5,322 research outputs found
The influence of coronal EUV irradiance on the emission in the He I 10830 A and D3 multiplets
Two of the most attractive spectral windows for spectropolarimetric
investigations of the physical properties of the plasma structures in the solar
chromosphere and corona are the ones provided by the spectral lines of the He I
10830 A and 5876 A (or D3) multiplets, whose polarization signals are sensitive
to the Hanle and Zeeman effects. However, in order to be able to carry out
reliable diagnostics, it is crucial to have a good physical understanding of
the sensitivity of the observed spectral line radiation to the various
competing driving mechanisms. Here we report a series of off-the-limb non-LTE
calculations of the He I D3 and 10830 A emission profiles, focusing our
investigation on their sensitivity to the EUV coronal irradiation and the model
atmosphere used in the calculations. We show in particular that the intensity
ratio of the blue to the red components in the emission profiles of the He I
10830 A multiplet turns out to be a good candidate as a diagnostic tool for the
coronal irradiance. Measurements of this observable as a function of the
distance to the limb and its confrontation with radiative transfer modeling
might give us valuable information on the physical properties of the solar
atmosphere and on the amount of EUV radiation at relevant wavelengths
penetrating the chromosphere from above.Comment: 19 pages, 11 figures (pre-print format). Accepted for publication in
Ap
Search for synchrotron emission from secondary leptons in dense cold starless cores
We report radio continuum observations with the Australia Telescope Compact
Array of two molecular clouds. The impetus for these observations is a search
for synchrotron radiation by cosmic ray secondary electrons/positrons in a
region of enhanced density and possibly high magnetic field. We present
modelling which shows that there should be an appreciable flux of synchrotron
above the more diffuse, galactic synchrotron background.
The starless core G333.125-0.562 and infrared source IRAS 15596-5301 were
observed at 1384 and 2368 MHz. For G333.125-0.562, we find no significant
levels of radio emission from this source at either frequency, nor any
appreciable polarisation: we place an upper limit on the radio continuum flux
from this source of 0.5 mJy per beam at both 1384 and 2368 MHz. Due to the
higher than expected flux density limits, we also obtained archival ATCA data
at 8640 MHz for this cloud and place an upper limit on the flux density of 50
micro-Jy per beam. Assuming the cosmic ray spectrum is similar to that near the
Sun, and given the cloud's molecular density and mass, we place an upper limit
on the magnetic field of 500 micro-G. IRAS 15596-5301, with an RMS of 50
micro-Jy per beam at 1384 MHz, shows an HII region consistent with optically
thin free-free emission already detected at 4800 MHz. We use the same
prescription as G333 to constrain the magnetic field from this cloud to be less
than 500 micro-G. We find that these values are not inconsistent with the view
that magnetic field values scale with the average density of the molecular
cloud.Comment: 6 pages, 5 pdf figures, accepted for publication in PAS
A dark matter interpretation for the ARCADE excess?
The ARCADE 2 Collaboration has recently measured an isotropic radio emission
which is significantly brighter than the expected contributions from known
extra-galactic sources. The simplest explanation of such excess involves a
"new" population of unresolved sources which become the most numerous at very
low (observationally unreached) brightness. We investigate this scenario in
terms of synchrotron radiation induced by WIMP annihilations or decays in
extragalactic halos. Intriguingly, for light-mass WIMPs with thermal
annihilation cross-section, and fairly conservative clustering assumptions, the
level of expected radio emission matches the ARCADE observations.Comment: 5 pages, 3 figures. v2: one benchmark model added, comments and
references expanded, to appear in PR
Synchrotron Radiation from the Galactic Center in Decaying Dark Matter Scenario
We discuss the synchrotron radiation flux from the Galactic center in
unstable dark matter scenario. Motivated by the anomalous excess of the
positron fraction recently reported by the PAMELA collaboration, we consider
the case that the dark matter particle is unstable (and long-lived), and that
energetic electron and positron are produced by the decay of dark matter. Then,
the emitted electron and positron becomes the source of the synchrotron
radiation. We calculate the synchrotron radiation flux for models of decaying
dark matter, which can explain the PAMELA positron excess. Taking the lifetime
of the dark matter of O(10^26 sec), which is the suggested value to explain the
PAMELA anomaly, the synchrotron radiation flux is found to be O(1 kJy/str) or
smaller, depending on the particle-physics and cosmological parameters.Comment: 20 pages, 6 figure
Magnification Ratio of the Fluctuating Light in Gravitational Lens 0957+561
