15,062 research outputs found
A CLEAN-based Method for Deconvolving Interstellar Pulse Broadening from Radio Pulses
Multipath propagation in the interstellar medium distorts radio pulses, an
effect predominant for distant pulsars observed at low frequencies. Typically,
broadened pulses are analyzed to determine the amount of propagation-induced
pulse broadening, but with little interest in determining the undistorted pulse
shapes. In this paper we develop and apply a method that recovers both the
intrinsic pulse shape and the pulse broadening function that describes the
scattering of an impulse. The method resembles the CLEAN algorithm used in
synthesis imaging applications, although we search for the best pulse
broadening function, and perform a true deconvolution to recover intrinsic
pulse structre. As figures of merit to optimize the deconvolution, we use the
positivity and symmetry of the deconvolved result along with the mean square
residual and the number of points below a given threshold. Our method makes no
prior assumptions about the intrinsic pulse shape and can be used for a range
of scattering functions for the interstellar medium. It can therefore be
applied to a wider variety of measured pulse shapes and degrees of scattering
than the previous approaches. We apply the technique to both simulated data and
data from Arecibo observations.Comment: 9 pages, 6 figures, Accepted for publication in the Astrophysical
Journa
Computer model calibration with large non-stationary spatial outputs: application to the calibration of a climate model
Bayesian calibration of computer models tunes unknown input parameters by
comparing outputs with observations. For model outputs that are distributed
over space, this becomes computationally expensive because of the output size.
To overcome this challenge, we employ a basis representation of the model
outputs and observations: we match these decompositions to carry out the
calibration efficiently. In the second step, we incorporate the non-stationary
behaviour, in terms of spatial variations of both variance and correlations, in
the calibration. We insert two integrated nested Laplace
approximation-stochastic partial differential equation parameters into the
calibration. A synthetic example and a climate model illustration highlight the
benefits of our approach
Long term variability of the cosmic ray intensity
In a previous paper Bhat, et al., assess the evidence for the continuing acceleration of cosmic rays in the Loop I supernova remnant. The enhanced gamma-ray emission is found consistent with the Blandford and Cowie model for particle acceleration at the remnant shock wave. The contributions of other supernovae remnants to the galactic cosmic ray energy density are now considered, paying anisotropy of cosmic rays accelerated by local supernovae ( 100 pc). The results are compared with geophysical data on the fluctuations in the cosmic ray intensity over the previous one billion years
Modified HLLC-VOF solver for incompressible two-phase fluid flows
A modified HLLC-type contact preserving Riemann solver for incompressible
two-phase flows using the artificial compressibility formulation is presented.
Here, the density is omitted from the pressure evolution equation. Also, while
calculating the eigenvalues and eigenvectors, the variations of the volume
fraction is taken into account. Hence, the equations for the intermediate
states and the intermediate wave speed are different from the previous HLLC-VOF
formulation [Bhat S P and Mandal J C, J. Comput. Phys. 379 (2019), pp.
173-191]. Additionally, an interface compression algorithm is used in tandem to
ensure sharp interfaces. The modified Riemann solver is found to be robust
compared to the previous HLLC-VOF solver, and the results produced are superior
compared to non-contact preserving solver. Several test problems in two- and
three-dimensions are solved to evaluate the efficacy of the solver on
structured and unstructured meshes
Monolithic InP-Based Grating Spectrometer for Wavelength-Division Multiplexed Systems at 1.5 ÎŒm
A monolithic InP-based grating spectrometer for use in wavelength-division multiplexed systems at 1.5 ÎŒm is reported.
The spectrometer uses a single etched reflective focusing diffraction grating and resolves >50 channels at 1 nm spacing with a ~0.3nm channel width and at least 19dB channel isolation. Operation is essentially of the state of the input polarisation
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