199 research outputs found
Visibility based angular power spectrum estimation in low frequency radio interferometric observations
We present two estimators to quantify the angular power spectrum of the sky
signal directly from the visibilities measured in radio interferometric
observations. This is relevant for both the foregrounds and the cosmological
21-cm signal buried therein. The discussion here is restricted to the Galactic
synchrotron radiation, the most dominant foreground component after point
source removal. Our theoretical analysis is validated using simulations at 150
MHz, mainly for GMRT and also briefly for LOFAR. The Bare Estimator uses
pairwise correlations of the measured visibilities, while the Tapered Gridded
Estimator uses the visibilities after gridding in the uv plane. The former is
very precise, but computationally expensive for large data. The latter has a
lower precision, but takes less computation time which is proportional to the
data volume. The latter also allows tapering of the sky response leading to
sidelobe suppression, an useful ingredient for foreground removal. Both
estimators avoid the positive bias that arises due to the system noise. We
consider amplitude and phase errors of the gain, and the w-term as possible
sources of errors . We find that the estimated angular power spectrum is
exponentially sensitive to the variance of the phase errors but insensitive to
amplitude errors. The statistical uncertainties of the estimators are affected
by both amplitude and phase errors. The w-term does not have a significant
effect at the angular scales of our interest. We propose the Tapered Gridded
Estimator as an effective tool to observationally quantify both foregrounds and
the cosmological 21-cm signal.Comment: 20 pages, 15 figures, 1 table.One typo corrected in Fig.13. Accepted
for publication in MNRA
The prospects of measuring the angular power spectrum of the diffuse Galactic synchrotron emission with SKA1 Low
The diffuse Galactic synchrotron emission (DGSE) is the most important
diffuse foreground component for future cosmological 21-cm observations. The
DGSE is also an important probe of the cosmic ray electron and magnetic field
distributions in the turbulent interstellar medium (ISM) of our Galaxy. In this
paper we briefly review the Tapered Gridded Estimator (TGE) which can be used
to quantify the angular power spectrum of the sky signal directly from the
visibilities measured in radio-interferometric observations. The salient
features of the TGE are (1.) it deals with the gridded data which makes it
computationally very fast (2.) it avoids a positive noise bias which normally
arises from the system noise inherent to the visibility data, and (3.) it
allows us to taper the sky response and thereby suppresses the contribution
from unsubtracted point sources in the outer parts and the sidelobes of the
antenna beam pattern. We also summarize earlier work where the TGE was used to
measure the C_l of the DGSE using 150 MHz GMRT data. Earlier measurements of
the angular power spectrum are restricted to smaller angular multipole l ~ 10^3
for the DGSE, the signal at the larger l values is dominated by the residual
point sources after source subtraction. The higher sensitivity of the upcoming
SKA1 Low will allow the point sources to be subtracted to a fainter level than
possible with existing telescopes. We predict that it will be possible to
measure the angular power spectrum of the DGSE to larger values of l with SKA1
Low. Our results show that it should be possible to achieve l_{max} ~ 10^4 and
~ 10^5 with 2 minutes and 10 hours of observations respectively.Comment: 10 pages, 3 figures; Accepted for publication in Journal of
Astrophysics and Astronomy (JOAA) special issue on "Science with the SKA: an
Indian perspective
The visibility based Tapered Gridded Estimator (TGE) for the redshifted 21-cm power spectrum
We present the improved visibility based Tapered Gridded Estimator (TGE) for
the power spectrum of the diffuse sky signal. The visibilities are gridded to
reduce the computation, and tapered through a convolution to suppress the
contribution from the outer regions of the telescope's field of view. The TGE
also internally estimates the noise bias, and subtracts this out to give an
unbiased estimate of the power spectrum. An earlier version of the 2D TGE for
the angular power spectrum is improved and then extended to obtain
the 3D TGE for the power spectrum of the 21-cm brightness
temperature fluctuations. Analytic formulas are also presented for predicting
the variance of the binned power spectrum. The estimator and its variance
predictions are validated using simulations of GMRT
observations. We find that the estimator accurately recovers the input model
for the 1D Spherical Power Spectrum and the 2D Cylindrical Power
Spectrum , and the predicted variance is also in
