5,541 research outputs found
Probing the Epoch of Early Baryonic Infall Through 21cm Fluctuations
After cosmological recombination, the primordial hydrogen gas decoupled from
the cosmic microwave background (CMB) and fell into the gravitational potential
wells of the dark matter. The neutral hydrogen imprinted acoustic oscillations
on the pattern of brightness fluctuations due to its redshifted 21cm absorption
of the CMB. Unlike CMB temperature fluctuations which probe the power spectrum
at cosmic recombination, we show that observations of the 21cm fluctuations at
z ~ 20-200 can measure four separate fluctuation modes (with a fifth mode
requiring very high precision), thus providing a unique probe of the geometry
and composition of the universe.Comment: 5 pages, 4 figures, MNRAS Letters, accepte
C.V.D. annual report: January, 1967 research project ru27-1 : analogue study of semiconductor device structures
The e::tension of the resistance network analogue method to the study
of a M.O.S.T. structure is described. By means of an iterative technique,
data regarding channel current, field distribution, surface charge and
position of pinch-off point as function of gate and drain voltagen can be
obtained which do not involve the usual 'gradual' channel approximation
Results for a particular device geometry are presented.
A discussion of a digital computer approach to the solution of semiconductor
device current flow problems is included, together with preliminary
results
The Cosmic Microwave Background and the Ionization History of the Universe
Details of how the primordial plasma recombined and how the universe later
reionized are currently somewhat uncertain. This uncertainty can restrict the
accuracy of cosmological parameter measurements from the Cosmic Microwave
Background (CMB). More positively, future CMB data can be used to constrain the
ionization history using observations. We first discuss how current
uncertainties in the recombination history impact parameter constraints, and
show how suitable parameterizations can be used to obtain unbiased parameter
estimates from future data. Some parameters can be constrained robustly,
however there is clear motivation to model recombination more accurately with
quantified errors. We then discuss constraints on the ionization fraction
binned in redshift during reionization. Perfect CMB polarization data could in
principle distinguish different histories that have the same optical depth. We
discuss how well the Planck satellite may be able to constrain the ionization
history, and show the currently very weak constraints from WMAP three-year
data.Comment: Changes to match MNRAS accepted versio
Constraining the unexplored period between reionization and the dark ages with observations of the global 21 cm signal
Observations of the frequency dependence of the global brightness temperature
of the redshifted 21 cm line of neutral hydrogen may be possible with single
dipole experiments. In this paper, we develop a Fisher matrix formalism for
calculating the sensitivity of such instruments to the 21 cm signal from
reionization and the dark ages. We show that rapid reionization histories with
duration delta z< 2 can be constrained, provided that local foregrounds can be
well modelled by low order polynomials. It is then shown that observations in
the range nu = 50 - 100 MHz can feasibly constrain the Lyman alpha and X-ray
emissivity of the first stars forming at z = 15 - 25, provided that systematic
temperature residuals can be controlled to less than 1 mK. Finally, we
demonstrate the difficulty of detecting the 21 cm signal from the dark ages
before star formation.Comment: 11 pages, 14 figures, submitted to PR
Constraints on Off-Axis X-Ray Emission from Beamed GRBs
We calculate the prompt x-ray emission as a function of viewing angle for
beamed Gamma-Ray Burst (GRB) sources. Prompt x-rays are inevitable due to the
less highly blueshifted photons emitted at angles greater than 1/gamma relative
to the beam symmetry axis, where gamma is the expansion Lorentz factor. The
observed flux depends on the combinations (gamma Delta theta) and (gamma
theta_v), where (Delta theta) is the beaming angle and theta_v is the viewing
angle. We use the observed source counts of gamma-ray-selected GRBs to predict
the minimum detection rate of prompt x-ray bursts as a function of limiting
sensitivity. We compare our predictions with the results from the Ariel V
catalog of fast x-ray transients, and find that Ariel's sensitivity is not
great enough to place significant constraints on gamma and (Delta theta). We
estimate that a detector with fluence limit ~10^{-7} erg/cm^2 in the 2-10 keV
channel will be necessary to distinguish between geometries. Because the x-ray
emission is simultaneous with the GRB emission, our predicted constraints do
not involve any model assumptions about the emission physics but simply follow
from special-relativistic considerations.Comment: Submitted to Ap
Variability of GRB Afterglows Due to Interstellar Turbulence
Gamma-Ray Burst (GRB) afterglows are commonly interpreted as synchrotron
emission from a relativistic blast wave produced by a point explosion in an
ambient medium, plausibly the interstellar medium of galaxies. We calculate the
amplitude of flux fluctuations in the lightcurve of afterglows due to
inhomogeneities in the surrounding medium. Such inhomogeneities are an
inevitable consequence of interstellar turbulence, but could also be generated
by variability and anisotropy in a precursor wind from the GRB progenitor.
