212 research outputs found
Dynamic power load simulator
Two independent models simulate dynamic and steady-state responses of electrical and electronic equipment under power load. One is resistance/capacitance/inductance network, and the other is variable resistance analog device. Resistance, inductance, and/or capacitance are selected by iterative process; time-domain response is compared with that of real equipment to select optimal values
High-power ac/dc variable load simulator
Design of medium-power dynamic electrical load simulator has been extended to permit simulation of ac as well as dc loads and to provide for operation at higher power levels. Simulator is internally protected against reverse voltage, overvoltage, overcurrent, and overload conditions
Automatic Detection of Expanding HI Shells Using Artificial Neural Networks
The identification of expanding HI shells is difficult because of their
variable morphological characteristics. The detection of HI bubbles on a global
scale therefore never has been attempted. In this paper, an automatic detector
for expanding HI shells is presented. The detection is based on the more stable
dynamical characteristics of expanding shells and is performed in two stages.
The first one is the recognition of the dynamical signature of an expanding
bubble in the velocity spectra, based on the classification of an artificial
neural network. The pixels associated with these recognized spectra are
identified on each velocity channel. The second stage consists in looking for
concentrations of those pixels that were firstly pointed out, and to decide if
they are potential detections by morphological and 21-cm emission variation
considerations. Two test bubbles are correctly detected and a potentially new
case of shell that is visually very convincing is discovered. About 0.6% of the
surveyed pixels are identified as part of a bubble. These may be false
detections, but still constitute regions of space with high probability of
finding an expanding shell. The subsequent search field is thus significantly
reduced. We intend to conduct in the near future a large scale HI shells
detection over the Perseus Arm using our detector.Comment: 39 pages, 11 figures, accepted by PAS
The SNR G106.3+2.7 and its Pulsar Wind Nebula: relics of triggered star formation in a complex environment
We propose that the pulsar nebula associated with the pulsar J2229+6114 and
the supernova remnant (SNR) G106.3+2.7 are the result of the same supernova
explosion. The whole structure is located at the edge of an HI bubble with
extended regions of molecular gas inside. The radial velocities of both the
atomic hydrogen and the molecular material suggest a distance of 800 pc. At
this distance the SNR is 14 pc long and 6 pc wide. Apparently the bubble was
created by the stellar wind and supernova explosions of a group of stars in its
center which also triggered the formation of the progenitor star of G106.3+2.7.
The progenitor star exploded at or close to the current position of the pulsar,
which is at one end of the SNR rather than at its center. The expanding shock
wave of the supernova explosion created a comet shaped supernova remnant by
running into dense material and then breaking out into the inner part of the HI
bubble. A synchrotron nebula with a shell-like structure (the ``Boomerang'') of
length 0.8 pc was created by the pulsar wind interacting with the dense ambient
medium. The expanding shock wave created an HI shell of mass 0.4 Msun around
this nebula by ionizing the atomic hydrogen in its vicinity.Comment: 10 pages, Latex, with aastex and emulateapj5, 5 figures. ApJ,
accepted, scheduled for the v560 n1 p1 Oct 10, 2001 issu
Radio Polarimetry of the ELAIS N1 Field: Polarized Compact Sources
We present deep polarimetric observations at 1420 MHz of the European Large
Area ISO Survey North 1 region (ELAIS N1) as part of the Dominion Radio
Astrophysical Observatory Planck Deep Fields project. By combining closely
spaced aperture synthesis fields, we image a region of 7.43 square degrees to a
maximum sensitivity in Stokes Q and U of 78 microJy/beam, and detect 786
compact sources in Stokes I. Of these, 83 exhibit polarized emission. We find
that the differential source counts (log N - log p) for polarized sources are
nearly constant down to p > 500 microJy, and that these faint polarized radio
sources are more highly polarized than the strong source population. The median
