20,468 research outputs found
Time-scales of Radio Emission in PSR J0437-4715 at 327 MHz
Time-scales of radio emission are studied in PSR J0437-4715 at 327 MHz using
almost half a million periods of high quality data from Ooty Radio Telescope.
The radio emission in this milli second pulsar occurs on a short (s) time-scale
of approximately 0.026 +- 0.001 periods, and on a (l) time-scale that is much
longer than the widths of the components of the integrated profile
(approximately 0.05 periods). The width of the s emission increases with its
increasing relative contribution to the total radio emission. This may provide
constraints for the details of discharge of vacuum gaps above pulsar polar
caps. The s emission occasionally takes place in the form of intense spikes,
which are confined to the main component of the integrated profile for 90 per
cent of the time. The positions of spikes within a component of the integrated
profile have no simple relation to the shape of that component. This may have
impact on the interpretation of the integrated profile components in terms of
independent regions of emission on the polar cap.Comment: Accepted for publication in Vol 543 (1 Nov 2000) of The Astrophysical
Journa
Spark Model for Pulsar Radiation Modulation Patterns
A non-stationary polar gap model first proposed by Ruderman & Sutherland
(1975) is modified and applied to spark-associated pulsar emission at radio
wave-lengths. It is argued that under physical and geometrical conditions
prevailing above pulsar polar cap, highly non-stationary spark discharges do
not occur at random positions. Instead, sparks should tend to operate in well
determined preferred regions. At any instant the polar cap is populated as
densely as possible with a number of two-dimensional sparks with a
characteristic dimension as well as a typical distance between adjacent sparks
being about the polar gap height. Our model differs, however, markedly from its
original 'hollow cone' version. The key feature is the quasi-central spark
driven by pair production process and anchored to the local pole of a
sunspot-like surface magnetic field. This fixed spark prevents the motion of
other sparks towards the pole, restricting it to slow circumferential drift
across the planes of field lines converging at the local pole. We argue that
the polar spark constitutes the core pulsar emission, and that the annular
rings of drifting sparks contribute to conal components of the pulsar beam. We
found that the number of nested cones in the beam of typical pulsar should not
excced three; a number also found by Mitra & Deshpande (1999) using a
completely different analysis.Comment: 31 pages, 8 figures, accepted by Ap
Non-linear metric perturbation enhancement of primordial gravitational waves
We present the evolution of the full set of Einstein equations during
preheating after inflation. We study a generic supersymmetric model of hybrid
inflation, integrating fields and metric fluctuations in a 3-dimensional
lattice. We take initial conditions consistent with Eintein's constraint
equations. The induced preheating of the metric fluctuations is not large
enough to backreact onto the fields, but preheating of the scalar modes does
affect the evolution of vector and tensor modes. In particular, they do enhance
the induced stochastic background of gravitational waves during preheating,
giving an energy density in general an order of magnitude larger than that
obtained by evolving the tensors fluctuations in an homogeneous background
metric. This enhancement can improve the expectations for detection by planned
gravitational waves observatories.Comment: 5 pages, 4 eps figures, matches Phys. Rev. Lett. versio
Structural basis of complement membrane attack complex formation
In response to complement activation, the membrane attack complex (MAC) assembles from fluid-phase proteins to form pores in lipid bilayers. MAC directly lyses pathogens by a âmulti-hitâ mechanism; however, sublytic MAC pores on host cells activate signalling pathways. Previous studies have described the structures of individual MAC components and subcomplexes; however, the molecular details of its assembly and mechanism of action remain unresolved. Here we report the electron cryo-microscopy structure of human MAC at subnanometre resolution. Structural analyses define the stoichiometry of the complete pore and identify a network of interaction interfaces that determine its assembly mechanism. MAC adopts a âsplit-washerâ configuration, in contrast to the predicted closed ring observed for perforin and cholesterol-dependent cytolysins. Assembly precursors partially penetrate the lipid bilayer, resulting in an irregular ÎČ-barrel pore. Our results demonstrate how differences in symmetric and asymmetric components of the MAC underpin a molecular basis for pore formation and suggest a mechanism of action that extends beyond membrane penetration
