13,166 research outputs found
The late time radio emission from SN 1993J at meter wavelengths
We present the investigations of SN 1993J using low frequency observations
with the Giant Meterwave Radio Telescope. We analyze the light curves of SN
1993J at 1420, 610, 325 and 243 MHz during years since explosion.The
supernova has become optically thin early on in the 1420 MHz and 610 MHz bands
while it has only recently entered the optically thin phase in the 325 MHz
band. The radio light curve in the 235 MHz band is more or less flat. This
indicates that the supernova is undergoing a transition from an optically thick
to optically thin limit in this frequency band. In addition, we analyze the SN
radio spectra at five epochs on day 3000, 3200, 3266, 3460 and 3730 since
explosion. Day 3200 spectrum shows a synchrotron cooling break. SN 1993J is the
only young supernova for which the magnetic field and the size of the radio
emitting region are determined through unrelated methods. Thus the mechanism
that controls the evolution of the radio spectra can be identified. We suggest
that at all epochs, the synchrotron self absorption mechanism is primarily
responsible for the turn-over in the spectra. Light curve models based on free
free absorption in homogeneous or inhomogeneous media at high frequencies
overpredict the flux densities at low frequencies. The discrepancy is
increasingly larger at lower and lower frequencies. We suggest that an extra
opacity, sensitively dependent on frequency, is likely to account for the
difference at lower frequencies. The evolution of the magnetic field
(determined from synchrotron self absorption turn-over) is roughly consistent
with . Radio spectral index in the optically thin part
evolves from at few tens of days to in about
10 years.Comment: 37 pages, 9 figures in LaTex; scheduled for ApJ 10 September 2004,
v612 issue; send comments to: [email protected]
The Deep Diffuse Extragalactic Radio Sky at 1.75 GHz
We present a study of diffuse extragalactic radio emission at GHz
from part of the ELAIS-S1 field using the Australia Telescope Compact Array.
The resulting mosaic is deg, with a roughly constant noise region
of deg used for analysis. The image has a beam size of arcsec and instrumental Jy beam. Using point-source models from the ATLAS survey, we
subtract the discrete emission in this field for Jy
beam. Comparison of the source-subtracted probability distribution, or
\pd, with the predicted distribution from unsubtracted discrete emission and
noise, yields an excess of Jy beam. Taking this as
an upper limit on any extended emission we constrain several models of extended
source counts, assuming arcmin. The best-fitting
models yield temperatures of the radio background from extended emission of
mK, giving an upper limit on the total temperature at
GHz of mK. Further modelling shows that our data are
inconsistent with the reported excess temperature of ARCADE2 to a source-count
limit of Jy. Our new data close a loop-hole in the previous
constraints, because of the possibility of extended emission being resolved out
at higher resolution. Additionally, we look at a model of cluster halo emission
and two WIMP dark matter annihilation source-count models, and discuss general
constraints on any predicted counts from such sources. Finally, we report the
derived integral count at GHz using the deepest discrete count plus our
new extended-emission limits, providing numbers that can be used for planning
future ultra-deep surveys.Comment: 18 pages, 15 figures, 7 tables, Accepted by MNRA
Chromospheric Inversions of a Micro-flaring Region
We use spectropolarimetric observations of the Ca II 8542~\AA\ line, taken
from the Swedish 1-m Solar Telescope (SST), in an attempt to recover dynamic
activity in a micro-flaring region near a sunspot via inversions. These
inversions show localized mean temperature enhancements of 1000~K in the
chromosphere and upper photosphere, along with co-spatial bi-directional
Doppler shifting of 5 - 10 km s. This heating also extends along a
nearby chromospheric fibril, co-spatial to 10 - 15 km s down-flows.
