210 research outputs found
Two decades of pulsar timing of Vela
Pulsar timing at the Mt Pleasant observatory has focused on Vela, which can
be tracked for 18 hours of the day. These nearly continuous timing records
extend over 24 years allowing a greater insight into details of timing noise,
micro glitches and other more exotic effects. In particular we report the
glitch parameters of the 2004 event, along with the reconfirmation that the
spin up for the Vela pulsar occurs instantaneously to the accuracy of the data.
This places a lower limit of about 30 seconds for the acceleration of the
pulsar to the new rotational frequency. We also confirm of the low braking
index for Vela, and the continued fall in the DM for this pulsar.Comment: Isolated Neutron Stars conference, London, April 24-28 200
Fermat Principle in Finsler Spacetimes
It is shown that, on a manifold with a Finsler metric of Lorentzian
signature, the lightlike geodesics satisfy the following variational principle.
Among all lightlike curves from a point (emission event) to a timelike curve
(worldline of receiver), the lightlike geodesics make the arrival time
stationary. Here ``arrival time'' refers to a parametrization of the timelike
curve. This variational principle can be applied (i) to the vacuum light rays
in an alternative spacetime theory, based on Finsler geometry, and (ii) to
light rays in an anisotropic non-dispersive medium with a general-relativistic
spacetime as background.Comment: 18 pages, submitted to Gen. Rel. Gra
Polarization-Correlated Photon Pairs from a Single Quantum Dot
Polarization correlation in a linear basis, but not entanglement, is observed
between the biexciton and single-exciton photons emitted by a single InAs
quantum dot in a two-photon cascade. The results are well described
quantitatively by a probabilistic model that includes two decay paths for a
biexciton through a non-degenerate pair of one-exciton states, with the
polarization of the emitted photons depending on the decay path. The results
show that spin non-degeneracy due to quantum-dot asymmetry is a significant
obstacle to the realization of an entangled-photon generation device.Comment: 4 pages, 4 figures, revised discussio
Reversible maps and composites of involutions in groups of piecewise linear homeomorphisms of the real line
An element of a group is reversible if it is conjugate to its own inverse, and it is strongly reversible if it is conjugate to its inverse by an involution. A group element is strongly reversible if and only if it can be expressed as a composite of two involutions. In this paper the reversible maps, the strongly reversible maps, and those maps that can be expressed as a composite of involutions are determined in certain groups of piecewise linear homeomorphisms of the real line
Particle decays and stability on the de Sitter universe
We study particle decay in de Sitter space-time as given by first order
perturbation theory in a Lagrangian interacting quantum field theory. We study
in detail the adiabatic limit of the perturbative amplitude and compute the
"phase space" coefficient exactly in the case of two equal particles produced
in the disintegration. We show that for fields with masses above a critical
mass there is no such thing as particle stability, so that decays
forbidden in flat space-time do occur here. The lifetime of such a particle
also turns out to be independent of its velocity when that lifetime is
comparable with de Sitter radius. Particles with mass lower than critical have
a completely different behavior: the masses of their decay products must obey
quantification rules, and their lifetime is zero.Comment: Latex, 38 pages, 1 PostScript figure; added references, minor
corrections and remark
Active region formation through the negative effective magnetic pressure instability
The negative effective magnetic pressure instability operates on scales
encompassing many turbulent eddies and is here discussed in connection with the
formation of active regions near the surface layers of the Sun. This
instability is related to the negative contribution of turbulence to the mean
magnetic pressure that causes the formation of large-scale magnetic structures.
For an isothermal layer, direct numerical simulations and mean-field
simulations of this phenomenon are shown to agree in many details in that their
onset occurs at the same depth. This depth increases with increasing field
strength, such that the maximum growth rate of this instability is independent
of the field strength, provided the magnetic structures are fully contained
within the domain. A linear stability analysis is shown to support this
finding. The instability also leads to a redistribution of turbulent intensity
and gas pressure that could provide direct observational signatures.Comment: 19 pages, 10 figures, submitted to Solar Physic
Vortices in superfluid trapped Fermi gases at zero temperature
We discuss various aspects of the vortex state of a dilute superfluid atomic
Fermi gas at T=0. The energy of the vortex in a trapped gas is calculated and
we provide an expression for the thermodynamic critical rotation frequency of
the trap for its formation. Furthermore, we propose a method to detect the
presence of a vortex by calculating the effect of its associated velocity field
on the collective mode spectrum of the gas
Gravitational waves from inspiralling compact binaries: Energy loss and waveform to second--post-Newtonian order
Gravitational waves generated by inspiralling compact binaries are
investigated to the second--post-Newtonian (2PN) approximation of general
relativity. Using a recently developed 2PN-accurate wave generation formalism,
we compute the gravitational waveform and associated energy loss rate from a
binary system of point-masses moving on a quasi-circular orbit. The crucial new
input is our computation of the 2PN-accurate ``source'' quadrupole moment of
the binary. Tails in both the waveform and energy loss rate at infinity are
explicitly computed. Gravitational radiation reaction effects on the orbital
frequency and phase of the binary are deduced from the energy loss. In the
limiting case of a very small mass ratio between the two bodies we recover the
results obtained by black hole perturbation methods. We find that finite mass
ratio effects are very significant as they increase the 2PN contribution to the
phase by up to 52\%. The results of this paper should be of use when
deciphering the signals observed by the future LIGO/VIRGO network of
gravitational-wave detectors.Comment: 43 pages, LaTeX-ReVTeX, no figures
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