16,384 research outputs found
Observations of microglitches in HartRAO radio pulsars
A detailed observation of microglitch phenomenon in relatively slow radio
pulsars is presented. Our analyses for these small amplitude jumps in pulse
rotation frequency () and/or spin down rate () combine the
traditional manual detection method (which hinges on careful visual inspections
of the residuals of pulse phase residuals) and a new, and perhaps more
objective, automated search technique (which exploits the power of the
computer, rather than the eyes, for resolving discrete events in pulsar spin
parameters). The results of the analyses of a sample of 26 radio pulsars reveal
that: (i) only 20 pulsars exhibit significant fluctuations in their arrival
times to be considered suitable for meaningful microglitch analyses; (ii) a
phenomenal 299 microglitch events were identified in and/or :
266 of these events were found to be simultaneously significant in and
, while 19 and 14 were noticeable only in and ,
respectively; (iii) irrespective of sign, the microglitches have fractional
sizes which cover about 3 orders of magnitude in and
( and ) with median values as
and , respectively.Comment: 12 pages, 3 figures, 2 Tables. Accepted for publication in Monthly
Notices of the Royal Astronomical Society Main Journa
Kalman Filter Track Fits and Track Breakpoint Analysis
We give an overview of track fitting using the Kalman filter method in the
NOMAD detector at CERN, and emphasize how the wealth of by-product information
can be used to analyze track breakpoints (discontinuities in track parameters
caused by scattering, decay, etc.). After reviewing how this information has
been previously exploited by others, we describe extensions which add power to
breakpoint detection and characterization. We show how complete fits to the
entire track, with breakpoint parameters added, can be easily obtained from the
information from unbroken fits. Tests inspired by the Fisher F-test can then be
used to judge breakpoints. Signed quantities (such as change in momentum at the
breakpoint) can supplement unsigned quantities such as the various chisquares.
We illustrate the method with electrons from real data, and with Monte Carlo
simulations of pion decays.Comment: 27 pages including 10 figures. To appear in NI
High-order conservative finite difference GLM-MHD schemes for cell-centered MHD
We present and compare third- as well as fifth-order accurate finite
difference schemes for the numerical solution of the compressible ideal MHD
equations in multiple spatial dimensions. The selected methods lean on four
different reconstruction techniques based on recently improved versions of the
weighted essentially non-oscillatory (WENO) schemes, monotonicity preserving
(MP) schemes as well as slope-limited polynomial reconstruction. The proposed
numerical methods are highly accurate in smooth regions of the flow, avoid loss
of accuracy in proximity of smooth extrema and provide sharp non-oscillatory
transitions at discontinuities. We suggest a numerical formulation based on a
cell-centered approach where all of the primary flow variables are discretized
at the zone center. The divergence-free condition is enforced by augmenting the
MHD equations with a generalized Lagrange multiplier yielding a mixed
hyperbolic/parabolic correction, as in Dedner et al. (J. Comput. Phys. 175
(2002) 645-673). The resulting family of schemes is robust, cost-effective and
straightforward to implement. Compared to previous existing approaches, it
completely avoids the CPU intensive workload associated with an elliptic
divergence cleaning step and the additional complexities required by staggered
mesh algorithms. Extensive numerical testing demonstrate the robustness and
reliability of the proposed framework for computations involving both smooth
and discontinuous features.Comment: 32 pages, 14 figure, submitted to Journal of Computational Physics
(Aug 7 2009
Application of a Zero-latency Whitening Filter to Compact Binary Coalescence Gravitational-wave Searches
Joint electromagnetic and gravitational-wave (GW) observation is a major goal
of both the GW astronomy and electromagnetic astronomy communities for the
coming decade. One way to accomplish this goal is to direct follow-up of GW
candidates. Prompt electromagnetic emission may fade quickly, therefore it is
desirable to have GW detection happen as quickly as possible. A leading source
of latency in GW detection is the whitening of the data. We examine the
performance of a zero-latency whitening filter in a detection pipeline for
compact binary coalescence (CBC) GW signals. We find that the filter reproduces
signal-to-noise ratio (SNR) sufficiently consistent with the results of the
original high-latency and phase-preserving filter for both noise and artificial
GW signals (called "injections"). Additionally, we demonstrate that these two
whitening filters show excellent agreement in value, a discriminator
for GW signals.Comment: 8 pages, 12 figure
Hydrodynamic simulations with the Godunov SPH
We present results based on an implementation of the Godunov Smoothed
Particle Hydrodynamics (GSPH), originally developed by Inutsuka (2002), in the
GADGET-3 hydrodynamic code. We first review the derivation of the GSPH
discretization of the equations of moment and energy conservation, starting
from the convolution of these equations with the interpolating kernel. The two
most important aspects of the numerical implementation of these equations are
(a) the appearance of fluid velocity and pressure obtained from the solution of
the Riemann problem between each pair of particles, and (b the absence of an
artificial viscosity term. We carry out three different controlled
hydrodynamical three-dimensional tests, namely the Sod shock tube, the
development of Kelvin-Helmholtz instabilities in a shear flow test, and the
"blob" test describing the evolution of a cold cloud moving against a hot wind.
The results of our tests confirm and extend in a number of aspects those
recently obtained by Cha (2010): (i) GSPH provides a much improved description
of contact discontinuities, with respect to SPH, thus avoiding the appearance
of spurious pressure forces; (ii) GSPH is able to follow the development of
gas-dynamical instabilities, such as the Kevin--Helmholtz and the
Rayleigh-Taylor ones; (iii) as a result, GSPH describes the development of curl
structures in the shear-flow test and the dissolution of the cold cloud in the
"blob" test.
We also discuss in detail the effect on the performances of GSPH of changing
different aspects of its implementation. The results of our tests demonstrate
that GSPH is in fact a highly promising hydrodynamic scheme, also to be coupled
to an N-body solver, for astrophysical and cosmological applications.
[abridged]Comment: 19 pages, 13 figures, MNRAS accepted, high resolution version can be
obtained at
http://adlibitum.oats.inaf.it/borgani/html/papers/gsph_hydrosim.pd
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