4,870 research outputs found
Expansion-maximization-compression algorithm with spherical harmonics for single particle imaging with X-ray lasers
In 3D single particle imaging with X-ray free-electron lasers, particle
orientation is not recorded during measurement but is instead recovered as a
necessary step in the reconstruction of a 3D image from the diffraction data.
Here we use harmonic analysis on the sphere to cleanly separate the angu- lar
and radial degrees of freedom of this problem, providing new opportunities to
efficiently use data and computational resources. We develop the
Expansion-Maximization-Compression algorithm into a shell-by-shell approach and
implement an angular bandwidth limit that can be gradually raised during the
reconstruction. We study the minimum number of patterns and minimum rotation
sampling required for a desired angular and radial resolution. These extensions
provide new av- enues to improve computational efficiency and speed of
convergence, which are critically important considering the very large datasets
expected from experiment
A Sensitivity and Array-Configuration Study for Measuring the Power Spectrum of 21cm Emission from Reionization
Telescopes aiming to measure 21cm emission from the Epoch of Reionization
must toe a careful line, balancing the need for raw sensitivity against the
stringent calibration requirements for removing bright foregrounds. It is
unclear what the optimal design is for achieving both of these goals. Via a
pedagogical derivation of an interferometer's response to the power spectrum of
21cm reionization fluctuations, we show that even under optimistic scenarios,
first-generation arrays will yield low-SNR detections, and that different
compact array configurations can substantially alter sensitivity. We explore
the sensitivity gains of array configurations that yield high redundancy in the
uv-plane -- configurations that have been largely ignored since the advent of
self-calibration for high-dynamic-range imaging. We first introduce a
mathematical framework to generate optimal minimum-redundancy configurations
for imaging. We contrast the sensitivity of such configurations with
high-redundancy configurations, finding that high-redundancy configurations can
improve power-spectrum sensitivity by more than an order of magnitude. We
explore how high-redundancy array configurations can be tuned to various
angular scales, enabling array sensitivity to be directed away from regions of
the uv-plane (such as the origin) where foregrounds are brighter and where
instrumental systematics are more problematic. We demonstrate that a
132-antenna deployment of the Precision Array for Probing the Epoch of
Reionization (PAPER) observing for 120 days in a high-redundancy configuration
will, under ideal conditions, have the requisite sensitivity to detect the
power spectrum of the 21cm signal from reionization at a 3\sigma level at
k<0.25h Mpc^{-1} in a bin of \Delta ln k=1. We discuss the tradeoffs of low-
versus high-redundancy configurations.Comment: 34 pages, 5 figures, 2 appendices. Version accepted to Ap
An Electron-Tracking Compton Telescope for a Survey of the Deep Universe by MeV gamma-rays
Photon imaging for MeV gammas has serious difficulties due to huge
backgrounds and unclearness in images, which are originated from incompleteness
in determining the physical parameters of Compton scattering in detection,
e.g., lack of the directional information of the recoil electrons. The recent
major mission/instrument in the MeV band, Compton Gamma Ray
Observatory/COMPTEL, which was Compton Camera (CC), detected mere
persistent sources. It is in stark contrast with 2000 sources in the GeV
band. Here we report the performance of an Electron-Tracking Compton Camera
(ETCC), and prove that it has a good potential to break through this stagnation
in MeV gamma-ray astronomy. The ETCC provides all the parameters of
Compton-scattering by measuring 3-D recoil electron tracks; then the Scatter
Plane Deviation (SPD) lost in CCs is recovered. The energy loss rate (dE/dx),
which CCs cannot measure, is also obtained, and is found to be indeed helpful
to reduce the background under conditions similar to space. Accordingly the
significance in gamma detection is improved severalfold. On the other hand, SPD
is essential to determine the point-spread function (PSF) quantitatively. The
SPD resolution is improved close to the theoretical limit for multiple
scattering of recoil electrons. With such a well-determined PSF, we demonstrate
for the first time that it is possible to provide reliable sensitivity in
Compton imaging without utilizing an optimization algorithm. As such, this
study highlights the fundamental weak-points of CCs. In contrast we demonstrate
the possibility of ETCC reaching the sensitivity below erg
cm s at 1 MeV.Comment: 19 pages, 12 figures, Accepted to the Astrophysical Journa
Hydrogen Epoch of Reionization Array (HERA)
The Hydrogen Epoch of Reionization Array (HERA) is a staged experiment to
measure 21 cm emission from the primordial intergalactic medium (IGM)
throughout cosmic reionization (), and to explore earlier epochs of our
Cosmic Dawn (). During these epochs, early stars and black holes
heated and ionized the IGM, introducing fluctuations in 21 cm emission. HERA is
designed to characterize the evolution of the 21 cm power spectrum to constrain
the timing and morphology of reionization, the properties of the first
galaxies, the evolution of large-scale structure, and the early sources of
heating. The full HERA instrument will be a 350-element interferometer in South
Africa consisting of 14-m parabolic dishes observing from 50 to 250 MHz.