fractional polarization is (4.8 +/- 0.7)% for polarized sources with Stokes I
flux density between 1 and 30 mJy; approximately three times larger than
sources with I > 100 mJy. The majority of the polarized sources have been
identified with galaxies in the Spitzer Wide Area Infrared Extragalactic Survey
(SWIRE) image of ELAIS N1. Most of the galaxies occupy regions in the IRAC
5.8/3.6 micron vs. 8.0/4.5 micron color-color diagram associated with dusty
AGNs, or with ellipticals with an aging stellar population. A few host galaxies
have colors that suggests significant PAH emission in the near-infrared. A
small fraction, 12%, of the polarized sources are not detected in the SWIRE
data. None of the polarized sources in our sample appears to be associated with
an actively star-forming galaxy.Comment: 28 pages, 8 Figures. Figures 2 and 3 as separate gif images. Accepted
for publication in the Astrophysical Journa
Limits on Enhanced Radio Wave Scattering by Supernova Remnants
We report multifrequency observations with the NRAO Very Long Baseline Array
(VLBA) of the compact radio sources J0128+6306 and J0547+2721, which are viewed
through the supernova remnants G127.1+0.5 and S147, respectively. Observations
were made at frequencies of 1.427, 1.667, 2.271, and 4.987 GHz. The lines of
sight to these sources pass through the shock wave and upstream and downstream
turbulent layers of their respective supernova remnants, and thus might detect
cosmic-ray generated turbulence produced during the Fermi acceleration process.
For both sources, we detect interstellar scattering, characterized by a
component of the angular size which scales as the square of the observing
wavelength. The magnitude of the scattering is characterized by an effective
scattering angular size theta_S0 at a frequency of 1 GHz of 13.2 +/- 2.6
milliarcseconds (mas) for J0128+6306 and 6.7 +/- 2.2 mas for J0547+2721. These
angular sizes are consistent with the ``incidental'' scattering for any line of
sight out of the galaxy at similar galactic latitudes and longitudes. There is
therefore no evidence for enhanced turbulence at these supernova remnants. We
establish upper limits to the supernova remnant-associated scattering measures
of 8.1-14.8 m^-20/3-pc for J0128+6306 and 3.0 m^-20/3-pc for J0547+2721.Comment: To be published in ApJ, 25 pages, 4 figures, 2 table
Direct Estimate of Cirrus Noise in Herschel Hi-GAL Images
In Herschel images of the Galactic plane and many star forming regions, a
major factor limiting our ability to extract faint compact sources is cirrus
confusion noise, operationally defined as the "statistical error to be expected
in photometric measurements due to confusion in a background of fluctuating
surface brightness". The histogram of the flux densities of extracted sources
shows a distinctive faint-end cutoff below which the catalog suffers from
incompleteness and the flux densities become unreliable. This empirical cutoff
should be closely related to the estimated cirrus noise and we show that this
is the case. We compute the cirrus noise directly, both on Herschel images from
which the bright sources have been removed and on simulated images of cirrus
with statistically similar fluctuations. We connect these direct estimates with
those from power spectrum analysis, which has been used extensively to predict
the cirrus noise and provides insight into how it depends on various
statistical properties and photometric operational parameters. We report
multi-wavelength power spectra of diffuse Galactic dust emission from Hi-GAL
observations at 70 to 500 microns within Galactic plane fields at l= 30 degrees
and l= 59 degrees. We find that the exponent of the power spectrum is about -3.
At 250 microns, the amplitude of the power spectrum increases roughly as the
square of the median brightness of the map and so the expected cirrus noise
scales linearly with the median brightness. Generally, the confusion noise will
be a worse problem at longer wavelengths, because of the combination of lower
angular resolution and the rising power spectrum of cirrus toward lower spatial
frequencies, but the photometric signal to noise will also depend on the
relative spectral energy distribution of the source compared to the cirrus.Comment: 4 pages (in journal), 3 figures, Astronomy and Astrophysics, accepted
for publication 13 May 201
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