145 research outputs found
A Resistive Wideband Space Beam Splitter
We present the design, construction and measurements of the electromagnetic
performance of a wideband space beam splitter. The beam splitter is designed to
power divide the incident radiation into reflected and transmitted components
for interferometer measurement of spectral features in the mean cosmic radio
background. Analysis of a 2-element interferometer configuration with a
vertical beam splitter between a pair of antennas leads to the requirement that
the beam splitter be a resistive sheet with sheet resistance {\eta}o /2, where
{\eta}o is the impedance of free space. The transmission and reflection
properties of such a sheet is computed for normal and oblique incidences and
for orthogonal polarizations of the incident electric field. We have
constructed such an electromagnetic beam splitter as a square soldered grid of
resistors of value 180 Ohms (approximately {\eta}o /2) and a grid size of 0.1
m, and present measurements of the reflection and transmission coefficients
over a wide frequency range between 50 and 250 MHz in which the wavelength well
exceeds the mesh size. Our measurements of the coefficients for voltage
transmission and reflection agree to within 5% with physical optics modeling of
the wave propagation, which takes into account edge diffraction.Comment: 14 pages,17 figure
SARAS: a precision system for measurement of the Cosmic Radio Background and signatures from the Epoch of Reionization
SARAS is a correlation spectrometer purpose designed for precision
measurements of the cosmic radio background and faint features in the sky
spectrum at long wavelengths that arise from redshifted 21-cm from gas in the
reionization epoch. SARAS operates in the octave band 87.5-175 MHz. We present
herein the system design arguing for a complex correlation spectrometer
concept. The SARAS design concept provides a differential measurement between
the antenna temperature and that of an internal reference termination, with
measurements in switched system states allowing for cancellation of additive
contaminants from a large part of the signal flow path including the digital
spectrometer. A switched noise injection scheme provides absolute spectral
calibration. Additionally, we argue for an electrically small
frequency-independent antenna over an absorber ground. Various critical design
features that aid in avoidance of systematics and in providing calibration
products for the parametrization of other unavoidable systematics are described
and the rationale discussed. The signal flow and processing is analyzed and the
response to noise temperatures of the antenna, reference termination and
amplifiers is computed. Multi-path propagation arising from internal
reflections are considered in the analysis, which includes a harmonic series of
internal reflections. We opine that the SARAS design concept is advantageous
for precision measurement of the absolute cosmic radio background spectrum;
therefore, the design features and analysis methods presented here are expected
to serve as a basis for implementations tailored to measurements of a
multiplicity of features in the background sky at long wavelengths, which may
arise from events in the dark ages and subsequent reionization era.Comment: 49 pages, 17 figure
GMOSS: All-sky model of spectral radio brightness based on physical components and associated radiative processes
We present Global MOdel for the radio Sky Spectrum (GMOSS) -- a novel,
physically motivated model of the low-frequency radio sky from 22 MHz to 23
GHz. GMOSS invokes different physical components and associated radiative
processes to describe the sky spectrum over 3072 pixels of
resolution. The spectra are allowed to be convex, concave or of more complex
form with contributions from synchrotron emission, thermal emission and
free-free absorption included. Physical parameters that describe the model are
optimized to best fit four all-sky maps at 150 MHz, 408 MHz, 1420 MHz and 23
GHz and two maps at 22 MHz and 45 MHz generated using the Global Sky Model of
de Oliveira-Costa et al. (2008). The fractional deviation of model to data has
a median value of and is less than for of the pixels.
Though aimed at modeling of foregrounds for the global signal arising from the
redshifted 21-cm line of Hydrogen during Cosmic Dawn and Epoch of Reionization
(EoR) - over redshifts , GMOSS is well suited for any
application that requires simulating spectra of the low-frequency radio sky as
would be observed by the beam of any instrument. The complexity in spectral
structure that naturally arises from the underlying physics of the model
provides a useful expectation for departures from smoothness in EoR foreground
spectra and hence may guide the development of algorithms for EoR signal
detection. This aspect is further explored in a subsequent paper.Comment: 19 pages, 7 figure
All-sky signals from recombination to reionization with the SKA
Cosmic evolution in the hydrogen content of the Universe through
recombination and up to the end of reionization is expected to be revealed as
subtle spectral features in the uniform extragalactic cosmic radio background.