Currently, 19 dishes have been deployed on site and the next 18 are under
construction. HERA has been designated as an SKA Precursor instrument.
In this paper, we summarize HERA's scientific context and provide forecasts
for its key science results. After reviewing the current state of the art in
foreground mitigation, we use the delay-spectrum technique to motivate
high-level performance requirements for the HERA instrument. Next, we present
the HERA instrument design, along with the subsystem specifications that ensure
that HERA meets its performance requirements. Finally, we summarize the
schedule and status of the project. We conclude by suggesting that, given the
realities of foreground contamination, current-generation 21 cm instruments are
approaching their sensitivity limits. HERA is designed to bring both the
sensitivity and the precision to deliver its primary science on the basis of
proven foreground filtering techniques, while developing new subtraction
techniques to unlock new capabilities. The result will be a major step toward
realizing the widely recognized scientific potential of 21 cm cosmology.Comment: 26 pages, 24 figures, 2 table
Wide-Field InfraRed Survey Telescope (WFIRST) Final Report
In December 2010, NASA created a Science Definition Team (SDT) for WFIRST,
the Wide Field Infra-Red Survey Telescope, recommended by the Astro 2010
Decadal Survey as the highest priority for a large space mission. The SDT was
chartered to work with the WFIRST Project Office at GSFC and the Program Office
at JPL to produce a Design Reference Mission (DRM) for WFIRST. Part of the
original charge was to produce an interim design reference mission by mid-2011.
That document was delivered to NASA and widely circulated within the
astronomical community. In late 2011 the Astrophysics Division augmented its
original charge, asking for two design reference missions. The first of these,
DRM1, was to be a finalized version of the interim DRM, reducing overall
mission costs where possible. The second of these, DRM2, was to identify and
eliminate capabilities that overlapped with those of NASA's James Webb Space
Telescope (henceforth JWST), ESA's Euclid mission, and the NSF's ground-based
Large Synoptic Survey Telescope (henceforth LSST), and again to reduce overall
mission cost, while staying faithful to NWNH. This report presents both DRM1
and DRM2.Comment: 102 pages, 57 figures, 17 table
Report by the ESA-ESO Working Group on Fundamental Cosmology
ESO and ESA agreed to establish a number of Working Groups to explore
possible synergies between these two major European astronomical institutions.
This Working Group's mandate was to concentrate on fundamental questions in
cosmology, and the scope for tackling these in Europe over the next ~15 years.
One major resulting recommendation concerns the provision of new generations of
imaging survey, where the image quality and near-IR sensitivity that can be
attained only in space are naturally matched by ground-based imaging and
spectroscopy to yield massive datasets with well-understood photometric
redshifts (photo-z's). Such information is essential for a range of new
cosmological tests using gravitational lensing, large-scale structure, clusters
of galaxies, and supernovae. Great scope in future cosmology also exists for
ELT studies of the intergalactic medium and space-based studies of the CMB and
gravitational waves; here the synergy is less direct, but these areas will
remain of the highest mutual interest to the agencies. All these recommended
facilities will produce vast datasets of general applicability, which will have
a tremendous impact on broad areas of astronomy.Comment: ESA-ESO Working Groups Report No. 3, 125 pages, 28 figures. A PDF
version including the cover is available from
http://www.stecf.org/coordination/esa_eso/cosmology/report_cover.pdf and a
printed version (A5 booklet) is available in limited numbers from the Space
Telescope-European Coordinating Facility (ST-ECF): [email protected]
Can we measure the neutrino mass hierarchy in the sky?
Cosmological probes are steadily reducing the total neutrino mass window,
resulting in constraints on the neutrino-mass degeneracy as the most
significant outcome. In this work we explore the discovery potential of
cosmological probes to constrain the neutrino hierarchy, and point out some
subtleties that could yield spurious claims of detection. This has an important
implication for next generation of double beta decay experiments, that will be
able to achieve a positive signal in the case of degenerate or inverted
hierarchy of Majorana neutrinos. We find that cosmological experiments that
nearly cover the whole sky could in principle distinguish the neutrino
hierarchy by yielding 'substantial' evidence for one scenario over the another,
via precise measurements of the shape of the matter power spectrum from large
scale structure and weak gravitational lensing.Comment: Submitted to JCA
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