155 research outputs found
Rosetta-Alice Observations of Exospheric Hydrogen and Oxygen on Mars
The European Space Agency's Rosetta spacecraft, en route to a 2014 encounter
with comet 67P/Churyumov-Gerasimenko, made a gravity assist swing-by of Mars on
25 February 2007, closest approach being at 01:54UT. The Alice instrument on
board Rosetta, a lightweight far-ultraviolet imaging spectrograph optimized for
in situ cometary spectroscopy in the 750-2000 A spectral band, was used to
study the daytime Mars upper atmosphere including emissions from exospheric
hydrogen and oxygen. Offset pointing, obtained five hours before closest
approach, enabled us to detect and map the HI Lyman-alpha and Lyman-beta
emissions from exospheric hydrogen out beyond 30,000 km from the planet's
center. These data are fit with a Chamberlain exospheric model from which we
derive the hydrogen density at the 200 km exobase and the H escape flux. The
results are comparable to those found from the the Ultraviolet Spectrometer
experiment on the Mariner 6 and 7 fly-bys of Mars in 1969. Atomic oxygen
emission at 1304 A is detected at altitudes of 400 to 1000 km above the limb
during limb scans shortly after closest approach. However, the derived oxygen
scale height is not consistent with recent models of oxygen escape based on the
production of suprathermal oxygen atoms by the dissociative recombination of
O2+.Comment: 17 pages, 8 figures, accepted for publication in Icaru
Strongly aligned gas-phase molecules at Free-Electron Lasers
We demonstrate a novel experimental implementation to strongly align
molecules at full repetition rates of free-electron lasers. We utilized the
available in-house laser system at the coherent x-ray imaging beamline at the
Linac Coherent Light Source. Chirped laser pulses, i. e., the direct output
from the regenerative amplifier of the Ti:Sa chirped pulse amplification laser
system, were used to strongly align 2,5-diiodothiophene molecules in a
molecular beam. The alignment laser pulses had pulse energies of a few mJ and a
pulse duration of 94 ps. A degree of alignment of
\left = 0.85 was measured, limited by the
intrinsic temperature of the molecular beam rather than by the available laser
system. With the general availability of synchronized chirped-pulse-amplified
near-infrared laser systems at short-wavelength laser facilities, our approach
allows for the universal preparation of molecules tightly fixed in space for
experiments with x-ray pulses.Comment: 10 pages, 5 figure
Silent progression in disease activity-free relapsing multiple sclerosis.
ObjectiveRates of worsening and evolution to secondary progressive multiple sclerosis (MS) may be substantially lower in actively treated patients compared to natural history studies from the pretreatment era. Nonetheless, in our recently reported prospective cohort, more than half of patients with relapsing MS accumulated significant new disability by the 10th year of follow-up. Notably, "no evidence of disease activity" at 2âyears did not predict long-term stability. Here, we determined to what extent clinical relapses and radiographic evidence of disease activity contribute to long-term disability accumulation.MethodsDisability progression was defined as an increase in Expanded Disability Status Scale (EDSS) of 1.5, 1.0, or 0.5 (or greater) from baseline EDSSâ=â0, 1.0-5.0, and 5.5 or higher, respectively, assessed from baseline to year 5 (±1âyear) and sustained to year 10 (±1âyear). Longitudinal analysis of relative brain volume loss used a linear mixed model with sex, age, disease duration, and HLA-DRB1*15:01 as covariates.ResultsRelapses were associated with a transient increase in disability over 1-year intervals (pâ=â0.012) but not with confirmed disability progression (pâ=â0.551). Relative brain volume declined at a greater rate among individuals with disability progression compared to those who remained stable (pâ<â0.05).InterpretationLong-term worsening is common in relapsing MS patients, is largely independent of relapse activity, and is associated with accelerated brain atrophy. We propose the term silent progression to describe the insidious disability that accrues in many patients who satisfy traditional criteria for relapsing-remitting MS. Ann Neurol 2019;85:653-666
Clustering of the IR Background Light with Spitzer: Contribution from Resolved Sources
We describe the angular power spectrum of resolved sources at 3.6 microns
(L-band) in Spitzer imaging data of the GOODS HDF-N, the GOODS CDF-S, and the
NDWFS Bootes field in several source magnitude bins. We also measure angular