The redshift evolution in the excitation temperature of the 21-cm spin flip
transition of neutral hydrogen appears as redshifted emission and absorption
against the cosmic microwave background. The precise signature of the spectral
trace from cosmic dawn and the epoch of reionization are dependent on the
spectral radiance, abundance and distribution of the first bound systems of
stars and early galaxies, which govern the evolution in the spin-flip level
populations. Redshifted 21 cm from these epochs when the spin temperature
deviates from the temperature of the ambient relic cosmic microwave background
results in an all-sky spectral structure in the 40-200 MHz range, almost wholly
within the band of SKA-Low. Another spectral structure from gas evolution is
redshifted recombination lines from epoch of recombination of hydrogen and
helium; the weak all-sky spectral structure arising from this event is best
detected at the upper end of the 350-3050 MHz band of SKA-mid. Total power
spectra of SKA interferometer elements form the measurement set for these faint
signals from recombination and reionization; the inter-element interferometer
visibilities form a calibration set. The challenge is in precision polarimetric
calibration of the element spectral response and solving for additives and
unwanted confusing leakages of sky angular structure modes into spectral modes.
Herein we discuss observing methods and design requirements that make possible
these all-sky SKA measurements of the cosmic evolution of hydrogen.Comment: Accepted for publication in the SKA Science Book 'Advancing
Astrophysics with the Square Kilometre Array', to appear in 201
On the detection of spectral ripples from the Recombination Epoch
Photons emitted during the epochs of Hydrogen () and Helium recombination ( for HeII
HeI, for HeIII
HeII) are predicted to appear as broad, weak spectral distortions of the Cosmic
Microwave Background. We present a feasibility study for a ground-based
experimental detection of these recombination lines, which would provide an
observational constraint on the thermal ionization history of the Universe,
uniquely probing astrophysical cosmology beyond the last scattering surface. We
find that an octave band in the 2--6 GHz window is optimal for such an
experiment, both maximizing signal-to-noise ratio and including sufficient line
spectral structure. At these frequencies the predicted signal appears as an
additive quasi-sinusoidal component with amplitude about nK that is
embedded in a sky spectrum some nine orders of magnitude brighter. We discuss
an algorithm to detect these tiny spectral fluctuations in the sky spectrum by
foreground modeling. We introduce a \textit{Maximally Smooth} function capable
of describing the foreground spectrum and distinguishing the signal of
interest. With Bayesian statistical tests and mock data we estimate that a
detection of the predicted distortions is possible with 90\% confidence by
observing for 255 days with an array of 128 radiometers using cryogenically
cooled state-of-the-art receivers. We conclude that detection is in principle
feasible in realistic observing times; we propose APSERa---Array of Precision
Spectrometers for the Epoch of Recombination---a dedicated radio telescope to
detect these recombination lines.Comment: 33 pages, 16 figures, submitted to ApJ, comments welcom
Radio recombination lines from the largest bound atoms in space
In this paper, we report the detection of a series of radio recombination
lines (RRLs) in absorption near 26 MHz arising from the largest bound carbon
atoms detected in space. These atoms, which are more than a million times
larger than the ground state atoms are undergoing delta transitions (n~1009,
Delta n=4) in the cool tenuous medium located in the Perseus arm in front of
the supernova remnant, Cassiopeia A. Theoretical estimates had shown that atoms
which recombined in tenuous media are stable up to quantum levels n~1500. Our
data indicates that we have detected radiation from atoms in states very close
to this theoretical limit. We also report high signal-to-noise detections of
alpha, beta and gamma transitions in carbon atoms arising in the same clouds.
