30,011 research outputs found
Constraining Quasar and IGM Properties Through Bubble Detection in Redshifted 21-cm Maps
The infrared detection of a z>7 quasar has opened up a new window to directly
probe the IGM during the epoch of reionization. In this paper we theoretically
consider the possibility of detecting the ionized bubble around a z=8 quasar
using targeted redshifted 21-cm observations with the GMRT. The apparent shape
and size of the ionized bubble, as seen by a distant observer, depends on the
parameters \dot{N}_{phs}/C, x_HI/C and \tau_Q where \dot{N}_{phs}, \tau_Q, x_HI
and C are respectively the photon emission rate, age of the quasar, the neutral
fraction and clumping factor of the IGM.Here we have analytically estimated the
shape and size of a quasar's ionized bubble assuming an uniform IGM and
ignoring other ionizing sources besides the quasar, and used this as a template
for matched filter bubble search with the GMRT visibility data. We have assumed
that \dot{N}_{phs} is known from the infrared spectrum and C from theoretical
considerations, which gives us two free parameters x_HI and \tau_Q for bubble.
Considering 1,000 hr of observation, we find that there is a reasonably large
region of parameter space where a 3\sigma detection is possible. We also find
that it will be possible to place lower limits on x_HI and \tau_Q with this
observation. Deeper follow up observations can place upper limits on \tau_Q and
x_HI. Value of C affect the estimation of x_HI but the estimation of \tau_Q
remains unaffected.We have used a semi-numerical technique to simulate the
apparent shape and size of quasar ionized bubbles considering the presence of
other ionizing sources and inhomogeneities in the IGM. The presence of other
sources increase the size of the quasar bubble, leading to underestimation of
x_HI. Clustering of other ionizing sources around the quasar can produce severe
distortions in bubble's shape. However, this does not severely affect parameter
estimation in the bubbles that are large.Comment: 18 pages, 16 figures, 3 tables. Minor change in text. Accepted for
publication in MNRA
Pulsar Scintillation and the Local Bubble
We present here the results from an extensive scintillation study of twenty
pulsars in the dispersion measure (DM) range 3 - 35 pc cm^-3 caried out using
the Ooty Radio Telescope (ORT) at 327 MHz, to investigate the distribution of
ionized material in the local interstellar medium. Observations were made
during the period January 1993 to August 1995, in which the dynamic
scintillation spectra of these pulsars were regularly monitored over 10 - 90
epochs spanning 100 days. Reliable and accurate estimates of strengths of
scattering have been deduced from the scintillation parameters averaged out for
their long-term fluctuations arising from refractive scintillation (RISS)
effects. Our analysis reveals several anomalies in the scattering strength,
which suggest tht the distribution of scattering material in the Solar
neighborhood is not uniform.
We have modelled these anomalous scattering effects in terms of
inhomogeneities in the distribution of electron dnsity fluctuations in the
local interstellar medium (LISM). Our model suggests the presence of a low
density bubble surrounded by a shell of much higher density fluctuations. We
are able to put constraints on geometrical and scattering properties of such a
structure, and find it to be morphologically similar to the Local Bubble known
from other studies.Comment: 35 pages, 12 figure
Nature of Sonoluminescence: Noble Gas Radiation Excited by Hot Electrons in "Cold" Water
We show that strong electric fields occurring in water near the surface of
collapsing gas bubbles because of the flexoelectric effect can provoke dynamic
electric breakdown in a micron-size region near the bubble and consider the
scenario of the SBSL. The scenario is: (i) at the last stage of incomplete
collapse of the bubble the gradient of pressure in water near the bubble
surface has such a value and sign that the electric field arising from the
flexoelectric effect exceeds the threshold field of the dynamic electrical
breakdown of water and is directed to the bubble center; (ii) mobile electrons
are generated because of thermal ionization of water molecules near the bubble
surface; (iii) these electrons are accelerated in ''cold'' water by the strong
electric fields; (iv) these hot electrons transfer noble gas atoms dissolved in
water to high-energy excited states and optical transitions between these
states produce SBSL UV flashes in the trasparency window of water; (v) the
breakdown can be repeated several times and the power and duration of the UV
flash are determined by the multiplicity of the breakdowns. The SBSL spectrum
is found to resemble a black-body spectrum where temperature is given by the
effective temperature of the hot electrons. The pulse energy and some other
characteristics of the SBSL are found to be in agreement with the experimental
data when realistic estimations are made.Comment: 11 pages (RevTex), 1 figure (.ps
First Observational Tests of Eternal Inflation: Analysis Methods and WMAP 7-Year Results
In the picture of eternal inflation, our observable universe resides inside a
