14,593 research outputs found
Comparison of available measurements of the absolute air-fluorescence yield and determination of its global average value
Experimental results of the absolute air-fluorescence yield are given very
often in different units (photons/MeV or photons/m) and for different
wavelength intervals. In this work we present a comparison of available results
normalized to its value in photons/MeV for the 337 nm band at 1013 hPa and 293
K. The conversion of photons/m to photons/MeV requires an accurate
determination of the energy deposited by the electrons in the field of view of
the experimental set-up. We have calculated the energy deposition for each
experiment by means of a detailed Monte Carlo simulation and the results have
been compared with those assumed or calculated by the authors. As a result,
corrections to the reported fluorescence yields are proposed. These corrections
improve the compatibility between measurements in such a way that a reliable
average value with uncertainty at the level of 5% is obtained.Comment: 13 pages, 6 figures. To appear in the Proocedings of the
International Symposium on the Recent Progress of Ultra-high Energy Cosmic
Ray Observations (UHECR2010), Nagoya, Japan, 201
On the absolute value of the air-fluorescence yield
The absolute value of the air-fluorescence yield is a key parameter for the
energy reconstruction of extensive air showers registered by fluorescence
telescopes. In previous publications, we reported a detailed Monte Carlo
simulation of the air-fluorescence generation that allowed the theoretical
evaluation of this parameter. This simulation has been upgraded in the present
work. As a result, we determined an updated absolute value of the fluorescence
yield of 7.9+-2.0 ph/MeV for the band at 337 nm in dry air at 800 hPa and 293
K, in agreement with experimental values. We have also performed a critical
analysis of available absolute measurements of the fluorescence yield with the
assistance of our simulation. Corrections have been applied to some
measurements to account for a bias in the evaluation of the energy deposition.
Possible effects of other experimental aspects have also been discussed. From
this analysis, we determined an average fluorescence yield of 7.04+-0.24 ph/MeV
at the above conditions.Comment: Submitted to Astroparticle Physic
Improved model for the analysis of air fluorescence induced by electrons
A model recently proposed for the calculation of air-fluorescence yield
excited by electrons is revisited. Improved energy distributions of secondary
electrons and a more realistic Monte Carlo simulation including some additional
processes have allowed us to obtain more accurate results. The model is used to
study in detail the relationship between fluorescence intensity and deposited
energy in a wide range of primary energy (keVs - GeVs). In addition,
predictions on the absolute value of the fluorescence efficiency in the absence
of collisional quenching will be presented and compared with available
experimental data.Comment: Contribution to the 5th Fluorescence Workshop, El Escorial, Madrid,
Spain, September 2007, to appear in Nuclear Instruments and Methods A.
Revised version.- More details on the comparison with experimental dat
Total electron scattering cross sections from thiophene for the (1-300 eV) impact energy range
Experimental electron scattering cross sections for thiophene in the impact energy range from 1 to 300 eV have been measured with a magnetically confined electron transmission-beam apparatus. Random uncertainty limits have been estimated to be less than 5%, and systematic errors derived from acceptance angle limitations have also been identified and evaluated. Experimental values are compared with our previous low energy (1-15 eV) R-matrix and intermediate/high energy (15-300 eV) IAM-SCAR+I calculations finding reasonable agreement, within the combined uncertainty limits. Some of the low energy shape and core-excited resonances predicted by previous calculations are experimentally confirmed in this study
Large parallel cosmic string simulations: New results on loop production
Using a new parallel computing technique, we have run the largest cosmic
string simulations ever performed. Our results confirm the existence of a long
transient period where a non-scaling distribution of small loops is produced at
lengths depending on the initial correlation scale. As time passes, this
initial population gives way to the true scaling regime, where loops of size
approximately equal to one-twentieth the horizon distance become a significant
component. We observe similar behavior in matter and radiation eras, as well as
in flat space. In the matter era, the scaling population of large loops becomes
the dominant component; we expect this to eventually happen in the other eras
as well.Comment: 23 pages, 10 figures, 2 tables. V2: combine 3 figures, add 1 table,
better discussion + citation of prev. wor
VISIR-VLT high resolution study of the extended emission of four obscured post-AGB candidates
The onset of the asymmetry of planetary nebulae (PNe) is expected to occur
during the late Asymptotic Giant Branch (AGB) and early post-AGB phases of low-
and intermediate-mass stars. Among all post-AGB objects, the most heavily
obscured ones might have escaped the selection criteria of previous studies
detecting extreme axysimmetric structures in young PNe. Since the most heavily
obscured post-AGB sources can be expected to descend from the most massive PN
progenitors, these should exhibit clear asymmetric morphologies. We have
obtained VISIR-VLT mid-IR images of four heavily obscured post-AGB objects
barely resolved in previous Spitzer IRAC observations to analyze their
morphology and physical conditions across the mid-IR. The VISIR-VLT images have
been deconvolved, flux calibrated, and used to construct RGB composite pictures
as well as color and optical depth maps that allow us to study the morphology
and physical properties of the extended emission of these sources. We have
detected extended emission from the four objects in our sample and resolved it
into several structural components that are greatly enhanced in the temperature
and optical depth maps. They reveal the presence of asymmetry in three young
PNe (IRAS 15534-5422, IRAS 17009-4154, and IRAS 18454+0001), where the
asymmetries can be associated with dusty torii and slightly bipolar outflows.
The fourth source (IRAS 18229-1127), a possible post-AGB star, is better
described as a rhomboidal detached shell. The heavily obscured sources in our
sample do not show extreme axisymmetric morphologies. This is at odds with the
expectation of highly asymmetrical morphologies in post-AGB sources descending
from massive PN progenitors. The sources presented in this paper may be
sampling critical early phases in the evolution of massive PN progenitors,
before extreme asymmetries develop.Comment: 9 pages, 4 figure
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