57 research outputs found
Image Reconstruction with a LaBr3-based Rotational Modulator
A rotational modulator (RM) gamma-ray imager is capable of obtaining
significantly better angular resolution than the fundamental geometric
resolution defined by the ratio of detector diameter to mask-detector
separation. An RM imager consisting of a single grid of absorbing slats
rotating ahead of an array of a small number of position-insensitive detectors
has the advantage of fewer detector elements (i.e., detector plane pixels) than
required by a coded aperture imaging system with comparable angular resolution.
The RM therefore offers the possibility of a major reduction in instrument
complexity, cost, and power. A novel image reconstruction technique makes it
possible to deconvolve the raw images, remove sidelobes, reduce the effects of
noise, and provide resolving power a factor of 6 - 8 times better than the
geometric resolution. A 19-channel prototype RM developed in our laboratory at
Louisiana State University features 13.8 deg full-angle field of view, 1.9 deg
geometric angular resolution, and the capability of resolving sources to within
35' separation. We describe the technique, demonstrate the measured performance
of the prototype instrument, and describe the prospects for applying the
technique to either a high-sensitivity standoff gamma-ray imaging detector or a
satellite- or balloon-borne gamma-ray astronomy telescope.Comment: submitted to Nuclear Instrument & Methods, special edition: SORMA
2010 on June 16, 201
Cosmic-ray acceleration in supernova remnants: non-linear theory revised
A rapidly growing amount of evidences, mostly coming from the recent
gamma-ray observations of Galactic supernova remnants (SNRs), is seriously
challenging our understanding of how particles are accelerated at fast shocks.
The cosmic-ray (CR) spectra required to account for the observed phenomenology
are in fact as steep as , i.e., steeper than the
test-particle prediction of first-order Fermi acceleration, and significantly
steeper than what expected in a more refined non-linear theory of diffusive
shock acceleration. By accounting for the dynamical back-reaction of the
non-thermal particles, such a theory in fact predicts that the more efficient
the particle acceleration, the flatter the CR spectrum. In this work we put
forward a self-consistent scenario in which the account for the magnetic field
amplification induced by CR streaming produces the conditions for reversing
such a trend, allowing --- at the same time --- for rather steep spectra and CR
acceleration efficiencies (about 20%) consistent with the hypothesis that SNRs
are the sources of Galactic CRs. In particular, we quantitatively work out the
details of instantaneous and cumulative CR spectra during the evolution of a
typical SNR, also stressing the implications of the observed levels of
magnetization on both the expected maximum energy and the predicted CR
acceleration efficiency. The latter naturally turns out to saturate around
10-30%, almost independently of the fraction of particles injected into the
acceleration process as long as this fraction is larger than about .Comment: 24 pages, 5 figures, accepted for publication in JCA
SRAO CO Observation of 11 Supernova Remnants in l = 70 to 190 deg
We present the results of 12CO J = 1-0 line observations of eleven Galactic
supernova remnants (SNRs) obtained using the Seoul Radio Astronomy Observatory
(SRAO) 6-m radio telescope. The observation was made as a part of the SRAO CO
survey of SNRs between l = 70 and 190 deg, which is intended to identify SNRs
interacting with molecular clouds. The mapping areas for the individual SNRs
are determined to cover their full extent in the radio continuum. We used
halfbeam grid spacing (60") for 9 SNRs and full-beam grid spacing (120") for
the rest. We detected CO emission towards most of the remnants. In six SNRs,
molecular clouds showed a good spatial relation with their radio morphology,
although no direct evidence for the interaction was detected. Two SNRs are
particularly interesting: G85.4+0.7, where there is a filamentary molecular
cloud along the radio shell, and 3C434.1, where a large molecular cloud appears
to block the western half of the remnant. We briefly summarize the results
obtained for individual SNRs.Comment: Accepted for publication in Astrophysics & Space Science. 12 pages,
12 figures, and 3 table
Background model systematics for the Fermi GeV excess
The possible gamma-ray excess in the inner Galaxy and the Galactic center
(GC) suggested by Fermi-LAT observations has triggered a large number of
studies. It has been interpreted as a variety of different phenomena such as a
signal from WIMP dark matter annihilation, gamma-ray emission from a population
of millisecond pulsars, or emission from cosmic rays injected in a sequence of
burst-like events or continuously at the GC. We present the first comprehensive
study of model systematics coming from the Galactic diffuse emission in the
inner part of our Galaxy and their impact on the inferred properties of the
excess emission at Galactic latitudes and 300 MeV to 500
GeV. We study both theoretical and empirical model systematics, which we deduce
from a large range of Galactic diffuse emission models and a principal
component analysis of residuals in numerous test regions along the Galactic
plane. We show that the hypothesis of an extended spherical excess emission
with a uniform energy spectrum is compatible with the Fermi-LAT data in our
region of interest at CL. Assuming that this excess is the extended
counterpart of the one seen in the inner few degrees of the Galaxy, we derive a
lower limit of ( CL) on its extension away from the GC. We
show that, in light of the large correlated uncertainties that affect the
subtraction of the Galactic diffuse emission in the relevant regions, the
energy spectrum of the excess is equally compatible with both a simple broken
power-law of break energy GeV, and with spectra predicted by the
self-annihilation of dark matter, implying in the case of final
states a dark matter mass of GeV.Comment: 65 pages, 28 figures, 7 table
A Statistical Study of Galactic SNRs using the PMN Survey
The Parkes-MIT-NRAO (PMN) radio survey has been used to generate a quasi
all-sky study of Galactic Supernova Remnants (SNRs) at a common frequency of
4.85 GHz. We present flux densities estimated for the sample of 110 Southern
Galactic SNRs (up to Dec = - 65 deg.) observed with the Parkes 64-m radio
telescope and an additional sample of 54 from the Northern PMN (up to Dec = +64
deg.) survey undertaken with the Green Bank 43-m (20 SNRs) and 91-m (34 SNRs)
radio telescopes. Out of this total sample of 164 selected SNRs (representing
71% of the 231 known SNRs in the Green catalogue) we consider 138 to provide
reliable estimates of flux density and surface brightness distribution. This
sub-sample represents those SNRs which fall within carefully chosen selection
criteria which minimises the effects of the known problems in establishing
reliable fluxes from the PMN survey data. Our selection criteria are based on a
judicious restriction of source angular size and telescope beam together with
careful evaluation of fluxes on a case by case basis. This gives confidence in
the newly derived PMN fluxes when the selection criteria are respected. We find
a sharp drop off in the flux densities for Galactic SNRs beyond 4 Jy and then a
fairly flat distribution from 5-9 Jy, a slight decline and a further flat
distribution from 9-20 Jy though the numbers of SNR in each Jy bin are low. We
also re-visit the contentious Sigma-D relation to determine a new power law
index for a sub-sample of shell type SNRs which yields beta= -2.2 +/- 0.6. This
new evaluation of the Sigma-D relation, applied to the restricted sample,
provides new distance estimates and their Galactic scale height distribution.