power spectra of resolved sources in the Bootes field at K_S and J-bands using
ground-based IR imaging data. In the three bands, J, K_S, and L, we detect the
clustering of galaxies on top of the shot-noise power spectrum at multipoles
between ell ~ 10^2 and 10^5. The angular power spectra range from the large,
linear scales to small, non-linear scales of galaxy clustering, and in some
magnitude ranges, show departure from a power-law clustering spectrum. We
consider a halo model to describe clustering measurements and to establish the
halo occup ation number parameters of IR bright galaxies at redshifts around
one. We also extend our clustering results and completeness-corrected faint
source number counts in GOODS fields to understand the underlying nature of
unresolved sources responsible for IR background (IRB) anisotropies that were
detected in deep Spitzer images. While these unresolved fluctuations were
measured at sub-arcminute angular scales, if a high-redshift diffuse component
associated with first galaxies exists in the IRB, then it's clustering
properties are best studied with shallow, wide-field images that allow a
measurement of the clustering spectrum from a few degrees to arcminute angular
scales.Comment: 12 pages, 11 figures. Accepted version in press with ApJ. Revised
version includes conditional luminosity function models for IR galaxy LFs,
counts and clustering spectra. The faint, unresolved galaxy counts in these
models can reproduce excess anisotropy fluctuations reported in
astro-ph/0511105. Conditional luminosity function code is available at
http://www.cooray.org/lumfunc.html V3: Includes all data from
astro-ph/0511105 in revised Fig.8 and minor changes to tex
X-ray diffractive imaging of controlled gas-phase molecules: Toward imaging of dynamics in the molecular frame
We report experimental results on the diffractive imaging of
three-dimensionally aligned 2,5-diiodothiophene molecules. The molecules were
aligned by chirped near-infrared laser pulses, and their structure was probed
at a photon energy of 9.5 keV () provided by the
Linac Coherent Light Source. Diffracted photons were recorded on the CSPAD
detector and a two-dimensional diffraction pattern of the equilibrium structure
of 2,5-diiodothiophene was recorded. The retrieved distance between the two
iodine atoms agrees with the quantum-chemically calculated molecular structure
to within 5 %. The experimental approach allows for the imaging of intrinsic
molecular dynamics in the molecular frame, albeit this requires more
experimental data which should be readily available at upcoming
high-repetition-rate facilities
Strong gravitational lensing probes of the particle nature of dark matter
There is a vast menagerie of plausible candidates for the constituents of
dark matter, both within and beyond extensions of the Standard Model of
particle physics. Each of these candidates may have scattering (and other)
cross section properties that are consistent with the dark matter abundance,
BBN, and the most scales in the matter power spectrum; but which may have
vastly different behavior at sub-galactic "cutoff" scales, below which dark
matter density fluctuations are smoothed out. The only way to quantitatively
measure the power spectrum behavior at sub-galactic scales at distances beyond
the local universe, and indeed over cosmic time, is through probes available in
multiply imaged strong gravitational lenses. Gravitational potential
perturbations by dark matter substructure encode information in the observed
relative magnifications, positions, and time delays in a strong lens. Each of
these is sensitive to a different moment of the substructure mass function and
to different effective mass ranges of the substructure. The time delay
perturbations, in particular, are proving to be largely immune to the
degeneracies and systematic uncertainties that have impacted exploitation of
strong lenses for such studies. There is great potential for a coordinated
theoretical and observational effort to enable a sophisticated exploitation of
strong gravitational lenses as direct probes of dark matter properties. This
opportunity motivates this white paper, and drives the need for: a) strong
support of the theoretical work necessary to understand all astrophysical
consequences for different dark matter candidates; and b) tailored
observational campaigns, and even a fully dedicated mission, to obtain the
requisite data.Comment: Science white paper submitted to the Astro2010 Decadal Cosmology &
Fundamental Physics Science Frontier Pane
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