19,070 research outputs found
A measurement of the radiation dose to LDEF by passive dosimetry
The results from a pair of thermoluminescent dosimeter experiments flown aboard the Long Duration Exposure Facility (LDEF) show an integrated dose several times smaller than that predicted by the NASA environmental models for shielding thicknesses much greater than 0.10 gm/sq cm aluminum. For thicknesses between 0.01 and 0.1 gm/sq cm, the measured dose was in agreement with predictions. The Space and Environment Technology Center of The Aerospace Corporation fielded two related experiments on LDEF to measure the energetic radiation dose by means of passive dosimetry. The sensors were LiF thermoluminescent dosimeters mounted behind various thicknesses of shielding. The details of the experiment are described first, followed by the results of the observations. A comparison is made with the predictions based upon the NASA environmental models and the actual mission profile flown by LDEF; conclusions follow
Sources of the ultraheavy cosmic rays
The suggestions that the source abundances of cosmic ray nuclei heavier then Fe differ significantly from Solar System abundances are not well supported by the data without assuming preferential acceleration. The Solar System abundances of Pb and Bi are split into r-, standard s-, and cyclic 8-process components; the apprarent deficiency of Pb seen in the HEAO-3 Heavy Nuclei Experiment data might indicate an absence of Pb from the recycling 8-process
Determining the HI content of galaxies via intensity mapping cross-correlations
We propose an innovative method for measuring the neutral hydrogen (HI)
content of an optically-selected spectroscopic sample of galaxies through
cross-correlation with HI intensity mapping measurements. We show that the
HI-galaxy cross-power spectrum contains an additive shot noise term which
scales with the average HI brightness temperature of the optically-selected
galaxies, allowing constraints to be placed on the average HI mass per galaxy.
This approach can estimate the HI content of populations too faint to directly
observe through their 21cm emission over a wide range of redshifts. This
cross-correlation, as a function of optical luminosity or colour, can be used
to derive HI-scaling relations. We demonstrate that this signal will be
detectable by cross-correlating upcoming Australian SKA Pathfinder (ASKAP)
observations with existing optically-selected samples. We also use
semi-analytic simulations to verify that the HI mass can be successfully
recovered by our technique in the range M_HI > 10^8 M_solar, in a manner
independent of the underlying power spectrum shape. We conclude that this
method is a powerful tool to study galaxy evolution, which only requires a
single intensity mapping dataset to infer complementary HI gas information from
existing optical and infra-red observations.Comment: 8 pages, 4 figures, submitted to MNRA
Intensity mapping cross-correlations II: HI halo models including shot noise
HI intensity mapping data traces the large-scale structure matter
distribution using the integrated emission of neutral hydrogen gas (HI). The
cross-correlation of the intensity maps with optical galaxy surveys can
mitigate foreground and systematic effects, but has been shown to significantly
depend on galaxy evolution parameters of the HI and the optical sample.
Previously, we have shown that the shot noise of the cross-correlation scales
with the HI content of the optical samples, such that the shot noise estimation
infers the average HI masses of these samples. In this article, we present an
adaptive framework for the cross-correlation of HI intensity maps with galaxy
samples using our implementation of the halo model formalism (Murray et al
2018, in prep) which utilises the halo occupation distribution of galaxies to
predict their power spectra. We compare two HI population models, tracing the
spatial halo and the galaxy distribution respectively, and present their auto-
and cross-power spectra with an associated galaxy sample. We find that the
choice of the HI model and the distribution of the HI within the galaxy sample
have minor significance for the shape of the auto- and cross-correlations, but
highly impact the measured shot noise amplitude of the estimators, a finding we
confirm with simulations. We demonstrate parameter estimation of the HI halo
occupation models and advocate this framework for the interpretation of future
experimental data, with the prospect of determining the HI masses of optical
galaxy samples via the cross-correlation shot noise.Comment: 15 pages, 8 figures, 3 tables. Comments welcom
Measurement of energetic particle radiation at the synchronous altitude aboard ATS-6
The Aerospace Corporation energetic electron-proton spectrometer operating on ATS-6 is described. This experiment detects energetic electrons in four channels between 140 keV and greater than 3.9 MeV, measures energetic protons in five energy channels between 2.3 and 80 MeV and energetic alpha particles in three channels between 9.4 and 94 MeV. After more than a year of operation in orbit, the experiment continues to return excellent data on the behavior of energetic magnetospheric electrons as well as information regarding the fluxes of solar protons and alpha particles
Cosmological Constraints from Moments of the Thermal Sunyaev-Zel'dovich Effect
In this paper, we explain how moments of the thermal Sunyaev-Zel'dovich (tSZ)
effect can constrain both cosmological parameters and the astrophysics of the
intracluster medium (ICM). As the tSZ signal is strongly non-Gaussian, higher
moments of tSZ maps contain useful information. We first calculate the
dependence of the tSZ moments on cosmological parameters, finding that higher
moments scale more steeply with sigma_8 and are sourced by more massive galaxy
clusters. Taking advantage of the different dependence of the variance and
skewness on cosmological and astrophysical parameters, we construct a
statistic, ||/^1.4, which cancels much of the dependence on cosmology
(i.e., sigma_8) yet remains sensitive to the astrophysics of intracluster gas
(in particular, to the gas fraction in low-mass clusters). Constraining the ICM
astrophysics using this statistic could break the well-known degeneracy between
cosmology and gas physics in tSZ measurements, allowing for tight constraints
on cosmological parameters. Although detailed simulations will be needed to
fully characterize the accuracy of this technique, we provide a first
application to data from the Atacama Cosmology Telescope and the South Pole
Telescope. We estimate that a Planck-like full-sky tSZ map could achieve a <1%
constraint on sigma_8 and a 1-sigma error on the sum of the neutrino masses
that is comparable to the existing lower bound from oscillation measurements.Comment: 11 pages, 12 figures, to be submitted to Phys. Rev. D; v2: 14 pages,
16 figures, matches PRD accepted version (changes from v1 include additional
calculations with primordial non-Gaussianity and a new appendix discussing
the tSZ kurtosis
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Air quality in Mecca and surrounding holy places in Saudi Arabia during Hajj: initial survey.
The Arabian Peninsula experiences severe air pollution, the extent and sources of which are poorly documented. Each year in Saudi Arabia this situation is intensified during Hajj, the Holy Pilgrimage of Islam that draws millions of pilgrims to Mecca. An initial study of air quality in Mecca and surrounding holy sites during the 2012 Hajj (October 24-27) revealed strongly elevated levels of the combustion tracer carbon monoxide (CO, up to 57 ppmv) and volatile organic compounds (VOCs) along the pilgrimage route-especially in the tunnels of Mecca-that are a concern for human health. The most abundant VOC was the gasoline evaporation tracer i-pentane, which exceeded 1200 ppbv in the tunnels. Even though VOC concentrations were generally lower during a follow-up non-Hajj sampling period (April 2013), many were still comparable to other large cities suffering from poor air quality. Major VOC sources during the 2012 Hajj study included vehicular exhaust, gasoline evaporation, liquefied petroleum gas, and air conditioners. Of the measured compounds, reactive alkenes and CO showed the strongest potential to form ground-level ozone. Because the number of pilgrims is expected to increase in the future, we present emission reduction strategies to target both combustive and evaporative fossil fuel sources
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