reasonably good agreement with the simulations.Comment: 19 pages, 13 figures. Accepted for publication in MNRAS. The
definitive version will be available at http://mnrasl.oxfordjournals.org
Validating a novel angular power spectrum estimator using simulated low frequency radio-interferometric data
The "Tapered Gridded Estimator" (TGE) is a novel way to directly estimate the
angular power spectrum from radio-interferometric visibility data that reduces
the computation by efficiently gridding the data, consistently removes the
noise bias, and suppresses the foreground contamination to a large extent by
tapering the primary beam response through an appropriate convolution in the
visibility domain. Here we demonstrate the effectiveness of TGE in recovering
the diffuse emission power spectrum through numerical simulations. We present
details of the simulation used to generate low frequency visibility data for
sky model with extragalactic compact radio sources and diffuse Galactic
synchrotron emission. We then use different imaging strategies to identify the
most effective option of point source subtraction and to study the underlying
diffuse emission. Finally, we apply TGE to the residual data to measure the
angular power spectrum, and assess the impact of incomplete point source
subtraction in recovering the input power spectrum of the
synchrotron emission. This estimator is found to successfully recovers the
of input model from the residual visibility data. These results are
relevant for measuring the diffuse emission like the Galactic synchrotron
emission. It is also an important step towards characterizing and removing both
diffuse and compact foreground emission in order to detect the redshifted signal from the Epoch of Reionization.Comment: 18 pages, 1 table, 9 figures, Accepted for publication in New
Astronom
Centrosymmetric-noncentrosymmetric Structural Phase Transition in Quasi one-dimensional compound, (TaSe)I
(TaSe)I, a compound belonging to the family of quasi-one-dimensional
transition-metal tetrachalcogenides, has drawn significant attention due to a
recent report on possible coexistence of two antagonistic phenomena,
superconductivity and magnetism below 2.5~K (Bera et. al, arXiv:2111.14525).
Here, we report a structural phase transition of the trimerized phase at
temperature, ~145~K using Raman scattering, specific heat, and
electrical transport measurements. The temperature-dependent single-crystal
X-ray diffraction experiments establish the phase transition from a
high-temperature centrosymmetric to a low-temperature non-centrosymmetric
structure, belonging to the same tetragonal crystal family. The first-principle
calculation finds the aforementioned inversion symmetry-breaking structural
transition to be driven by the hybridization energy gain due to the off-centric
movement of the Ta atoms, which wins over the elastic energy loss.Comment: 11 pages, 5 figures, Under review as a regular articl
Towards 21-cm Intensity Mapping at with uGMRT using the Tapered Gridded Estimator I: Foreground Avoidance
The post-reionization neutral hydrogen (HI) 21-cm intensity
mapping signal holds the potential to probe the large scale structures, study
the expansion history and constrain various cosmological parameters. Here we
apply the Tapered Gridded Estimator (TGE) to estimate
the power spectrum of the redshifted 21-cm signal using a sub-band drawn
from uGMRT Band 3 observations of European Large-Area ISO Survey-North 1
(ELAIS-N1). The TGE allows us to taper the sky response which suppresses the
foreground contribution from sources in the periphery of the telescope's field
of view. We apply the TGE on the measured visibility data to estimate the
multi-frequency angular power spectrum (MAPS) from which
we determine using maximum-likelihood which
naturally overcomes the issue of missing frequency channels (55 \% here). The
entire methodology is validated using simulations. For the data, using the
foreground avoidance technique, we obtain a upper limit of
for the 21-cm brightness
temperature fluctuation at . This corresponds
to , where and respectively denote the cosmic \HI mass density and the \HI bias
parameter. A previous work has analyzed of the same data at
, and reported and
at . The upper
limits presented here are still orders of magnitude larger than the expected
signal corresponding to and .Comment: 13 pages, 11 figures, accepted for publication in MNRA
Towards -cm intensity mapping at with uGMRT using the tapered gridded estimator III: Foreground removal
Neutral hydrogen (\ion{H}{i}) -cm intensity mapping (IM) is a promising
probe of the large-scale structures in the Universe. However, a few orders of
magnitude brighter foregrounds obscure the IM signal. Here we use the Tapered
Gridded Estimator (TGE) to estimate the multi-frequency angular power spectrum
(MAPS) from a bandwidth uGMRT Band
data at . In foregrounds remain
correlated across the entire range, whereas the -cm signal is
localized within (typically ).
