246 research outputs found
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
Theory of cooling neutron stars versus observations
We review current state of neutron star cooling theory and discuss the
prospects to constrain the equation of state, neutrino emission and superfluid
properties of neutron star cores by comparing the cooling theory with
observations of thermal radiation from isolated neutron stars.Comment: 9 pages, 4 figures, 3 tables, to appear in the proceedings of "40
Years of Pulsars" held in Montreal, Canada, August 12-17, 2007, eds. C.
Bassa, Z. Wang, A. Cumming, V. Kaspi, AIP, in press (v.2 - minor bibliography
corrections
Equation of state and opacities for hydrogen atmospheres of magnetars
The equation of state and radiative opacities of partially ionized, strongly
magnetized hydrogen plasmas, presented in a previous paper [ApJ 585, 955
(2003), astro-ph/0212062] for the magnetic field strengths 8.e11 G < B < 3.e13
G, are extended to the field strengths 3.e13 G < B < 1.e15 G, relevant for
magnetars. The first- and second-order thermodynamic functions and radiative
opacities are calculated and tabulated for 5.e5 < T < 4.e7 K in a wide range of
densities. We show that bound-free transitions give an important contribution
to the opacities in the considered range of B in the outer neutron-star
atmosphere layers. Unlike the case of weaker fields, bound-bound transitions
are unimportant.Comment: 7 pages, 6 figures, LaTeX using emulateapj.cls (included). Accepted
by Ap
Neutrino emission rates in highly magnetized neutron stars revisited
Magnetars are a subclass of neutron stars whose intense soft-gamma-ray bursts
and quiescent X-ray emission are believed to be powered by the decay of a
strong internal magnetic field. We reanalyze neutrino emission in such stars in
the plausibly relevant regime in which the Landau band spacing of both protons
and electrons is much larger than kT (where k is the Boltzmann constant and T
is the temperature), but still much smaller than the Fermi energies. Focusing
on the direct Urca process, we find that the emissivity oscillates as a
function of density or magnetic field, peaking when the Fermi level of the
protons or electrons lies about 3kT above the bottom of any of their Landau
bands. The oscillation amplitude is comparable to the average emissivity when
the Landau band spacing mentioned above is roughly the geometric mean of kT and
the Fermi energy (excluding mass), i. e., at fields much weaker than required
to confine all particles to the lowest Landau band. Since the density and
magnetic field strength vary continuously inside the neutron star, there will
be alternating surfaces of high and low emissivity. Globally, these
oscillations tend to average out, making it unclear whether there will be any
observable effects.Comment: 7 pages, 2 figures; accepted for publication in Astronomy &
Astrophysic
Local Time Dependence of Turbulent Magnetic Fields in Saturn\u27s Magnetodisc
Net plasma transport in magnetodiscs around giant planets is outward. Observations of plasma temperature have shown that the expanding plasma is heating nonadiabatically during this process. Turbulence has been suggested as a source of heating. However, the mechanism and distribution of magnetic fluctuations in giant magnetospheres are poorly understood. In this study we attempt to quantify the radial and local time dependence of fluctuating magnetic field signatures that are suggestive of turbulence, quantifying the fluctuations in terms of a plasma heating rate density. In addition, the inferred heating rate density is correlated with magnetic field configurations that include azimuthal bend forward/back and magnitude of the equatorial normal component of magnetic field relative to the dipole. We find a significant local time dependence in magnetic fluctuations that is consistent with flux transport triggered in the subsolar and dusk sectors due to magnetodisc reconnection
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
Coulomb crystals in the magnetic field
The body-centered cubic Coulomb crystal of ions in the presence of a uniform
magnetic field is studied using the rigid electron background approximation.
The phonon mode spectra are calculated for a wide range of magnetic field
strengths and for several orientations of the field in the crystal. The phonon
spectra are used to calculate the phonon contribution to the crystal energy,
entropy, specific heat, Debye-Waller factor of ions, and the rms ion
displacements from the lattice nodes for a broad range of densities,
temperatures, chemical compositions, and magnetic fields. Strong magnetic field
dramatically alters the properties of quantum crystals. The phonon specific
heat increases by many orders of magnitude. The ion displacements from their
equilibrium positions become strongly anisotropic. The results can be relevant
for dusty plasmas, ion plasmas in Penning traps, and especially for the crust
of magnetars (neutron stars with superstrong magnetic fields G). The effect of the magnetic field on ion displacements in a
strongly magnetized neutron star crust can suppress the nuclear reaction rates
and make them extremely sensitive to the magnetic field direction.Comment: 25 pages, 8 figures; accepted to Phys. Rev.
More than meets the eye: magnetars in disguise
It has recently been proposed that radio emission from magnetars can be
evaluated using a "fundamental plane" in parameter space between pulsar voltage
gap and ratio of X-ray luminosity Lx to rotational energy loss rate Edot. In
particular, radio emission from magnetars will occur if Lx/Edot<1 and the
voltage gap is large, and there is no radio emission if Lx/Edot>1. Here we
clarify several issues regarding this fundamental plane, including
demonstrating that the fundamental plane is not uniquely defined. We also show
that, if magnetars and all other pulsars are different manifestations of a
unified picture of neutron stars, then pulsar radio activity (inactivity)
appears to be determined by the ratio Lx/Edot1), although
observational bias and uncertainty in the ratio for some sources may still
invalidate this conclusion. Finally, we comment on the use of other pulsar
parameters that are constructed from the three observables: spin period P,
period derivative Pdot, and Lx.Comment: 6 pages, 3 figures; to appear in MNRAS; added notes on magnetar
differences in radio and recent works on magnetosphere
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
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