624 research outputs found
Short-Term Leaf Elongation Kinetics of Maize in Response to Salinity Are Independent of the Root
The Precision Array for Probing the Epoch of Reionization: 8 Station Results
We are developing the Precision Array for Probing the Epoch of Reionization
(PAPER) to detect 21cm emission from the early Universe, when the first stars
and galaxies were forming. We describe the overall experiment strategy and
architecture and summarize two PAPER deployments: a 4-antenna array in the
low-RFI environment of Western Australia and an 8-antenna array at our
prototyping site in Green Bank, WV. From these activities we report on system
performance, including primary beam model verification, dependence of system
gain on ambient temperature, measurements of receiver and overall system
temperatures, and characterization of the RFI environment at each deployment
site.
We present an all-sky map synthesized between 139 MHz and 174 MHz using data
from both arrays that reaches down to 80 mJy (4.9 K, for a beam size of 2.15e-5
steradians at 154 MHz), with a 10 mJy (620 mK) thermal noise level that
indicates what would be achievable with better foreground subtraction. We
calculate angular power spectra () in a cold patch and determine them
to be dominated by point sources, but with contributions from galactic
synchrotron emission at lower radio frequencies and angular wavemodes. Although
the cosmic variance of foregrounds dominates errors in these power spectra, we
measure a thermal noise level of 310 mK at for a 1.46-MHz band
centered at 164.5 MHz. This sensitivity level is approximately three orders of
magnitude in temperature above the level of the fluctuations in 21cm emission
associated with reionization.Comment: 13 pages, 14 figures, submitted to AJ. Revision 2 corrects a scaling
error in the x axis of Fig. 12 that lowers the calculated power spectrum
temperatur
Measurements of one-point statistics in 21 cm intensity maps via foreground avoidance strategy
Measurements of the one-point probability distribution function and
higher-order moments (variance, skewness, and kurtosis) of the high-redshift 21
cm fluctuations are among the most direct statistical probes of the
non-Gaussian nature of structure formation and evolution during reionization.
However, contamination from astrophysical foregrounds and instrument
systematics pose significant challenges in measuring these statistics in real
observations. In this work, we use forward modelling to investigate the
feasibility of measuring 21 cm one-point statistics through a foreground
avoidance strategy. Leveraging the well-known characteristic of foreground
contamination in which it occupies a wedge-shape region in k-space, we apply a
foreground wedge-cut filter that removes the contaminated modes from a mock
data set based on the Hydrogen Epoch of Reionization Array (HERA) instrument,
and measure the one-point statistics from the image-space representation of the
remaining non-contaminated modes. We experiment with wedge-cutting over
different frequency bandwidths and varying degrees of removal that correspond
to different assumptions on the extent of the foreground sources on the sky and
leakage from the Fourier Transform window function. We find that the centre of
the band is the least biased from wedge-cutting while the edges of the band are
unusable due to being highly down-weighted by the window function. Based on
this finding, we introduce a rolling filter method that allows reconstruction
of an optimal wedge-cut 21~cm intensity map over the full bandwidth using
outputs from wedge-cutting over multiple sub-bands. We perform Monte Carlo
simulations to show that HERA should be able to measure the rise in skewness
and kurtosis near the end of reionization with the rolling wedge-cut method if
foreground leakage from the Fourier transform window function can be
controlled.Comment: 12 pages, 8 figures, submitted to MNRA
What Next-Generation 21 cm Power Spectrum Measurements Can Teach Us About the Epoch of Reionization
A number of experiments are currently working towards a measurement of the 21
cm signal from the Epoch of Reionization. Whether or not these experiments
deliver a detection of cosmological emission, their limited sensitivity will
prevent them from providing detailed information about the astrophysics of
reionization. In this work, we consider what types of measurements will be
enabled by a next-generation of larger 21 cm EoR telescopes. To calculate the
type of constraints that will be possible with such arrays, we use simple
models for the instrument, foreground emission, and the reionization history.
We focus primarily on an instrument modeled after the
collecting area Hydrogen Epoch of Reionization Array (HERA) concept design, and
parameterize the uncertainties with regard to foreground emission by
considering different limits to the recently described "wedge" footprint in
k-space. Uncertainties in the reionization history are accounted for using a
series of simulations which vary the ionizing efficiency and minimum virial
temperature of the galaxies responsible for reionization, as well as the mean
free path of ionizing photons through the IGM. Given various combinations of
models, we consider the significance of the possible power spectrum detections,
the ability to trace the power spectrum evolution versus redshift, the
detectability of salient power spectrum features, and the achievable level of
quantitative constraints on astrophysical parameters. Ultimately, we find that
of collecting area is enough to ensure a very high significance
() detection of the reionization power spectrum in even the
most pessimistic scenarios. This sensitivity should allow for meaningful
constraints on the reionization history and astrophysical parameters,
especially if foreground subtraction techniques can be improved and
successfully implemented.Comment: 27 pages, 18 figures, updated SKA numbers in appendi
Report of the committee on a commercially developed space facility
Major facilities that could support significant microgravity research and applications activity are discussed. The ground-based facilities include drop towers, aircraft flying parabolic trajectories, and sounding rockets. Facilities that are intrinsically tied to the Space Shuttle range from Get-Away-Special canisters to Spacelab long modules. There are also orbital facilities which include recoverable capsules launched on expendable launch vehicles, free-flying spacecraft, and space stations. Some of these existing, planned, and proposed facilities are non-U.S. in origin, but potentially available to U.S. investigators. In addition, some are governmentally developed and operated whereas others are planned to be privately developed and/or operated. Tables are provided to show the facility, developer, duration, estimated gravity level, crew interaction, flight frequency, year available, power to payload, payload volume, and maximum payload mass. The potential of direct and indirect benefits of manufacturing in space are presented
Infrasound and Gravity Waves Over the Andes Observed by a Pressure Sensor on Board a Stratospheric Balloon
The study of infrasound (acoustic) and gravity waves sources and propagation in the atmosphere of a planet gives us precious insight on atmosphere dynamics, climate, and even internal structure. The implementation of modern pressure sensors with high rate sampling on stratospheric balloons is improving their study. We analyzed the data from the National Aeronautics and Space Administration Ultra Long Duration Balloon mission (16 May to 30 June 2016). Here, we focus on the balloon's transit of the Andes Mountains. We detected gravity waves that are associated to troposphere convective activity and mountain waves. An increase of the horizontal wavelengths from 50 to 70 km with increasing distance to the mountains is favoring the presence of mountain waves. We also report on the detection of infrasounds generated by the mountains in the 0.01–0.1 Hz range with a pressure amplitude increase by a factor 2 relative background signal. Besides, we characterized the decrease of microbaroms power when the balloon was flying away from the ocean coast. These observations suggest, in a way similar to microseisms for seismometers, that microbaroms are the main background noise sources recorded in the stratosphere even far from the ocean sources. Finally, we observed a broadband signal above the Andes, between 0.45 and 2 Hz, probably associated with a thunderstorm. The diversity of geophysical phenomena captured in less than a day of observation stresses the interest of high rate pressure sensors on board long-duration balloon missions
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