86 research outputs found
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
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
Understanding the diversity of 21Â cm cosmology analyses
21 cm power spectrum observations have the potential to revolutionize our understanding of the epoch of reionization and dark energy, but require extraordinarily precise data analysis methods to separate the cosmological signal from the astrophysical and instrumental contaminants. This analysis challenge has led to a diversity of proposed analyses, including delay spectra, imaging power spectra, m-mode analysis, and numerous others. This diversity of approach is a strength, but has also led to a confusion within the community about whether insights gleaned by one group are applicable to teams working in different analysis frameworks. In this paper, we show that all existing analysis proposals can be classified into two distinct families based on whether they estimate the power spectrum of the measured or reconstructed sky. This subtle difference in the statistical question posed largely determines the susceptibility of the analyses to foreground emission and calibration errors, and ultimately the science different analyses can pursue. In this paper, we detail the origin of the two analysis families, categorize the analyses being actively developed, and explore their relative sensitivities to foreground contamination and calibration errors.National Science Foundation (NSF) [1613855, 1613040, 1506024, 1636646]; National Aeronautivcal and Space Administration [80NSSC18K0389]; NSF Astronomy and Astrophysics Postdoctoral Fellowship [1701440]This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]
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
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