98 research outputs found
Hydromagnetic and gravitomagnetic crust-core coupling in a precessing neutron star
We consider two types of mechanical coupling between the crust and the core
of a precessing neutron star. First, we find that a hydromagnetic (MHD)
coupling between the crust and the core strongly modifies the star's
precessional modes when ; here is the
Alfven crossing timescale, and and are the star's spin and
precession periods, respectively. We argue that in a precessing pulsar PSR
B1828-11 the restoring MHD stress prevents a free wobble of the crust relative
to the non-precessing core. Instead, the crust and the proton-electron plasma
in the core must precess in unison, and their combined ellipticity determines
the period of precession. Link has recently shown that the neutron superfluid
vortices in the core of PSR B1828-11 cannot be pinned to the plasma; he has
also argued that this lack of pinning is expected if the proton Fermi liquid in
the core is type-I superconductor. In this case, the neutron superfluid is
dynamically decoupled from the precessing motion. The pulsar's precession
decays due to the mutual friction between the neutron superfluid and the plasma
in the core. The decay is expected to occur over tens to hundreds of precession
periods and may be measurable over a human lifetime. Such a measurement would
provide information about the strong n-p interaction in the neutron-star core.
Second, we consider the effect of gravitomagnetic coupling between the neutron
superfluid in the core and the rest of the star and show that this coupling
changes the rate of precession by about 10%. The general formalism developed in
this paper may be useful for other applications.Comment: 6 page
Electromagnetic power of merging and collapsing compact objects
[Abridged] Electromagnetic emission can be produced as a precursor to the
merger, as a prompt emission during the collapse of a NS and at the spin-down
stage of the resulting BH. We demonstrate that the time evolution of the
axisymmetric force-free magnetic fields can be expressed in terms of the
hyperbolic Grad-Shafranov equation. We find exact non-linear time-dependent
split-monopole structure of magnetosphere driven by spinning and collapsing NS
in Schwarzschild geometry. Based on this solution, we argue that the collapse
of a NS into the BH happens smoothly, without natural formation of current
sheets or other dissipative structures on the open field lines and, thus, does
not allow the magnetic field to become disconnected from the star and escape to
infinity. Thus, as long as an isolated Kerr BH can produce plasma and currents,
it does not lose its open magnetic field lines, its magnetospheric structure
evolved towards a split monopole and the BH spins down electromagnetically. The
"no hair theorem", which assumes that the outside medium is a vacuum, is not
applicable in this case: highly conducting plasma introduces a topological
constraint forbidding the disconnection of the magnetic field lines from the
BH. Eventually, a single random large scale spontaneous reconnection event will
lead to magnetic field release, shutting down the electromagnetic BH engine
forever. We also discuss the nature of short Gamma Ray Bursts and suggest that
the similarity of the early afterglows properties of long and short GRBs can be
related to the fact that in both cases a spinning BH can retains magnetic field
for sufficiently long time to extract a large fraction of its rotation energy
and produce high energy emission via the internal dissipation in the wind
Schwarzschild black holes as unipolar inductors: expected electromagnetic power of a merger
(Abridged) The motion of a Schwarzschild black hole with velocity through a constant magnetic field in vacuum induces a
component of the electric field along the magnetic field, generating a non-zero
second Poincare electromagnetic invariant . This will
produce (e.g., via radiative effects and vacuum breakdown) an electric charge
density of the order of , where
is the Schwarzschild radius and is the mass of the black
hole; the charge density is similar to the Goldreich-Julian
density. The magnetospheres of moving black holes resemble in many respects the
magnetospheres of rotationally-powered pulsars, with pair formation fronts and
outer gaps, where the sign of the induced charge changes. As a result, the
black hole will generate bipolar electromagnetic jets each consisting of two
counter-aligned current flows (four current flows total), each carrying an
electric current of the order . The
electromagnetic power of the jets is ;
for a particular case of merging black holes the resulting Poynting power is , where is the radius of the orbit. In
addition, in limited regions near the horizon the first electromagnetic
invariant changes sign, so that the induced electric field becomes larger than
the magnetic field, . The total energy loss from a system of merging BHs
is a sum of two components with similar powers, one due to the rotation of
space-time within the orbit, driven by the non-zero angular momentum in the
system, and the other due to the linear motion of the BHs through the magnetic
field.Comment: Phys. Rev. D accepte
Analytic Solutions to the Constraint Equation for a Force-Free Magnetosphere around a Kerr Black Hole
The Blandford-Znajek constraint equation for a stationary, axisymmetric
black-hole force-free magnetosphere is cast in a 3+1 absolute space and time
formulation, following Komissarov (2004). We derive an analytic solution for
fields and currents to the constraint equation in the far-field limit that
satisfies the Znajek condition at the event horizon. This solution generalizes
