64 research outputs found
A Self-Learning Neural Network Approach for RFI Detection and Removal in Radio Astronomy
We present a novel neural network (NN) method for the detection and removal
of Radio Frequency Interference (RFI) from the raw digitized signal in the
signal processing chain of a typical radio astronomy experiment. The main
advantage of our method is that it does not require a training set. Instead,
our method relies on the fact that the true signal of interest coming from
astronomical sources is thermal and therefore described as a Gaussian random
process, which cannot be compressed. We employ a variational encoder/decoder
network to find the compressible information in the datastream that can explain
the most variance with the fewest degrees of freedom. We demonstrate it on a
set of toy problems and stored ringbuffers from the Baryon Mapping eXperiment
(BMX) prototype. We find that the RFI subtraction is effective at cleaning
simulated timestreams: while we find that the power spectra of the RFI-cleaned
timestreams output by the NN suffer from extra signal consistent with additive
noise, we find that it is generally around percent level across the band and
sub 10 percent in contaminated spectral channels even when RFI power is an
order of magnitude larger than the signal. We discuss advantages and
limitations of this method and possible implementation in the front-end of
future radio experiments.Comment: 16 pages, 6 figures, Accepted for publication in PAS
The Hydrogen Epoch of Reionization Array Dish II: Characterization of Spectral Structure with Electromagnetic Simulations and its science Implications
We use time-domain electromagnetic simulations to determine the spectral
characteristics of the Hydrogen Epoch of Reionization Arrays (HERA) antenna.
These simulations are part of a multi-faceted campaign to determine the
effectiveness of the dish's design for obtaining a detection of redshifted 21
cm emission from the epoch of reionization. Our simulations show the existence
of reflections between HERA's suspended feed and its parabolic dish reflector
that fall below -40 dB at 150 ns and, for reasonable impedance matches, have a
negligible impact on HERA's ability to constrain EoR parameters. It follows
that despite the reflections they introduce, dishes are effective for
increasing the sensitivity of EoR experiments at relatively low cost. We find
that electromagnetic resonances in the HERA feed's cylindrical skirt, which is
intended to reduce cross coupling and beam ellipticity, introduces significant
power at large delays ( dB at 200 ns) which can lead to some loss of
measurable Fourier modes and a modest reduction in sensitivity. Even in the
presence of this structure, we find that the spectral response of the antenna
is sufficiently smooth for delay filtering to contain foreground emission at
line-of-sight wave numbers below Mpc, in
the region where the current PAPER experiment operates. Incorporating these
results into a Fisher Matrix analysis, we find that the spectral structure
observed in our simulations has only a small effect on the tight constraints
HERA can achieve on parameters associated with the astrophysics of
reionization.Comment: Accepted to ApJ, 18 pages, 17 Figures. Replacement matches accepted
manuscrip
The Hydrogen Epoch of Reionization Array Dish I: Beam Pattern Measurements and Science Implications
The Hydrogen Epoch of Reionization Array (HERA) is a radio interferometer
aiming to detect the power spectrum of 21 cm fluctuations from neutral hydrogen
from the Epoch of Reionization (EOR). Drawing on lessons from the Murchison
Widefield Array (MWA) and the Precision Array for Probing the Epoch of
Reionization (PAPER), HERA is a hexagonal array of large (14 m diameter) dishes
with suspended dipole feeds. Not only does the dish determine overall
sensitivity, it affects the observed frequency structure of foregrounds in the
interferometer. This is the first of a series of four papers characterizing the
frequency and angular response of the dish with simulations and measurements.
We focus in this paper on the angular response (i.e., power pattern), which
sets the relative weighting between sky regions of high and low delay, and
thus, apparent source frequency structure. We measure the angular response at
137 MHz using the ORBCOMM beam mapping system of Neben et al. We measure a
collecting area of 93 m^2 in the optimal dish/feed configuration, implying
HERA-320 should detect the EOR power spectrum at z~9 with a signal-to-noise
ratio of 12.7 using a foreground avoidance approach with a single season of
observations, and 74.3 using a foreground subtraction approach. Lastly we study
the impact of these beam measurements on the distribution of foregrounds in
Fourier space.Comment: 13 pages, 9 figures. Replaced to match accepted ApJ versio
A cryogenic rotation stage with a large clear aperture for the half-wave plates in the Spider instrument
We describe the cryogenic half-wave plate rotation mechanisms built for and
used in Spider, a polarization-sensitive balloon-borne telescope array that
observed the Cosmic Microwave Background at 95 GHz and 150 GHz during a
stratospheric balloon flight from Antarctica in January 2015. The mechanisms
operate at liquid helium temperature in flight. A three-point contact design
keeps the mechanical bearings relatively small but allows for a large (305 mm)
diameter clear aperture. A worm gear driven by a cryogenic stepper motor allows
for precise positioning and prevents undesired rotation when the motors are
depowered. A custom-built optical encoder system monitors the bearing angle to
an absolute accuracy of +/- 0.1 degrees. The system performed well in Spider
during its successful 16 day flight.Comment: 11 pages, 7 figures, Published in Review of Scientific Instruments.
