438 research outputs found
Persistent crust-core spin lag in neutron stars
It is commonly believed that the magnetic field threading a neutron star
provides the ultimate mechanism (on top of fluid viscosity) for enforcing
long-term corotation between the slowly spun down solid crust and the liquid
core. We show that this argument fails for axisymmetric magnetic fields with
closed field lines in the core, the commonly used `twisted torus' field being
the most prominent example. The failure of such magnetic fields to enforce
global crust-core corotation leads to the development of a persistent spin lag
between the core region occupied by the closed field lines and the rest of the
crust and core. We discuss the repercussions of this spin lag for the evolution
of the magnetic field, suggesting that, in order for a neutron star to settle
to a stable state of crust-core corotation, the bulk of the toroidal field
component should be deposited into the crust soon after the neutron star's
birth.Comment: 17 pages, 1 figure; v2: minor corrections, matches the version to
appear in MNRA
Generalized Lemaitre-Tolman-Bondi Solutions with Pressure
Utilizing the ADM equations, we derive a metric and reduced field equations
describing a general, spherically symmetric perfect fluid. The metric describes
both the interior perfect fluid region and exterior vacuum Schwarzschild
spacetime in a single coordinate patch. The exterior spacetime is in
generalized Painleve-Gullstrand coordinates which is an infinite class of
coordinate systems. In the static limit the system reduces to a
Tolman-Oppenheimer-Volkoff equation on the interior with the exterior in
Schwarzschild coordinates. We show the coordinate transformation for the
non-static cases to comoving coordinates, where the metric is seen to be a
direct generalization of the Lemaitre-Tolman-Bondi spacetime to include
pressures.Comment: Accepted for publication by Physical Reviews
Black holes and neutron stars in the generalized tensor-vector-scalar theory
Bekenstein's Tensor-Vector-Scalar (TeVeS) theory has had considerable success
as a relativistic theory of Modified Newtonian Dynamics (MoND). However, recent
work suggests that the dynamics of the theory are fundamentally flawed and
numerous authors have subsequently begun to consider a generalization of TeVeS
where the vector field is given by an Einstein-Aether action. Herein, I develop
strong-field solutions of the generalized TeVeS theory, in particular exploring
neutron stars as well as neutral and charged black holes. I find that the
solutions are identical to the neutron star and black hole solutions of the
original TeVeS theory, given a mapping between the parameters of the two
theories, and hence provide constraints on these values of the coupling
constants. I discuss the consequences of these results in detail including the
stability of such spacetimes as well as generalizations to more complicated
geometries.Comment: Accepted for publication in Physical Review
A neutron star progenitor for FRBs? Insights from polarisation measurements
Fast Radio Bursts (FRBs) are intense, millisecond-duration broadband radio
transients, the emission mechanisms of which are not understood. Masui et al.
recently presented Green Bank Telescope observations of FRB 110523, which
displayed temporal variation of the linear polarisation position angle (PA).
This effect is commonly seen in radio pulsars and is attributed to a changing
projected magnetic field orientation in the emission region as the star
rotates. If a neutron star is the progenitor of this FRB, and the emission
mechanism is pulsar-like, we show that the progenitor is either rapidly
rotating, or the emission originates from a region of complex magnetic field
geometry. The observed PA variation could also be caused by propagation effects
within a neutron-star magnetosphere, or by spatially varying magnetic fields if
the progenitor lies in a dense, highly magnetised environment. Although we urge
caution in generalising results from FRB 110523 to the broader FRB population,
our analysis serves as a guide to interpreting future polarisation measurements
of FRBs, and presents another means of elucidating the origins of these
enigmatic ephemera.Comment: 7 pages, 2 figures, submitted to MNRA
The effect of pressure gradients on luminosity distance - redshift relations
Inhomogeneous cosmological models have had significant success in explaining
cosmological observations without the need for dark energy. Generally, these
models imply inhomogeneous matter distributions alter the observable relations
that are taken for granted when assuming the Universe evolves according to the
standard Friedmann equations. Moreover, it has recently been shown that both
inhomogeneous matter and pressure distributions are required in both early and
late stages of cosmological evolution. These associated pressure gradients are
required in the early Universe to sufficiently describe void formation, whilst
late-stage pressure gradients stop the appearance of anomalous singularities.
In this paper we investigate the effect of pressure gradients on cosmological
observations by deriving the luminosity distance - redshift relations in
spherically symmetric, inhomogeneous spacetimes endowed with a perfect fluid.
By applying this to a specific example for the energy density distribution and
using various equations of state, we are able to explicitly show that pressure
gradients may have a non-negligble effect on cosmological observations. In
particular, we show that a non-zero pressure gradient can imply significantly
different residual Hubble diagrams for compared to when the
pressure is ignored. This paper therefore highlights the need to properly
consider pressure gradients when interpreting cosmological observations.Comment: Accepted for publication in Classical and Quantum Gravit
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