the Blandford-Znajek monopole solution for a slowly rotating black hole to
black holes with arbitrary angular momentum. Energy and angular momentum
extraction through this solution occurs mostly along the equatorial plane. We
also present a nonphysical, reverse jet-like solution.Comment: 6 pages, accepted for publication in Ap
Compton Scattering by Static and Moving Media I. The Transfer Equation and Its Moments
Compton scattering of photons by nonrelativistic particles is thought to play
an important role in forming the radiation spectrum of many astrophysical
systems. Here we derive the time-dependent photon kinetic equation that
describes spontaneous and induced Compton scattering as well as absorption and
emission by static and moving media, the corresponding radiative transfer
equation, and their zeroth and first moments, in both the system frame and in
the frame comoving with the medium. We show that it is necessary to use the
correct relativistic differential scattering cross section in order to obtain a
photon kinetic equation that is correct to first order in epsilon/m_e, T_e/m_e,
and V, where epsilon is the photon energy, T_e and m_e are the electron
temperature and rest mass, and V is the electron bulk velocity in units of the
speed of light. We also demonstrate that the terms in the radiative transfer
equation that are second-order in V usually should be retained, because if the
radiation energy density is sufficiently large compared to the radiation flux,
the effects of bulk Comptonization described by the terms that are second-order
in V are at least as important as the effects described by the terms that are
first-order in V, even when V is small. Our equations are valid for systems of
arbitrary optical depth and can therefore be used in both the free-streaming
and the diffusion regimes. We demonstrate that Comptonization by the electron
bulk motion occurs whether or not the radiation field is isotropic or the bulk
flow converges and that it is more important than thermal Comptonization if V^2
> 3 T_e/m_e.Comment: 31 pages, accepted for publication in The Astrophysical Journa
Non-monotonic orbital velocity profiles around rapidly rotating Kerr-(anti-)de Sitter black holes
It has been recently demonstrated that the orbital velocity profile around
Kerr black holes in the equatorial plane as observed in the locally
non-rotating frame exhibits a non-monotonic radial behaviour. We show here that
this unexpected minimum-maximum feature of the orbital velocity remains if the
Kerr vacuum is generalized to the Kerr-de Sitter or Kerr-anti-de Sitter metric.
This is a new general relativity effect in Kerr spacetimes with non-vanishing
cosmological constant. Assuming that the profile of the orbital velocity is
known, this effect constrains the spacetime parameters.Comment: 9 pages, 4 figures, accepted for Class. Quant. Gra
High Resolution Ionization of Ultracold Neutral Plasmas
Collective effects, such as waves and instabilities, are integral to our
understanding of most plasma phenomena. We have been able to study these in
ultracold neutral plasmas by shaping the initial density distribution through
spatial modulation of the ionizing laser intensity. We describe a relay imaging
system for the photoionization beam that allows us to create higher resolution
features and its application to extend the observation of ion acoustic waves to
shorter wavelengths. We also describe the formation of sculpted density
profiles to create fast expansion of plasma into vacuum and streaming plasmas
Understanding possible electromagnetic counterparts to loud gravitational wave events: Binary black hole effects on electromagnetic fields
In addition to producing loud gravitational waves (GW), the dynamics of a
binary black hole system could induce emission of electromagnetic (EM)
radiation by affecting the behavior of plasmas and electromagnetic fields in
their vicinity. We here study how the electromagnetic fields are affected by a
pair of orbiting black holes through the merger. In particular, we show how the
binary's dynamics induce a variability in possible electromagnetically induced
emissions as well as an enhancement of electromagnetic fields during the
late-merge and merger epochs. These time dependent features will likely leave
their imprint in processes generating detectable emissions and can be exploited
in the detection of electromagnetic counterparts of gravitational waves.Comment: 12 page
Robustness of the Blandford-Znajek mechanism
The Blandford-Znajek mechanism has long been regarded as a key ingredient in
models attempting to explain powerful jets in AGNs, quasars, blazzars etc. In
such mechanism, energy is extracted from a rotating black hole and dissipated
at a load at far distances. In the current work we examine the behaviour of the
BZ mechanism with respect to different boundary conditions, revealing the
mechanism robustness upon variation of these conditions. Consequently, this
work closes a gap in our understanding of this important scenario.Comment: 7 pages, accepted in CQ
Timelike Geodesic Currents in the Stationary, Axisymmetric, Force-free Magnetosphere of a Kerr Black Hole
The structural properties of geodesic currents in an ambient Kerr background
is studied from an analytical point of view. The geodesics in the congruence
correspond to charged particles that carry energy and angular momentum from the
black hole through the Blandford-Znajek mechanism. It is shown that the
resulting magnetosphere naturally satisfies the Znajek regularity condition.
Particular attention is paid here to the energy extracted by matter currents
rather than by electromagnetic Poynting fluxes
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