single bubble nucleated from an inflating false vacuum. Many of the theories
giving rise to eternal inflation predict that we have causal access to
collisions with other bubble universes, providing an opportunity to confront
these theories with observation. We present the results from the first
observational search for the effects of bubble collisions, using cosmic
microwave background data from the WMAP satellite. Our search targets a generic
set of properties associated with a bubble collision spacetime, which we
describe in detail. We use a modular algorithm that is designed to avoid a
posteriori selection effects, automatically picking out the most promising
signals, performing a search for causal boundaries, and conducting a full
Bayesian parameter estimation and model selection analysis. We outline each
component of this algorithm, describing its response to simulated CMB skies
with and without bubble collisions. Comparing the results for simulated bubble
collisions to the results from an analysis of the WMAP 7-year data, we rule out
bubble collisions over a range of parameter space. Our model selection results
based on WMAP 7-year data do not warrant augmenting LCDM with bubble
collisions. Data from the Planck satellite can be used to more definitively
test the bubble collision hypothesis.Comment: Companion to arXiv:1012.1995. 41 pages, 23 figures. v2: replaced with
version accepted by PRD. Significant extensions to the Bayesian pipeline to
do the full-sky non-Gaussian source detection problem (previously restricted
to patches). Note that this has changed the normalization of evidence values
reported previously, as full-sky priors are now employed, but the conclusions
remain unchange
ROSAT and ASCA observations of the Crab-Like Supernova Remnant N157B in the Large Magellanic Cloud
We report the results of ROSAT and ASCA X-ray observations of the supernova
remnant N157B (or 30 Dor B, SNR 0539-69.1) in the Large Magellanic Cloud. For
comparison, we also briefly describe the results on SNR 0540-69.3, the only
confirmed Crab-like remnant in the Cloud. The X-ray emission from N157B can be
decomposed into a bright comet-shaped feature, superimposed on a diffuse
emission region of a dimension pc. The flat and nearly featureless
spectrum of the remnant is distinctly different from those of young shell-like
remnants, suggesting a predominantly Crab-like nature of N157B. Characterized
by a power law with an energy slope , the spectrum of N157B above
keV is, however, considerably steeper than that of SNR 0540-69.3,
which has a slope of . At lower energies, the spectrum of N157B
presents marginal evidence for emission lines, which if real most likely arise
in hot gas of the diffuse emission region. The hot gas has a characteristic
thermal temperature of 0.4-0.7 keV. No significant periodic signal is detected
from N157B in the period range of s. The pulsed
fraction is (99% confidence) in the keV range. We discuss
the nature of the individual X-ray components. In particular, we suggest that
the synchrotron radiation of relativistic particles from a fast-moving () pulsar explains the size, morphology, spectrum, and energetics
of the comet-shaped X-ray feature. We infer the age of the remnant as yrs. The lack of radio polarization of the remnant may be due to
Faraday dispersion by foreground \ion{H}{2} gas.Comment: To be published in The Astrophysical Journal, 21 pages, plus 11
images in the PS, GIF, or jpeg format. Postscript files of images are
available at http://www.astro.nwu.edu/astro/wqd/paper/n157b
Quantifying the non-Gaussianity in the EoR 21-cm signal through bispectrum
The epoch of reionization (EoR) 21-cm signal is expected to be highly
non-Gaussian in nature and this non-Gaussianity is also expected to evolve with
the progressing state of reionization. Therefore the signal will be correlated
between different Fourier modes (). The power spectrum will not be able
capture this correlation in the signal. We use a higher-order estimator -- the
bispectrum -- to quantify this evolving non-Gaussianity. We study the
bispectrum using an ensemble of simulated 21-cm signal and with a large variety
of triangles. We observe two competing sources driving the non-Gaussianity
in the signal: fluctuations in the neutral fraction () field and
fluctuations in the matter density field. We find that the non-Gaussian
contribution from these two sources vary, depending on the stage of
reionization and on which modes are being studied. We show that the sign of
the bispectrum works as a unique marker to identify which among these two
components is driving the non-Gaussianity. We propose that the sign change in
the bispectrum, when plotted as a function of triangle configuration
and at a certain stage of the EoR can be used as a confirmative
test for the detection of the 21-cm signal. We also propose a new consolidated
way to visualize the signal evolution (with evolving or
redshift), through the trajectories of the signal in a power spectrum and
equilateral bispectrum i.e. space.Comment: 18 pages, 11 figures. Accepted for publication in MNRAS. Replaced to
match the accepted versio
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