7 research outputs found
The impact of non-Gaussianity on the error covariance for observations of the Epoch of Reionization 21-cm power spectrum
For abstract see published article
Improving constraints on the reionization parameters using 21-cm bispectrum
Radio interferometric experiments aim to constrain the reionization model
parameters by measuring the 21-cm signal statistics, primarily the power
spectrum. However the Epoch of Reionization (EoR) 21-cm signal is highly
non-Gaussian, and this non-Gaussianity encodes important information about this
era. The bispectrum is the lowest order statistic able to capture this inherent
non-Gaussianity. Here we are the first to demonstrate that bispectra for large
and intermediate length scales and for all unique -triangle shapes provide
tighter constraints on the EoR parameters compared to the power spectrum or the
bispectra for a limited number of shapes of -triangles. We use the Bayesian
inference technique to constrain EoR parameters. We have also developed an
Artificial Neural Network (ANN) based emulator for the EoR 21-cm power spectrum
and bispectrum which we use to remarkably speed up our parameter inference
pipeline. Here we have considered the sample variance and the system noise
uncertainties corresponding to hrs of SKA-Low observations for
estimating errors in the signal statistics. We find that using all unique
-triangle bispectra improves the constraints on parameters by a factor of
(depending on the stage of reionization) over the constraints that are
obtained using power spectrum alone.Comment: 27 pages, 9 figures, Accepted for publication in JCA
The monopole and quadrupole moments of the Epoch of Reionization (EoR) 21-cm bispectrum
We study the monopole () and quadrupole () moments
of the 21-cm bispectrum (BS) from EoR simulations and present results for
squeezed and stretched triangles. Both and are
positive at the early stage of EoR where the mean neutral hydrogen (HI) density
fraction . The subsequent evolution of
and at large and intermediate scales and
respectively) is punctuated by two sign changes which
mark transitions in the HI distribution. The first sign flip where
becomes negative occurs in the intermediate stages of EoR
, at large scale first followed by the intermediate
scale. This marks the emergence of distinct ionized bubbles in the neutral
background. is relatively less affected by this transition, and
it mostly remains positive even when becomes negative. The second
sign flip, which affects both and , occurs at the
late stage of EoR . This marks a transition in the
topology of the HI distribution, after which we have distinct HI islands in an
ionized background. This causes to become positive. The negative
is a definite indication that the HI islands survive only in
under-dense regions.Comment: Accepted for publication in MNRA
Detecting galaxies in a large H{\sc i}~spectral cube
The upcoming Square Kilometer Array (SKA) is expected to produce humongous
amount of data for undertaking H{\sc i}~science. We have developed an MPI-based
{\sc Python} pipeline to deal with the large data efficiently with the present
computational resources. Our pipeline divides such large H{\sc i}~21-cm
spectral cubes into several small cubelets, and then processes them in parallel
using publicly available H{\sc i}~source finder {\sc SoFiA-}. The pipeline
also takes care of sources at the boundaries of the cubelets and also filters
out false and redundant detections. By comapring with the true source catalog,
we find that the detection efficiency depends on the {\sc SoFiA-} parameters
such as the smoothing kernel size, linking length and threshold values. We find
the optimal kernel size for all flux bins to be between to pixels and
to pixels, respectively in the spatial and frequency directions.
Comparing the recovered source parameters with the original values, we find
that the output of {\sc SoFiA-} is highly dependent on kernel sizes and a
single choice of kernel is not sufficient for all types of H{\sc i}~galaxies.
We also propose use of alternative methods to {\sc SoFiA-} which can be used
in our pipeline to find sources more robustly.Comment: 15 pages, 7 figures, Accepted for publication in the Special Issue of
Journal of Astrophysics and Astronomy on "Indian Participation in the SKA'',
comments are welcom
Studying the Multi-frequency Angular Power Spectrum of the Cosmic Dawn 21-cm Signal
International audienceThe light-cone (LC) anisotropy arises due to cosmic evolution of the cosmic dawn 21-cm signal along the line-of-sight (LoS) axis of the observation volume. The LC effect makes the signal statistically non-ergodic along the LoS axis. The multi-frequency angular power spectrum (MAPS) provides an unbiased alternative to the popular 3D power spectrum as it does not assume statistical ergodicity along every direction in the signal volume. Unlike the 3D power spectrum, MAPS captures the cosmological evolution of the intergalactic medium. Here we first study the impact of different underlying physical processes during cosmic dawn on the behaviour of the 21-cm MAPS using simulations of various different scenarios and models. We also make error predictions in 21-cm MAPS measurements considering only the system noise and cosmic variance for mock observations of HERA, NenuFAR and SKA-Low. We find that of HERA observations will be able to measure 21-cm MAPS at for with channel-width. The better sensitivity of SKA-Low allows reaching this sensitivity up to . Considering NenuFAR, measurements are possible only for with channel-width and for a ten times longer observation time of . However, for the range and more than measurements are still possible for NenuFAR when combining consecutive frequency channels within a band
Predictions for measuring the 21-cm multifrequency angular power spectrum using SKA-Low
The light-cone (LC) effect causes the mean as well as the statistical
properties of the redshifted 21-cm signal to
change with frequency (or cosmic time). Consequently, the statistical
homogeneity (ergodicity) of the signal along the line of sight (LoS) direction
is broken. This is a severe problem particularly during the Epoch of
Reionization (EoR) when the mean neutral hydrogen fraction ()
changes rapidly as the universe evolves. This will also pose complications for
large bandwidth observations. These effects imply that the 3D power spectrum
fails to quantify the entire second-order statistics of the signal as it
assumes the signal to be ergodic and periodic along the LoS. As a proper
alternative to , we use the multi-frequency angular power spectrum (MAPS)
which does not assume the signal to be
ergodic and periodic along the LoS. Here, we study the prospects for measuring
the EoR 21-cm MAPS using future observations with the upcoming SKA-Low.
Ignoring any contribution from the foregrounds, we find that the EoR 21-cm MAPS
can be measured at a confidence level at angular scales for total observation time across
observational bandwidth. We also quantitatively address
the effects of foregrounds on MAPS detectability forecast by avoiding signal
contained within the foreground wedge in plane. These
results are very relevant for the upcoming large bandwidth EoR experiments as
previous predictions were all restricted to individually analyzing the signal
over small frequency (or equivalently redshift) intervals.Comment: Published in Monthly Notices of the Royal Astronomical Society
(MNRAS). Available at https://doi.org/10.1093/mnras/staa102