We find a peak in the SNR distribution between 7-11 kpc with most restricted to
+/- 100 pc Galactic scale height.Comment: 14 pages, 7 figures. Accepted for publishing in Astrophysics and
Space Scienc
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Solar energy in Australia: a profile of renewable energy activity in its national context
The following topics are included: country overview; energy summary; geopolitical, economic, and cultural aspects of Australia; the energy profile; and international agreements, contacts, manufacturers, and projects. (MHR
The TIGRE Instrument for 0.3–100 MeV Gamma-Ray Astronomy
The Tracking and Imaging Gamma-Ray Experiment4 (TIGRE) uses multilayers of silicon strip detectors both as a gamma-ray converter and to track Compton recoil electrons and positron-electron pairs. The direction and energy of the Compton scattered gamma ray and pair particles are measured with arrays of small CsI(Tl)-photodiode detectors. With a large ?-steradian field-of-view, the prototype instrument described here is sensitive to gamma rays from 0.3 to 100 MeV with a typical energy resolution of 3% (FWHM) and a 1-? angular resolution of 120 arc min. at 2 MeV. It has a high absolute detection efficiency of 8% over the full energy range. The telescope will be described and Monte Carlo calculations will be presented. Measurements with a small desktop instrument are presented elsewhere in this volume. © 1995 IEEE.National Aeronautics and Space Administration: NAGW-1996 Sharif University of Technology1 visiting from Cukurova University, Adana, Turkey 2 also at Riverside Community College, Riverside, CA 92506 3 visiting from Sharif University of Technology, Tehran, Iran 4supported by NASA Grant NAGW-199
Dendritic Spine Alterations in Neocortical Pyramidal Neurons following Postnatal Neuronal Nogo-A Knockdown
The myelin-associated protein Nogo-A is a well-known inhibitor of axonal regeneration and compensatory plasticity, yet functions of neuronal Nogo-A are not as clear. The present study examined the effects of decreased levels of neuronal Nogo-A on dendritic spines of developing neocortical neurons. Decreased Nogo-A levels in these neurons resulted in lowered spine density and an increase in filopodial type protrusions. These results suggest a role for neuronal Nogo-A in maintaining a spine phenotype in neocortical pyramidal cells
Stakeholder perceptions of manure treatment technologies in Denmark, Italy, the Netherlands and Spain
Manure treatment technologies have been developed in Europe to better use animal manures and to reduce their environmental impact, but the adoption of these technologies in practice is regionally diverse and still limited. Also, little is known about the opinions of stakeholders towards manure treatment. This study aimed to identify stakeholder perceptions of (1) which factors can facilitate and hinder the implementation in practice, (2) which technologies have the most potential for successful adoption, and (3) how farm characteristics and scale of treatment operations affect priorities for technology adoption. This analysis used data from a survey of various stakeholders engaged in manure treatment in four European countries (Denmark, Italy, the Netherlands and Spain) that have large areas of high animal density, but diverse socio-economic, political and environmental conditions. Pressure from governmental regulations was perceived as a key factor that stimulated manure treatment in all four countries (70% of respondents). Processing manure to produce bioenergy was considered important in Denmark and Italy, but less important in Spain and the Netherlands. The major barriers to technology adoption were related to economic factors -lack of investment capital (60% of respondents), high processing cost (52%) and a long payback period (45%), while there was relatively little concern regarding transport and noise burden and health risks. Slurry separation and anaerobic digestion were perceived to have the greatest potential for a common adoption. Other preferred technologies were more country-specific (e.g. acidification in Denmark, composting in Spain, and drying and reverse osmosis in Netherlands). Manure treatment was considered to be less applicable at small livestock farms. Separation, composting and acidification were perceived to be more applicable at farm scale, while drying, anaerobic digestion, reverse osmosis at large, industrial scales. Our results imply that manure treatment will remain a regional activity. Policy measures and outreach strategies to alleviate the main barriers to the adoption of manure treatment are suggested.</p
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