282 research outputs found
Numerical Simulation of Radio Signal from Extended Air Showers
The burst of radio emission by the extensive air shower provides a promising
alternative for detecting ultra-high energy cosmic rays.We have developed an
independent numerical program to simulate these radio signals. Our code is
based on a microscopic treatment, with both the geosynchrotron radiation and
charge excess effect included. Here we make a first presentation of our basic
program and its results. The time signal for different polarizations are
computed, we find that the pulses take on a bipolar pattern, the spectrum is
suppressed towards the lower frequencies.We investigate how the shower at
different heights in atmosphere contribute to the total signal, and examine the
signal strength and distribution at sites of different elevations. We also
study the signal from showers of different inclination angles and azimuth
directions. In all these cases we find the charge excess effect important.Comment: 23 pages, 14 figure
TeV cosmic-ray proton and helium spectra in the myriad model II
Recent observations show that the cosmic ray nuclei spectra start to harden
above 100 GeV, in contradiction with the conventional steady-state cosmic ray
model. We had suggested that this anomaly is due to the propagation effect of
cosmic rays released from local young cosmic ray sources, the total flux of the
cosmic ray should be computed with the myriad model, where contribution from
sources in local catalog is added to the background. However, while the
hardening could be elegantly explained in this model, the model parameters
obtained from the fit skew toward a region with fast diffusion and low
supernova rate in the Galaxy, in tension with other observations. In this
paper, we further explore this model in order to set up a concordant picture.
Two possible improvements related to the cosmic ray sources have been
considered. Firstly, instead of the usual axisymmetric disk model, we
considered a spiral model of source distribution. Secondly, for the nearby and
young sources which are paramount to the hardening, we allow for an
energy-dependent escape time. We find that major improvement comes from the
energy-dependent escape time of the local sources, and with both modifications,
not only the cosmic ray proton and helium anomalies are solved, but also the
parameters attain reasonable range values compatible with other analysis.Comment: 13 pages, 7 figures, 1 table, accepted for publication in RA
Semi-Numerical Simulation of Reionization with Semi-Analytical Modeling of Galaxy Formation
In a semi-numerical model of reionization, the evolution of ionization
fraction is simulated approximately by the ionizing photon to baryon ratio
criterion. In this paper we incorporate a semi-analytical model of galaxy
formation based on the Millennium II N-body simulation into the semi-numerical
modeling of reionization. The semi-analytical model is used to predict the
production of ionizing photons, then we use the semi-numerical method to model
the reionization process. Such an approach allows more detailed modeling of the
reionization, and also connects observations of galaxies at low and high
redshifts to the reionization history. The galaxy formation model we use was
designed to match the low- observations, and it also fits the high redshift
luminosity function reasonably well, but its prediction on the star formation
falls below the observed value, and we find that it also underpredicts the
stellar ionizing photon production rate, hence the reionization can not be
completed at without taking into account some other potential
sources of ionization photons. We also considered simple modifications of the
model with more top heavy initial mass functions (IMF), with which the
reionization can occur at earlier epochs. The incorporation of the
semi-analytical model may also affect the topology of the HI regions during the
EoR, and the neutral regions produced by our simulations with the
semi-analytical model appeared less poriferous than the simple halo-based
models.Comment: 13 pages, 8 figures, RAA accepte
A Scalable Arrangement Method for Aperiodic Array Antennas to Reduce Peak Sidelobe Level
Peak sidelobe level reduction (PSLR) is crucial in the application of
large-scale array antenna, which directly determines the radiation performance
of array antenna. We study the PSLR of subarray level aperiodic arrays and
propose three array structures: dislocated subarrays with uniform elements
(DSUE), uniform subarrays with random elements (USRE), dislocated subarrays
with random elements (DSRE). To optimize the dislocation position of subarrays
and random position of elements, the improved Bat algorithm (IBA) is applied.
To draw the comparison of PSLR effect among these three array structures, we
take three size of array antennas from small to large as examples to simulate
and calculate the redundancy and peak sidelobe level (PSLL) of them. The
results show that DSRE is the optimal array structure by analyzing the
dislocation distance of subarray, scanning angle and applicable frequency. The
proposed design method is a universal and scalable method, which is of great
application value to the design of large-scale aperiodic array antenna
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