v2 includes reviewer changes and longer literature revie
Packed Ultra-wideband Mapping Array (PUMA): A Radio Telescope for Cosmology and Transients
PUMA is a proposal for an ultra-wideband, low-resolution and transit
interferometric radio telescope operating at . Its
design is driven by six science goals which span three science themes: the
physics of dark energy (measuring the expansion history and growth of the
universe up to ), the physics of inflation (constraining primordial
non-Gaussianity and primordial features) and the transient radio sky (detecting
one million fast radio bursts and following up SKA-discovered pulsars). We
propose two array configurations composed of hexagonally close-packed 6m dish
arrangements with 50% fill factor. The initial 5,000 element 'petite array' is
scientifically compelling, and can act as a demonstrator and a stepping stone
to the full 32,000 element 'full array'. Viewed as a 21cm intensity mapping
telescope, the program has the noise equivalent of a traditional spectroscopic
galaxy survey comprised of 0.6 and 2.5 billion galaxies at a comoving
wavenumber of spanning the redshift range for the petite and full configurations, respectively. At redshifts beyond
, the 21cm technique is a uniquely powerful way of mapping the universe,
while the low-redshift range will allow for numerous cross-correlations with
existing and upcoming surveys. This program is enabled by the development of
ultra-wideband radio feeds, cost-effective dish construction methods, commodity
radio-frequency electronics driven by the telecommunication industry and the
emergence of sufficient computing power to facilitate real-time signal
processing that exploits the full potential of massive radio arrays. The
project has an estimated construction cost of 55 and 330 million FY19 USD for
the petite and full array configurations. Including R&D, design, operations and
science analysis, the cost rises to 125 and 600 million FY19 USD, respectively.Comment: 10 pages + references, 3 figures, 3 tables; project white paper
submitted to the Astro2020 decadal survey; further details in updated
arXiv:1810.0957
Mapping Cosmic Dawn and Reionization: Challenges and Synergies
Cosmic dawn and the Epoch of Reionization (EoR) are among the least explored
observational eras in cosmology: a time at which the first galaxies and
supermassive black holes formed and reionized the cold, neutral Universe of the
post-recombination era. With current instruments, only a handful of the
brightest galaxies and quasars from that time are detectable as individual
objects, due to their extreme distances. Fortunately, a multitude of
multi-wavelength intensity mapping measurements, ranging from the redshifted 21
cm background in the radio to the unresolved X-ray background, contain a
plethora of synergistic information about this elusive era. The coming decade
will likely see direct detections of inhomogenous reionization with CMB and 21
cm observations, and a slew of other probes covering overlapping areas and
complementary physical processes will provide crucial additional information
and cross-validation. To maximize scientific discovery and return on
investment, coordinated survey planning and joint data analysis should be a
high priority, closely coupled to computational models and theoretical
predictions.Comment: 5 pages, 1 figure, submitted to the Astro2020 Decadal Survey Science
White Paper cal
Cosmology with the Highly Redshifted 21cm Line
In addition to being a probe of Cosmic Dawn and Epoch of Reionization
astrophysics, the 21cm line at is also a powerful way to constrain
cosmology. Its power derives from several unique capabilities. First, the 21cm
line is sensitive to energy injections into the intergalactic medium at high
redshifts. It also increases the number of measurable modes compared to
existing cosmological probes by orders of magnitude. Many of these modes are on
smaller scales than are accessible via the CMB, and moreover have the advantage
of being firmly in the linear regime (making them easy to model theoretically).
Finally, the 21cm line provides access to redshifts prior to the formation of
luminous objects. Together, these features of 21cm cosmology at provide
multiple pathways toward precise cosmological constraints. These include the
"marginalizing out" of astrophysical effects, the utilization of redshift space
distortions, the breaking of CMB degeneracies, the identification of signatures
of relative velocities between baryons and dark matter, and the discovery of
unexpected signs of physics beyond the CDM paradigm at high redshifts.Comment: Science white paper submitted to Decadal 2020 surve
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