Assuming the range to have minimal -cm signal, we
use in this range to model the foregrounds. This
foreground model is extrapolated to , and subtracted
from the measured . The residual
in the range is
used to constrain the -cm signal, compensating for the signal loss from
foreground subtraction. is found to be
noise-dominated without any trace of foregrounds. Using
we constrain the -cm brightness
temperature fluctuations , and obtain the upper limit
at . We
further obtain the upper limit
[\Omega_{\ion{H}{i}}b_{\ion{H}{i}}]_{\rm UL}\leq0.022 where
\Omega_{\ion{H}{i}} and b_{\ion{H}{i}} are the comoving \ion{H}{i} density
and bias parameters respectively. Although the upper limit is nearly times
larger than the expected -cm signal, it is times tighter over previous
works using foreground avoidance on the same data.Comment: Accepted for publication in MNRAS. 16 pages (including Appendix), 8
figures (plus 8 in Appendix), 5 Table
Defining the phenotypes of sickle cell disease.
The sickle cell gene is pleiotropic in nature. Although it is a single gene mutation, it has multiple phenotypic expressions that constitute the complications of sickle cell disease. The frequency and severity of these complications vary considerably both latitudinally in patients and longitudinally in the same patient over time. Thus, complications that occur in childhood may disappear, persist or get worse with age. Dactylitis and stroke, for example, occur mostly in childhood, whereas leg ulcers and renal failure typically occur in adults. It is essential that the phenotypic manifestations of sickle cell disease be defined accurately so that communication among providers and researchers facilitates the implementation of appropriate and cost-effective diagnostic and therapeutic modalities. The aim of this review is to define the complications that are specific to sickle cell disease based on available evidence in the literature and the experience of hematologists in this field
All-sky angular power spectrum – I. Estimating brightness temperature fluctuations using the 150-MHz TGSS survey
Measurements of the Galactic synchrotron emission are important for the 21-cm studies of the epoch of reionization. The study of synchrotron emission is also useful for quantifying the fluctuations in the magnetic field and the cosmic-ray electron density of the turbulent interstellar medium (ISM) of our Galaxy. Here, we present the all-sky angular power spectrum (Cℓ) measurements of the diffuse synchrotron emission obtained using the TIFR GMRT Sky Survey (TGSS) at 150 MHz
Identification of Serotype in Culture Negative Pneumococcal Meningitis Using Sequential Multiplex PCR: Implication for Surveillance and Vaccine Design
BACKGROUND: PCR-based serotyping of Streptococcus pneumoniae has been proposed as a simpler approach than conventional methods, but has not been applied to strains in Asia where serotypes are diverse and different from other part of the world. Furthermore, PCR has not been used to determine serotype distribution in culture-negative meningitis cases. METHODOLOGY: Thirty six serotype-specific primers, 7 newly designed and 29 previously published, were arranged in 7 multiplex PCR sets, each in new hierarchies designed for overall serotype distribution in Bangladesh, and specifically for meningitis and non-meningitis isolates. Culture-negative CSF specimens were then tested directly for serotype-specific sequences using the meningitis-specific set of primers. PCR-based serotyping of 367 strains of 56 known serotypes showed 100% concordance with quellung reaction test. The first 7 multiplex reactions revealed the serotype of 40% of all, and 31% and 48% non-meningitis and meningitis isolates, respectively. By redesigning the multiplex scheme specifically for non-meningitis or meningitis, the quellung reaction of 43% and 48% of respective isolates could be identified. Direct examination of 127 culture-negative CSF specimens, using the meningitis-specific set of primers, yielded serotype for 51 additional cases. CONCLUSIONS: This PCR approach, could improve ascertainment of pneumococcal serotype distributions, especially for meningitis in settings with high prior use of antibiotics
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