54,692 research outputs found
Giant Radio Pulses from the Crab Pulsar
Individual giant radio pulses (GRPs) from the Crab pulsar last only a few
microseconds. However, during that time they rank among the brightest objects
in the radio sky reaching peak flux densities of up to 1500 Jy even at high
radio frequencies. Our observations show that GRPs can be found in all phases
of ordinary radio emission including the two high frequency components (HFCs)
visible only between 5 and 9 GHz (Moffett & Hankins, 1996). This leads us to
believe that there is no difference in the emission mechanism of the main pulse
(MP), inter pulse (IP) and HFCs. High resolution dynamic spectra from our
recent observations of giant pulses with the Effelsberg telescope at a center
frequency of 8.35 GHz show distinct spectral maxima within our observational
bandwidth of 500 MHz for individual pulses. Their narrow band components appear
to be brighter at higher frequencies (8.6 GHz) than at lower ones (8.1 GHz).
Moreover, there is an evidence for spectral evolution within and between those
structures. High frequency features occur earlier than low frequency ones.
Strong plasma turbulence might be a feasible mechanism for the creation of the
high energy densities of ~6.7 x 10^4 erg cm^-3 and brightness temperatures of
10^31 K.Comment: accepted by Advances in Space Research, to appear in the 35th COSPAR
assembly proceeding
Boundary value problems for the stationary axisymmetric Einstein equations: a disk rotating around a black hole
We solve a class of boundary value problems for the stationary axisymmetric
Einstein equations corresponding to a disk of dust rotating uniformly around a
central black hole. The solutions are given explicitly in terms of theta
functions on a family of hyperelliptic Riemann surfaces of genus 4. In the
absence of a disk, they reduce to the Kerr black hole. In the absence of a
black hole, they reduce to the Neugebauer-Meinel disk.Comment: 46 page
The magnetoresistance tensor of La(0.8)Sr(0.2)MnO(3)
We measure the temperature dependence of the anisotropic magnetoresistance
(AMR) and the planar Hall effect (PHE) in c-axis oriented epitaxial thin films
of La(0.8)Sr(0.2)MnO(3), for different current directions relative to the
crystal axes, and show that both AMR and PHE depend strongly on current
orientation. We determine a magnetoresistance tensor, extracted to 4th order,
which reflects the crystal symmetry and provides a comprehensive description of
the data. We extend the applicability of the extracted tensor by determining
the bi-axial magnetocrystalline anisotropy in our samples
25 kHz narrow spectral bandwidth of a wavelength tunable diode laser with a short waveguide-based external cavity
We report on the spectral properties of a diode laser with a tunable external
cavity in integrated optics. Even though the external cavity is short compared
to other small-bandwidth external cavity lasers, the spectral bandwidth of this
tunable laser is as small as 25 kHz (FWHM), at a side-mode suppression ratio
(SMSR) of 50 dB. Our laser is also able to access preset wavelengths in as
little as 200 us and able to tune over the full telecom C-band (1530 nm - 1565
nm).Comment: 8 pages, 7 figure
Slowly rotating voids in cosmology
We consider a spacetime consisting of an empty void separated from an almost
Friedmann-Lema\^\i tre-Robertson-Walker (FLRW) dust universe by a spherically
symmetric, slowly rotating shell which is comoving with the cosmic dust. We
treat in a unified manner all types of the FLRW universes. The metric is
expressed in terms of a constant characterizing the angular momentum of the
shell, and parametrized by the comoving radius of the shell. Treating the
rotation as a first order perturbation, we compute the dragging of inertial
frames as well as the apparent motion of distant stars within the void.
Finally, we discuss, in terms of in principle measurable quantities, 'Machian'
features of the model.Comment: 21 pages, 5 figures, REVTex, accepted for publication in
Class.Quant.Gravit
Relativistic Static Thin Disks: The Counter-Rotating Model
A detailed study of the Counter-Rotating Model (CRM) for generic finite
static axially symmetric thin disks with nonzero radial pressure is presented.
We find a general constraint over the counter-rotating tangential velocities
needed to cast the surface energy-momentum tensor of the disk as the
superposition of two counter-rotating perfect fluids. We also found expressions
for the energy density and pressure of the counter-rotating fluids. Then we
shown that, in general, there is not possible to take the two counter-rotating
fluids as circulating along geodesics neither take the two counter-rotating
tangential velocities as equal and opposite. An specific example is studied
where we obtain some CRM with well defined counter-rotating tangential
velocities and stable against radial perturbations. The CRM obtained are in
agree with the strong energy condition, but there are regions of the disks with
negative energy density, in violation of the weak energy condition.Comment: 19 pages, 6 figures. Submitted to Physical Review
Geodesic Deviation in Kaluza-Klein Theories
We study in detail the equations of the geodesic deviation in
multidimensional theories of Kaluza-Klein type. We show that their
4-dimensional space-time projections are identical with the equations obtained
by direct variation of the usual geodesic equation in the presence of the
Lorentz force, provided that the fifth component of the deviation vector
satisfies an extra constraint derived here.Comment: 5 pages, Revtex, 1 figure. To appear in Phys. Rev. D (Brief Report
Retrodirective transponder feasibility experiment
Test program on feasibility of digital phase measuring subsystem of pulse-coherent retrodirective transponde
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