Strong magnetic flux cancellation is also apparent in one of the footpoints,
concentrated in the chromosphere. This event more closely resembles that of an
Ellerman Bomb (EB), though placed slightly higher in the atmosphere than is
typically observed.Comment: 9 pages, 9 figures, accepted in ApJ. Movies are stored here:
https://star.pst.qub.ac.uk/webdav/public/areid/Microflare
Aerodynamic performance and pressure distributions for a NASA SC(2)-0714 airfoil tested in the Langley 0.3-meter transonic cryogenic tunnel
This report presents in graphic and tabular forms the aerodynamic coefficient and surface pressure distribution data for a NASA SC(2)-0714 airfoil tested in the Langley 0.3-Meter Transonic Cryogenic Tunnel. The test was another in a series of tests involved in the joint NASA/U.S. Industry Advanced Technology Airfoil Tests program. This 14% thick supercritical airfoil was tested at Mach numbers from 0.6 to 0.76 and angles of attack from -2.0 to 6.0 degrees. The test Reynolds numbers were 4 million, 6 million, 10 million, 15 million, 30 million, 40 million, and 45 million
A Bethe Ansatz Study of Free Energy and Excitation Spectrum for Even Spin Fateev Zamolodchikov Model
A Bethe Ansatz study of a self dual Z_N spin model is undertaken for even
spin system. One has to solve a coupled system of Bethe Ansatz Equations (BAE)
involving zeroes of two families of transfer matrices. A numerical study on
finite size lattices is done for identification of elementary excitations over
the Ferromagnetic and Antiferromagnetic ground states. The free energies for
both Ferromagnetic and Antiferromagnetic ground states and dispersion relation
for elementary excitations are found.Comment: 25 pages, 4 figure
Studies of sidewall boundary layer in the Langley 0.3 meter transonic cryogenic tunnel with and without suction
Boundary layer measurements on the sidewalls of the Langley 0.3 Meter Transonic Cryogenic Tunnel were made to determine the effectiveness of the passive boundary layer bleed system over a Reynolds number range from 20 to 200 x 10 to the sixth power per meter at Mach numbers from 0.30 to 0.76. The tunnel sidewall boundary layer displacement thickness was about 2 percent of the width of the test section without the boundary layer bleed. Measured velocity profiles correlated well with the defect law of Hama. With the boundary layer bleed equivalent to about 2 percent of the test section mass flow, the boundary layer displacement thickness reduced to about 1 percent of the test section width, which is generally considered acceptable for testing airfoils. It was also noticed that effectiveness of the bleed was nearly independent of the Mach number and Reynolds number over the range of conditions tested. A comparison of the measured suction effectiveness of the bleed with the finite difference and integral methods of boundary layer calculation showed good agreement
Detection of Pulsed X-ray Emission from PSR B1706-44
We report the first detection of pulsed X-ray emission from the young,
energetic radio and Gamma-ray pulsar PSR B1706-44. We find a periodic signal at
a frequency of f = 9.7588088 +/- 0.0000026 Hz (at epoch 51585.34104 MJD),
consistent with the radio ephemeris, using data obtained with the High
Resolution Camera on-board the Chandra X-ray Observatory}. The probability that
this detection is a chance occurrence is 3.5E-5 as judged by the Rayleigh test.
The folded light curve has a broad, single-peaked profile with a pulsed
fraction of 23% +/- 6%. This result is consistent the ROSAT PSPC upper limit of
< 18% after allowing for the ability of Chandra to resolve the pulsar from a
surrounding synchrotron nebula. We also fitted Chandra spectroscopic data on
PSR B1706-44, which require at least two components, e.g., a blackbody of
temperature T(infinity) between 1.51E6 K and 1.83E6 K and a power-law of Gamma
= 2.0 +/- 0.5. The blackbody radius at the nominal 2.5 kpc distance is only
R(infinity) = 3.6 +/- 0.9 km, indicating either a hot region on a cooler
surface, or the need for a realistic atmosphere model that would allow a lower
temperature and larger area. Because the power-law and blackbody spectra each
contribute more than 23% of the observed flux, it is not possible to decide
which component is responsible for the modulation in the spectrally unresolved
light curve.Comment: 6 pages, 4 figures, Latex, emulateapj. Published version. Includes an
updated radio ephemeris and presents the absolute radio/X-ray phase alignmen
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The Interrelationship and Effect of Trust and Strong Cultures in Setting information Systems Security Goals
This paper investigates the interrelationship and effect of trust and culture on the level of goal setting within the context of information systems security. In doing so, it explores and discusses the concepts of trust and strong culture and seeks to demonstrate their importance in setting efficiently information systems security goals. The paper contributes to interpretive information systems research with the study of goal setting in a security management context and its grounding within an interpretive epistemology
Experience with some repeat tests on the 9 inch chord CAST-10-2/DOA 2 airfoil model in the Langley 0.3-m TCT adaptive wall test section
A co-operative testing program is in progress between the Langley Research Center (NASA) and the National Aeronautical Establishment (NAE, Canada) to validate two different techniques of airfoil testing at transonic speeds. The procedure employed is to test the same airfoil model in the NAE two-dimensional tunnel and the Langley 0.3-m Transonic Cryogenic Tunnel (0.3-m TCT). The airfoil model used in testing was CAST-10-2/DOA-2 super-critical airfoil. The Langley 0.3-m TCT has a relatively small cross section of 13 in x 13 in, giving a (h/c) ratio of 1.44 for the same 9 in chord model. The approach employed in the 0.3-m TCT aims towards eliminating the wall effects by using active walls. The top and bottom walls are flexible. By changing the wall shapes during a test in an iterative manner, the wall interference effects are reduced. The method employed to change the wall shapes is the adaptive wall technique. The current test program provided an opportunity to validate the adaptive wall technique in the 0.3-m TCT. The relatively long chord airfoil represents a severe test case to test the efficacy of the adaptive wall technique under cryogenic conditions. The program also involved removal of side wall boundary-layer thus increasing the complexity of the wall adaptation technique. This paper deals with some salient results obtained regarding repeatability of test data and possible residual interference effects
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