4,770 research outputs found
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
The CMB Quadrupole in a Polarized Light
The low quadrupole of the cosmic microwave background (CMB), measured by COBE
and confirmed by WMAP, has generated much discussion recently. We point out
that the well-known correlation between temperature and polarization
anisotropies of the CMB further constrains the low multipole anisotropy data.
This correlation originates from the fact that the low-multipole polarization
signal is sourced by the CMB quadrupole as seen by free electrons during the
relatively recent cosmic history. Consequently, the large-angle temperature
anisotropy data make restrictive predictions for the large-angle polarization
anisotropy, which depend primarily on the optical depth for electron scattering
after cosmological recombination, tau. We show that if current cosmological
models for the generation of large angle anisotropy are correct and the
COBE/WMAP data are not significantly contaminated by non-CMB signals, then the
observed C_te amplitude on the largest scales is discrepant at the 99.8% level
with the observed C_tt for the concordance LCDM model with tau=0.10. Using
tau=0.17, the preferred WMAP model-independent value, the discrepancy is at the
level of 98.5%.Comment: 6 pages, 6 figures, ApJ in pres
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
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
Scattered Lyman-alpha Radiation Around Sources Before Cosmological Reionization
The spectra of the first galaxies and quasars in the Universe should be
strongly absorbed shortward of their rest-frame Lyman-alpha wavelength by
neutral hydrogen (HI) in the intervening intergalactic medium. However, the
Lyman-alpha line photons emitted by these sources are not eliminated but rather
scatter until they redshift out of resonance and escape due to the Hubble
expansion of the surrounding intergalactic HI. We calculate the resulting
brightness distribution and the spectral shape of the diffuse Lyman-alpha line
emission around high redshift sources, before the intergalactic medium was
reionized. Typically, the Lyman-alpha photons emitted by a source at z=10
scatter over a characteristic angular radius of order 15 arcseconds around the
source and compose a line which is broadened and redshifted by about a thousand
km/s relative to the source. The scattered photons are highly polarized.
Detection of the diffuse Lyman-alpha halos around high redshift sources would
provide a unique tool for probing the neutral intergalactic medium before the
epoch of reionization. On sufficiently large scales where the Hubble flow is
smooth and the gas is neutral, the Lyman-alpha brightness distribution can be
used to determine the cosmological mass densities of baryons and matter.Comment: 21 pages, 5 Postscript figures, accepted by ApJ; figures 1--3
corrected; new section added on the detectability of Lyman alpha halos;
conclusions update
Redshifted 21cm Signatures Around the Highest Redshift Quasars
The Ly-alpha absorption spectrum of the highest redshift quasars indicates
that they are surrounded by giant HII regions, a few Mpc in size. The neutral
gas around these HII regions should emit 21cm radiation in excess of the Cosmic
Microwave Background, and enable future radio telescopes to measure the
transverse extent of these HII regions. At early times, the HII regions expand
with a relativistic speed. Consequently, their measured sizes along the
line-of-sight (via Ly-alpha absorption) and transverse to it (via 21 cm
emission) should have different observed values due to relativistic time-delay.
We show that the combined measurement of these sizes would directly constrain
the neutral fraction of the surrounding intergalactic medium (IGM) as well as
the quasar lifetime. Based on current number counts of luminous quasars at z>6,
an instrument like LOFAR should detect >2 redshifted 21cm shells per field
(with a radius of 11 degrees) around active quasars as bright as those already
discovered by SDSS, and >200 relic shells of inactive quasars per field. We
show that Ly-alpha photons from the quasar are unable to heat the IGM or to
couple the spin and kinetic temperatures of atomic hydrogen beyond the edge of
the HII region. The detection of the IGM in 21cm emission around high redshift
quasars would therefore gauge the presence of a cosmic Ly-alpha background
during the reionization epoch.Comment: 11 pages, 6 figures. Submitted to Ap
Gravitational Lensing of the SDSS High-Redshift Quasars
We predict the effects of gravitational lensing on the color-selected
flux-limited samples of z~4.3 and z>5.8 quasars, recently published by the
Sloan Digital Sky Survey (SDSS). Our main findings are: (i) The lensing
probability should be 1-2 orders of magnitude higher than for conventional
surveys. The expected fraction of multiply-imaged quasars is highly sensitive
to redshift and the uncertain slope of the bright end of the luminosity
function, beta_h. For beta_h=2.58 (3.43) we find that at z~4.3 and i*<20.0 the
fraction is ~4% (13%) while at z~6 and z*<20.2 the fraction is ~7% (30%). (ii)
The distribution of magnifications is heavily skewed; sources having the
redshift and luminosity of the SDSS z>5.8 quasars acquire median magnifications
of med(mu_obs)~1.1-1.3 and mean magnifications of ~5-50. Estimates of
the quasar luminosity density at high redshift must therefore filter out
gravitationally-lensed sources. (iii) The flux in the Gunn-Peterson trough of
the highest redshift (z=6.28) quasar is known to be f_lambda<3 10^-19
erg/sec/cm^2/Angstrom. Should this quasar be multiply imaged, we estimate a 40%
chance that light from the lens galaxy would have contaminated the same part of
the quasar spectrum with a higher flux. Hence, spectroscopic studies of the
epoch of reionization need to account for the possibility that a lens galaxy,
which boosts the quasar flux, also contaminates the Gunn-Peterson trough. (iv)
Microlensing by stars should result in ~1/3 of multiply imaged quasars in the
z>5.8 catalog varying by more than 0.5 magnitudes over the next decade. The
median equivalent width would be lowered by ~20% with respect to the intrinsic
value due to differential magnification of the continuum and emission-line
regions.Comment: 27 pages, 10 figures. Expansion on the discussion in
astro-ph/0203116. Replaced with version accepted for publication in Ap
Inconsistency with De Sitter Spacetime of "Gravitational Pair Production and Black Hole Evaporation"
We study the recent Physical Review Letter [1] which presents a new mechanism
for black hole evaporation through a spatially dependent temperature. This new
temperature is comparable to the Hawking result near the black hole, but is
very small far away, and therefore could be a small correction. Here we apply
the proposed reasoning to the case of de Sitter space, finding that it over
predicts the de Sitter temperature of a minimally coupled scalar by factor of
and therefore cannot be ignored in any limit. This indicates an
inconsistency in the proposed formalism.Comment: 2 pages, 1 figure, in double column forma
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