Radio observations establish the B/A magnification ratio of gravitational
lens 0957+561 at about 0.75. Yet, for more than 15 years, the optical
magnfication ratio has been between 0.9 and 1.12. The accepted explanation is
microlensing of the optical source. However, this explanation is mildly
discordant with (i) the relative constancy of the optical ratio, and (ii)
recent data indicating possible non-achromaticity in the ratio. To study these
issues, we develop a statistical formalism for separately measuring, in a
unified manner, the magnification ratio of the fluctuating and constant parts
of the light curve. Applying the formalism to the published data of Kundi\'c et
al. (1997), we find that the magnification ratios of fluctuating parts in both
the g and r colors agrees with the magnification ratio of the constant part in
g-band, and tends to disagree with the r-band value. One explanation could be
about 0.1 mag of consistently unsubtracted r light from the lensing galaxy G1,
which seems unlikely. Another could be that 0957+561 is approaching a caustic
in the microlensing pattern.Comment: 12 pages including 1 PostScript figur
A survey of backward proton and pion production in p+C interactions at beam momenta from 1 to 400 GeV/c
New data on proton and pion production in p+C interactions from the CERN PS
and SPS accelerators are used in conjunction with other available data sets to
perform a comprehensive survey of backward hadronic cross sections. This survey
covers the complete backward hemisphere in the range of lab angles from 10 to
180 degrees, from 0.2 to 1.4 GeV/c in lab momentum and from 1 to 400 GeV/c in
projectile momentum. Using the constraints of continuity and smoothness of the
angular, momentum and energy dependences a consistent description of the
inclusive cross sections is established which allows the control of the
internal consistency of the nineteen available data sets.Comment: 52 pages 47 figure
Fundamental analysis of the failure of polymer-based fiber reinforced composites
A mathematical model is described which will permit predictions of the strength of fiber reinforced composites containing known flaws to be made from the basic properties of their constituents. The approach was to embed a local heterogeneous region (LHR) surrounding the crack tip into an anisotropic elastic continuum. The model should (1) permit an explicit analysis of the micromechanical processes involved in the fracture process, and (2) remain simple enough to be useful in practical computations. Computations for arbitrary flaw size and orientation under arbitrary applied load combinations were performed from unidirectional composites with linear elastic-brittle constituent behavior. The mechanical properties were nominally those of graphite epoxy. With the rupture properties arbitrarily varied to test the capability of the model to reflect real fracture modes in fiber composites, it was shown that fiber breakage, matrix crazing, crack bridging, matrix-fiber debonding, and axial splitting can all occur during a period of (gradually) increasing load prior to catastrophic fracture. The computations reveal qualitatively the sequential nature of the stable crack process that precedes fracture
Two-photon transitions in primordial hydrogen recombination
The subject of cosmological hydrogen recombination has received much
attention recently because of its importance to predictions for and
cosmological constraints from CMB observations. While the central role of the
two-photon decay 2s->1s has been recognized for many decades, high-precision
calculations require us to consider two-photon decays from the higher states
ns,nd->1s (n>=3). Simple attempts to include these processes in recombination
calculations have suffered from physical problems associated with sequences of
one-photon decays, e.g. 3d->2p->1s, that technically also produce two photons.
These correspond to resonances in the two-photon spectrum that are optically
thick, necessitating a radiative transfer calculation. We derive the
appropriate equations, develop a numerical code to solve them, and verify the
results by finding agreement with analytic approximations to the radiative
transfer equation. The related processes of Raman scattering and two-photon
recombination are included using similar machinery. Our results show that early
in recombination the two-photon decays act to speed up recombination, reducing
the free electron abundance by 1.3% relative to the standard calculation at
z=1300. However we find that some photons between Ly-alpha and Ly-beta are
produced, mainly by 3d->1s two-photon decay and 2s->1s Raman scattering. At
later times these photons redshift down to Ly-alpha, excite hydrogen atoms, and
act to slow recombination. Thus the free electron abundance is increased by
1.3% relative to the standard calculation at z=900. The implied correction to
the CMB power spectrum is neligible for the recently released WMAP and ACBAR
data, but at Fisher matrix level will be 7 sigma for Planck. [ABRIDGED]Comment: Matches PRD accepted version. 28 pages, 12 figure
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