54 research outputs found
Why and When to Expect Gaussian Error Distributions in Epoch of Reionization 21-cm Power Spectrum Measurements
We explore error distributions in Epoch of Reionization 21-cm power spectrum
estimators using a combination of mathematical analysis and numerical
simulations. We provide closed form solutions for the error distributions of
individual bins in 3d-power spectra for two estimators currently in use in the
field, which we designate as ``straight-square" and ``cross-multiply"
estimators. We then demonstrate when the corresponding spherically binned power
spectra should (and should not) have Gaussian error distributions, which
requires appealing to nonstandard statements of the central limit theorem. This
has important implications for how upper limits are reported, as well as how
cosmological inferences are performed based on power spectrum measurements.
Specifically, assuming a Gaussian error distribution can over or underestimate
the upper limit depending on the type of estimator, and produces overly compact
likelihood functions for the power spectrum
The Statistics of Negative Power Spectrum Systematics in some 21 cm Analyses
Through a very careful analysis Kolopanis et al. (2022) identified a negative
power spectrum (PS) systematic. The 21 cm cosmology community has assumed that
any observational systematics would add power, as negative PS are non-physical.
In addition to the mystery of their origin, negative PS systematics raise the
spectre of artificially lowering upper limits on the 21 cm PS. It appears that
the source of the negative PS systematics is a subtle interaction between
choices in how the PS estimate is calculated and baseline-dependent systematic
power. In this paper we present a statistical model of baseline dependent
systematics to explore how negative PS systematics can appear and their
statistical characteristics. This leads us to recommendations on when and how
to consider negative PS systematics when reporting observational 21 cm
cosmology upper limit.Comment: Submitted to MNRA
Positron clouds within thunderstorms
We report the observation of two isolated clouds of positrons inside an
active thunderstorm. These observations were made by the Airborne Detector for
Energetic Lightning Emissions (ADELE), an array of six gamma-ray detectors,
which flew on a Gulfstream V jet aircraft through the top of an active
thunderstorm in August 2009. ADELE recorded two 511 keV gamma-ray count rate
enhancements, 35 seconds apart, each lasting approximately 0.2 seconds. The
enhancements, which were about a factor of 12 above background, were both
accompanied by electrical activity as measured by a flat-plate antenna on the
underside of the aircraft. The energy spectra were consistent with a source
mostly composed of positron annihilation gamma rays, with a prominent 511 keV
line clearly visible in the data. Model fits to the data suggest that the
aircraft was briefly immersed in clouds of positrons, more than a kilometer
across. It is not clear how the positron clouds were created within the
thunderstorm, but it is possible they were caused by the presence of the
aircraft in the electrified environment.Comment: Accepted for publication in the Journal of Plasma Physic
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
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
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