In these data, we find that the increase in line widths with quantum number
(proportional to n^5) due to pressure and radiation broadening of lines is much
gentler than expected from existing models which assume a power law background
radiation field. This discrepancy had also been noted earlier. The model line
widths had been overestimated since the turnover in radiation field of
Cassiopeia A at low frequencies had been ignored. In this paper, we show that,
once the spectral turnover is included in the modeling, the slower increase in
line width with quantum number is naturally explained.Comment: 5 pages, 4 figures, accepted for publication in MNRA
Homography-Based Correction of Positional Errors in MRT Survey
The Mauritius Radio Telescope (MRT) images show systematics in the positional
errors of sources when compared to source positions in the Molonglo Reference
Catalogue (MRC). We have applied two-dimensional homography to correct
positional errors in the image domain and avoid re-processing the visibility
data. Positions of bright (above 15-) sources, common to MRT and MRC
catalogues, are used to set up an over-determined system to solve for the 2-D
homography matrix. After correction, the errors are found to be within 10% of
the beamwidth for these bright sources and the systematics are eliminated from
the images.Comment: 4 pages, 4 figures, The Low-Frequency Radio Universe, Proceedings of
a conference held at NCRA-TIFR, Pune, 8-12 December 2008, ASP Conference
Series, Vol. 407, 2009, Eds: D.J. Saikia, D.A. Green, Y. Gupta and T. Ventur
On the detection of global 21-cm signal from reionization using interferometers
Detection of the global redshifted 21-cm signal is an excellent means of deciphering the physical processes during the Dark Ages and subsequent Epoch of Reionization (EoR). However, detection of this faint monopole is challenging due to the high precision required in instrumental calibration and modeling of substantially brighter foregrounds and instrumental systematics. In particular, modeling of receiver noise with mK accuracy and its separation remains a formidable task in experiments aiming to detect the global signal using single-element spectral radiometers. Interferometers do not respond to receiver noise; therefore, here we explore the theory of the response of interferometers to global signals. In other words, we discuss the spatial coherence in the electric field arising from the monopole component of the 21-cm signal and methods for its detection using sensor arrays. We proceed by first deriving the response to uniform sky of two-element interferometers made of unit dipole and resonant loop antennas, then extend the analysis to interferometers made of one-dimensional arrays and also consider two-dimensional aperture antennas. Finally, we describe methods by which the coherence might be enhanced so that the interferometer measurements yield improved sensitivity to the monopole component. We conclude (a) that it is indeed possible to measure the global 21-cm from EoR using interferometers, (b) that a practically useful configuration is with omnidirectional antennas as interferometer elements, and (c) that the spatial coherence may be enhanced using, for example, a space beam splitter between the interferometer elements
SARAS 2: A Spectral Radiometer for probing Cosmic Dawn and the Epoch of Reionization through detection of the global 21 cm signal
The global 21 cm signal from Cosmic Dawn (CD) and the Epoch of Reionization
(EoR), at redshifts , probes the nature of first sources of
radiation as well as physics of the Inter-Galactic Medium (IGM). Given that the
signal is predicted to be extremely weak, of wide fractional bandwidth, and
lies in a frequency range that is dominated by Galactic and Extragalactic
foregrounds as well as Radio Frequency Interference, detection of the signal is
a daunting task. Critical to the experiment is the manner in which the sky
signal is represented through the instrument. It is of utmost importance to
design a system whose spectral bandpass and additive spurious can be well
calibrated and any calibration residual does not mimic the signal. SARAS is an
ongoing experiment that aims to detect the global 21 cm signal. Here we present
the design philosophy of the SARAS 2 system and discuss its performance and
limitations based on laboratory and field measurements. Laboratory tests with
the antenna replaced with a variety of terminations, including a network model
for the antenna impedance, show that the gain calibration and modeling of
internal additives leave no residuals with Fourier amplitudes exceeding 2~mK,
or residual Gaussians of 25 MHz width with amplitudes exceeding 2~mK. Thus,
even accounting for reflection and radiation efficiency losses in the antenna,
the SARAS~2 system is capable of detection of complex 21-cm profiles at the
level predicted by currently favoured models for thermal baryon evolution.Comment: 44 pages, 17 figures; comments and suggestions are welcom
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