Detection of their properties could provide important clues about the
environments of GRB sources. We apply our calculations to GRB990510, where an
rms scatter of 2% was observed for the optical flux fluctuations on the 0.1--2
hour timescale during the first day of the afterglow, consistent with it being
entirely due to photometric noise (Stanek et al. 1999). The resulting upper
limits on the density fluctuations on scales of 20-200 AU around the source of
GRB990510, are lower than the inferred fluctuation amplitude on similar scales
in the Galactic interstellar medium. Hourly monitoring of future optical
afterglows might therefore reveal fractional flux fluctuations at the level of
a few percent.Comment: 18 pages, submitted to Ap
Global 21cm signal experiments: a designer's guide
[Abridged] The spatially averaged global spectrum of the redshifted 21cm line
has generated much experimental interest, for it is potentially a direct probe
of the Epoch of Reionization and the Dark Ages. Since the cosmological signal
here has a purely spectral signature, most proposed experiments have little
angular sensitivity. This is worrisome because with only spectra, the global
21cm signal can be difficult to distinguish from foregrounds such as Galactic
synchrotron radiation, as both are spectrally smooth and the latter is orders
of magnitude brighter. We establish a mathematical framework for global signal
data analysis in a way that removes foregrounds optimally, complementing
spectra with angular information. We explore various experimental design
trade-offs, and find that 1) with spectral-only methods, it is impossible to
mitigate errors that arise from uncertainties in foreground modeling; 2)
foreground contamination can be significantly reduced for experiments with fine
angular resolution; 3) most of the statistical significance in a positive
detection during the Dark Ages comes from a characteristic high-redshift trough
in the 21cm brightness temperature; and 4) Measurement errors decrease more
rapidly with integration time for instruments with fine angular resolution. We
show that if observations and algorithms are optimized based on these findings,
an instrument with a 5 degree beam can achieve highly significant detections
(greater than 5-sigma) of even extended (high Delta-z) reionization scenarios
after integrating for 500 hrs. This is in contrast to instruments without
angular resolution, which cannot detect gradual reionization. Abrupt ionization
histories can be detected at the level of 10-100's of sigma. The expected
errors are also low during the Dark Ages, with a 25-sigma detection of the
expected cosmological signal after only 100 hrs of integration.Comment: 34 pages, 30 figures. Replaced (v2) to match accepted PRD version
(minor pedagogical additions to text; methods, results, and conclusions
unchanged). Fixed two typos (v3); text, results, conclusions etc. completely
unchange
Separating out the Alcock-Paczynski Effect on 21cm Fluctuations
We reconsider the Alcock-Paczynski effect on 21cm fluctuations from high
redshift, focusing on the 21cm power spectrum. We show that at each accessible
redshift both the angular diameter distance and the Hubble constant can be
determined from the power spectrum. Furthermore, this is possible using
anisotropies that depend only on linear density perturbations and not on
astrophysical sources of 21cm fluctuations. We show that measuring these
quantities at high redshift would not just confirm results from the cosmic
microwave background but provide appreciable additional sensitivity to
cosmological parameters and dark energy.Comment: 6 pages, 3 figures, MNRAS, revised versio
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