identifying
rational K-homology and homology via the Chern character). Our result implies
homotopy invariance of higher signatures associated to these cohomology
classes. This consequence was first established by Connes-Gromov-Moscovici and
Mathai.
Our approach is based on the construction of flat twisting bundles out of
sequences of almost flat bundles as first described in our previous work. In
contrast to the argument of Mathai, our approach is independent of (and indeed
gives a new proof of) the result of Hilsum-Skandalis on the homotopy invariance
of the index of the signature operator twisted with bundles of small curvature.Comment: 11 page
The Crustal Rigidity of a Neutron Star, and Implications for PSR 1828-11 and other Precession Candidates
We calculate the crustal rigidity parameter, b, of a neutron star (NS), and
show that b is a factor 40 smaller than the standard estimate due to Baym &
Pines (1971). For a NS with a relaxed crust, the NS's free-precession frequency
is directly proportional to b. We apply our result for b to PSR 1828-11, a 2.5
Hz pulsar that appears to be precessing with period 511 d. Assuming this 511-d
period is set by crustal rigidity, we show that this NS's crust is not relaxed,
and that its reference spin (roughly, the spin for which the crust is most
relaxed) is 40 Hz, and that the average spindown strain in the crust is 5
\times 10^{-5}. We also briefly describe the implications of our b calculation
for other well-known precession candidates.Comment: 44 pages, 10 figures, submitted to Ap
Weak decay of uniformly accelerated protons and related processes
We investigate the weak interaction emission of spin-1/2 fermions from
accelerated currents. As particular applications, we analyze the decay of
uniformly accelerated protons and neutrons, and the neutrino-antineutrino
emission from uniformly accelerated electrons. The possible relevance of our
results to astrophysics is also discussed.Comment: 16 pages (REVTEX), 6 figures, to appear in Physical Review
Eddington-limited X-ray Bursts as Distance Indicators. I. Systematic Trends and Spherical Symmetry in Bursts from 4U 1728-34
We investigate the limitations of thermonuclear X-ray bursts as a distance
indicator for the weakly-magnetized accreting neutron star 4U 1728-34. We
measured the unabsorbed peak flux of 81 bursts in public data from the Rossi
X-Ray Timing Explorer (RXTE). The distribution of peak fluxes was bimodal: 66
bursts exhibited photospheric radius expansion and were distributed about a
mean bolometric flux of 9.2e-8 erg/cm^2/s, while the remaining (non-radius
expansion) bursts reached 4.5e-8 erg/cm^2/s, on average. The peak fluxes of the
radius-expansion bursts were not constant, exhibiting a standard deviation of
9.4% and a total variation of 46%. These bursts showed significant correlations
between their peak flux and the X-ray colors of the persistent emission
immediately prior to the burst. We also found evidence for quasi-periodic
variation of the peak fluxes of radius-expansion bursts, with a time scale of
approximately 40 d. The persistent flux observed with RXTE/ASM over 5.8 yr
exhibited quasi-periodic variability on a similar time scale. We suggest that
these variations may have a common origin in reflection from a warped accretion
disk. Once the systematic variation of the peak burst fluxes is subtracted, the
residual scatter is only approximately 3%, roughly consistent with the
measurement uncertainties. The narrowness of this distribution strongly
suggests that i) the radiation from the neutron star atmosphere during
radius-expansion episodes is nearly spherically symmetric, and ii) the
radius-expansion bursts reach a common peak flux which may be interpreted as a
standard candle intensity.Adopting the minimum peak flux for the
radius-expansion bursts as the Eddington flux limit, we derive a distance for
the source of 4.4-4.8 kpc.Comment: 9 pages, 7 figures, accepted by ApJ. Minor referee's revisions, also
includes 9 newly public X-ray burst
Neutron Star Structure and the Neutron Radius of 208Pb
We study relationships between the neutron-rich skin of a heavy nucleus and
the properties of neutron-star crusts. Relativistic effective field theories
with a thicker neutron skin in Pb have a larger electron fraction and a
lower liquid-to-solid transition density for neutron-rich matter. These
properties are determined by the density dependence of the symmetry energy
which we vary by adding nonlinear couplings between isoscalar and isovector
mesons. An accurate measurement of the neutron radius in Pb---via
parity violating electron scattering---may have important implications for the
structure of neutron stars.Comment: 5 pages 3 figures, added additional evidence of model independence,
Phys. Rev. Letters in pres
Cooling of Neutron Stars: Two Types of Triplet Neutron Pairing
We consider cooling of neutron stars (NSs) with superfluid cores composed of
neutrons, protons, and electrons (assuming singlet-state pairing of protons,
and triplet-state pairing of neutrons). We mainly focus on (nonstandard)
triplet-state pairing of neutrons with the projection of the total
angular momentum of Cooper pairs onto quantization axis. The specific feature
of this pairing is that it leads to a power-law (nonexponential) reduction of
the emissivity of the main neutrino processes by neutron superfluidity. For a
wide range of neutron critical temperatures , the cooling of NSs with
the superfluidity is either the same as the cooling with the superfluidity, considered in the majority of papers, or much faster. The
cooling of NSs with density dependent critical temperatures and
can be imitated by the cooling of the NSs with some effective
critical temperatures and constant over NS cores. The
hypothesis of strong neutron superfluidity with is inconsistent
with current observations of thermal emission from NSs, but the hypothesis of
weak neutron superfluidity of any type does not contradict to observations.Comment: 10 pages, 6 figure
Neutral weak currents in nucleon superfluid Fermi liquids: Larkin-Migdal and Leggett approaches
Neutrino emission in processes of breaking and formation of nucleon Cooper
pairs is calculated in the framework of the Larkin-Migdal and the Leggett
approaches to the description of superfluid Fermi liquids at finite
temperatures. We explain peculiarities of both approaches and explicitly
demonstrate that they lead to the same expression for the emissivity in pair
breaking and formation processes.Comment: 24 pages, 3 figure
On the Transport Properties of a Quark-Hadron Coulomb Lattice in the Cores of Neutron Stars
Already more that 40 years ago, it has been suggested that because of the
enormous mass densities in the cores of neutron stars, the hadrons in the
centers of neutron stars may undergo a phase transition to deconfined quark
matter. In this picture, neutron stars could contain cores made of pure (up,
down, strange) quark matter which are surrounded by a mixed phase of quarks and
hadrons. More than that, because of the competition between the Coulomb and the
surface energies associated with the positively charged regions of nuclear
matter and negatively charged regions of quark matter, the mixed phase may
develop geometrical structures, similarly to what is expected of the
sub-nuclear liquid-gas phase transition. In this paper we restrict ourselves to
considering the formation of rare phase blobs in the mixed quark-hadron phase.
The influence of rare phase blobs on the thermal and transport properties of
neutron star matter is investigated. The total specific heat, , thermal
conductivity, , and electron-blob Bremsstrahlung neutrino emissivities,
, of quark-hybrid matter are computed and the results
are compared with the associated thermal and transport properties of standard
neutron star matter. Our results show that the contribution of rare phase blobs
to the specific heat is negligibly small. This is different for the neutrino
emissivity from electron-blob Bremsstrahlung scattering, which turns out to be
of the same order of magnitude as the total contributions from other
Bremsstrahlung processes for temperatures below about K.Comment: minor changes, accepted by Phys. Rev.
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