Study of the Low Energy Dynamics in the ppK+K- System with the Cosy-11 Magnetic Spectrometer
The near threshold production of K+K- pairs in proton-proton collisions has
been investigated at the cooler synchrotron COSY below and above the threshold
for the \phi meson production. The experimental excitation function determined
for the pp-->ppK+K- reaction differs from theoretical expectations including
proton-proton final state interaction. The discrepancy may be assigned to the
influence of K+K- or pK interaction. In this article we present distributions
of the cross section for the pp-->ppK+K- reaction as a function of the
invariant masses of two and three particle subsystems at excess energies of
Q=10 MeV and 28 MeV.Comment: Presented at the Symposium on Meson Physics, Cracow, 01-04 October
200
Frequency dependence of pulsar radiation patterns
We report on new results from simultaneous, dual frequency, single pulse
observation of PSR B0329+54 using the Giant Metrewave Radio Telescope. We find
that the longitude separation of subpulses at two different frequencies (238
and 612 MHz) is less than that for the corresponding components in the average
profile. A similar behaviour has been noticed before in a number of pulsars. We
argue that subpulses are emitted within narrow flux tubes of the dipolar field
lines and that the mean pulsar beam has a conal structure. In such a model the
longitudes of profile components are determined by the intersection of the line
of sight trajectory with subpulse-associated emission beams. Thus, we show that
the difference in the frequency dependence of subpulse and profile component
longitudes is a natural property of the conal model of pulsar emission beam. We
support our conclusions by numerical modelling of pulsar emission, using the
known parameters for this pulsar, which produce results that agree very well
with our dual frequency observations.Comment: 24 pages, 8 figures. Accepted for publication in Ap
Strictly Anomaly Mediated Supersymmetry Breaking
We consider an MSSM extension with anomaly mediation as the source of
supersymmetry-breaking, and a U(1) symmetry which solves the tachyonic slepton
problem, and introduces both the see-saw mechanism for neutrino masses, and the
Higgs mu-term. We compare its spectra with those from so-called minimal anomaly
mediated supersymmetry breaking. We find a Standard Model-like Higgs of mass
125 GeV with a gravitino mass of 140 TeV and tan(beta)=16. However, the muon
anomalous magnetic moment is 3 sigma away from the experimental value.
The model naturally produces a period of hybrid inflation, which can exit to
a false vacuum characterised by large Higgs vevs, reaching the true ground
state after a period of thermal inflation. The scalar spectral index is reduced
to approximately 0.975, and the correct abundance of neutralino dark matter can
be produced by decays of thermally-produced gravitinos, provided the gravitino
mass (and hence the Higgs mass) is high. Naturally light cosmic strings are
produced, satisfying bounds from the Cosmic Microwave Background. The
complementary pulsar timing and cosmic ray bounds require that strings decay
primarily via loops into gravitational waves. Unless the loops are extremely
small, the next generation pulsar timing array will rule out or detect the
string-derived gravitational radiation background in this model.Comment: 38 pages, 1 figure. Discussion of 125GeV Higgs possibility, and of
U(1) decoupling limi
Non-linear Preheating with Scalar Metric Perturbations
We have studied preheating of field perturbations in a 3-dimensional lattice
including the effect of scalar metric perturbations, in two generic models of
inflation: chaotic inflation with a quartic potential, and standard hybrid
inflation. We have prepared the initial state for the classical evolution of
the system with vanishing vector and tensor metric perturbations, consistent
with the constraint equations, the energy and momentum constraints. The
non-linear evolution inevitably generates vector and tensor modes, and this
reflects on how well the constraint equations are fulfilled during the
evolution. The induced preheating of the scalar metric perturbations is not
large enough to backreact onto the fields, but it could affect the evolution of
vector and tensor modes. This is the case in hybrid inflation for some values
of the coupling and the height of potential . For example with
GeV, preheating of scalar perturbations is such that
their source term in the evolution equation of tensor and vector becomes
comparable to that of the field anisotropic stress.Comment: 15 pages, 12 eps figure
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