* Astronomy

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RE: Neutron stars

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Pulsars & Neutron Stars

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Title: Particle acceleration in the polar cap region of an oscillating neutron star
Authors: Olindo Zanotti, Viktoriya Morozova, Bobomurat Ahmedov

We have revisited the issue of particle acceleration in the polar cap region of a neutron star by taking into account both general relativistic effects and the presence of toroidal oscillations at the star surface. In particular, we address the question of whether toroidal oscillations of the star surface can affect the acceleration properties in the polar cap. We have solved numerically the relativistic electrodynamics equations in the stationary regime, focusing on the computation of the Lorentz factor of a space-charge-limited electron flow accelerated in the polar cap region of a rotating as well as oscillating pulsar. To this extent, the correct expression for the general relativistic Goldreich-Julian charge density in the presence of toroidal oscillations has been adopted. Depending on the ratio between the actual charge density of the pulsar magnetosphere and the Goldreich-Julian charge density, two different regimes of the Lorentz factor of the particle flow are found. Namely an oscillatory regime, which is produced for sub-GJ current density configurations, and which does not produce an efficient acceleration, and a true accelerating regime, which is produced for super-GJ current density configurations. We have found that star oscillations may become responsible of a significant asymmetry of the pulse profile, which will depend on the orientation of the oscillations with respect to the pulsar magnetic field. In particular, significant enhancements of the Lorentz factor are produced by star oscillations in the super-GJ current density regime.

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Neutron star seen forming exotic new state of matter

The dense core of a nearby neutron star is undergoing a rapid chill, providing the first direct evidence that such stars can become superfluid - a state of matter that cannot be created in laboratories on Earth.

Neutron stars are the remnants of exploded stars. Their cores are so dense that atomic nuclei dissolve, and protons and electrons combine to form a soup dominated by neutrons.

If conditions are right, these neutrons ought to be able to pair up to form a superfluid - a substance with quantum properties that mean it flows with zero friction. Superfluids formed in laboratories can do bizarre things such as creep up the walls of a cup, or remain still even while their container is made to spin.

Blobrana · L

Title: Can neutron stars constrain dark matter?
Authors: Chris Kouvaris and Peter Tinyakov

Because of their strong gravitational field, neutron stars capture weakly interacting dark matter particles (WIMPs) more efficiently compared to other stars, including the white dwarfs. Once captured, the WIMPs sink to the neutron star center and annihilate, heating the star. We find that this heat could lead to detectable effects on the surface temperature of old neutron stars, especially those in dark-matter-rich regions such as the Galactic center or cores of globular clusters. The capture and annihilation is fully efficient even for WIMP-to-nucleon cross sections (elastic or inelastic) as low as ~10^-45cm², and for the annihilation cross sections as small as ~10^-57cm². Thus, detection of a sufficiently cold neutron star in a dark-matter-rich environment would exclude a wide range of dark matter candidates, including those with extremely small cross sections.

Source

Blobrana · L

Title: Neutron Star Radius Measurement with the Quiescent Low-Mass X-ray Binary U24 in NGC 6397
Authors: Sebastien Guillot, Robert E. Rutledge, Edward F. Brown

This paper reports the spectral and timing analyses of the quiescent low-mass X-ray binary U24 observed during five archived Chandra-ACIS exposures of the nearby globular cluster NGC 6397, for a total of 350 ksec. We find that the X-ray flux and the parameters of the hydrogen atmosphere spectral model are consistent with those previously published. Following the timing analysis, we find no evidence of short or long-term intensity variability. We also report the improved neutron star physical radius measurements, with statistical accuracy of the order of ~10%: R_ns = 8.9(+0.9)(-0.6) km for M_ns = 1.4 Msun. Alternatively, we provide the best-fit projected radius R_infinity= 11.9(+2.2)(-2.5)km, as seen by an observer at infinity. The best-fit effective temperature, kTeff = 80(+4)(-5) eV, is used to estimate the neutron star core temperature which falls in the range T_core = (3.0 - 9.8) x10 7 K, depending on the atmosphere model considered. This makes U24 the fourth most precisely measured neutron star radius among qLMXBs, after those in OmCen, in M13 and the qLMXB 47Tuc X7.

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Title: Constraints on neutron star radii based on chiral effective field theory interactions
Authors: K. Hebeler, J.M. Lattimer, C.J. Pethick, A. Schwenk

We show that microscopic calculations based on chiral effective field theory interactions constrain the properties of dense matter below nuclear densities to a much higher degree than is reflected in current neutron star modelling. Combined with observed neutron star masses, our results lead to a radius R = 11.8 ± 2.1 km for a M = 1.4 M_{solar} neutron star, where the theoretical error is due, in about equal amounts, to uncertainties in many-body forces and to the extrapolation to high densities.

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Title: Have neutron stars a dark matter core?
Authors: Paolo Ciarcelluti, Fredrik Sandin

Recent observational results for the masses and radii of some neutron stars are in contrast with typical observations and theoretical predictions for "normal" neutron stars. We propose that their unusual properties can be interpreted as the signature of a dark matter core inside them. This interpretation requires that the dark matter is made of some form of stable, long-living or in general non-annihilating particles, that can accumulate in the star. In the proposed scenario all mass-radius measurements can be explained with one nuclear matter equation of state and a dark core of varying relative size. This hypothesis will be challenged by forthcoming observations and could eventually be a useful tool for the determination of dark matter.

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Dark matter may give neutron stars black hearts

Dark matter may be prompting black holes to appear spontaneously in the hearts of distant exotic stars. If so, this could hint at the nature of dark matter.
Arnaud de Lavallaz and Malcolm Fairbairn of King's College London wondered what would happen when dark matter - which makes up most of the mass of galaxies - is sucked into the heart of neutron stars. These stars, the remnants of supernova explosions, are the densest known stars in the universe. It turns out that the outcome depends on the nature of dark matter.
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Faster-Than-Light Polarisation Currents

Pulsars are neutron stars that emit amazingly regular, short bursts of radio waves, so regular that they were originally thought to be signals from little green men! Though their discovery over 40 years ago was very widely reported and resulted in a Nobel Prize, the reasons how and why they send these bursts has remained a mystery; to quote Jean Eilek of NRAO, "we know why they pulse, but why do they shine?"

However, in papers presented this week to the American Astronomical Society, Andrea Schmidt and John Singleton of Los Alamos National Laboratory provide detailed analyses of several pieces of observational data that suggest that pulsars emit the electromagnetic equivalent of the well-known "sonic boom" from accelerating supersonic aircraft. Just as the "boom" can be very loud a long way from the aircraft, the analogous signals from the pulsar remain intense over very long distances.
Schmidt and Singleton's presentations provide strong support for a pulsar emission mechanism (the superluminal model) due to circulating polarisation currents that travel faster than the speed of light. These superluminal polarisation currents are disturbances in the pulsar's plasma
atmosphere in which oppositely-charged particles are displaced by small amounts in opposite directions; they are induced by the neutron star's rotating magnetic field.

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Ed ~ This superluminal Sonic boom is an illusion. The underlying nature of spacetime dictates that nothing can travel faster than light speed.

-- Edited by Blobrana on Wednesday 6th of January 2010 05:52:50 PM