scholarly journals Improved upper limits on the 21 cm signal power spectrum of neutral hydrogen at z ≈ 9.1 from LOFAR

2020 ◽  
Vol 493 (2) ◽  
pp. 1662-1685 ◽  
Author(s):  
F G Mertens ◽  
M Mevius ◽  
L V E Koopmans ◽  
A R Offringa ◽  
G Mellema ◽  
...  
Keyword(s):  
Power Spectrum ◽  
Low Frequency ◽  
Neutral Hydrogen ◽  
Gaussian Process Regression ◽  
Spectral Structure ◽  
Signal Power ◽  
Spectral Correlation ◽  
Upper Limit ◽  
Upper Limits ◽  
Excess Power

ABSTRACT A new upper limit on the 21 cm signal power spectrum at a redshift of z ≈ 9.1 is presented, based on 141 h of data obtained with the Low-Frequency Array (LOFAR). The analysis includes significant improvements in spectrally smooth gain-calibration, Gaussian Process Regression (GPR) foreground mitigation and optimally weighted power spectrum inference. Previously seen ‘excess power’ due to spectral structure in the gain solutions has markedly reduced but some excess power still remains with a spectral correlation distinct from thermal noise. This excess has a spectral coherence scale of 0.25–0.45 MHz and is partially correlated between nights, especially in the foreground wedge region. The correlation is stronger between nights covering similar local sidereal times. A best 2-σ upper limit of $\Delta ^2_{21} \lt (73)^2\, \mathrm{mK^2}$ at $k = 0.075\, \mathrm{h\, cMpc^{-1}}$ is found, an improvement by a factor ≈8 in power compared to the previously reported upper limit. The remaining excess power could be due to residual foreground emission from sources or diffuse emission far away from the phase centre, polarization leakage, chromatic calibration errors, ionosphere, or low-level radiofrequency interference. We discuss future improvements to the signal processing chain that can further reduce or even eliminate these causes of excess power.

scholarly journals LOFAR 144-MHz follow-up observations of GW170817

2020 ◽  
Vol 494 (4) ◽  
pp. 5110-5117
Author(s):  
J W Broderick ◽  
T W Shimwell ◽  
K Gourdji ◽  
A Rowlinson ◽  
S Nissanke ◽  
...  
Keyword(s):  
Neutron Star ◽  
Low Frequency ◽  
Radio Spectrum ◽  
Central Frequency ◽  
Binary Neutron Star ◽  
Upper Limit ◽  
Upper Limits ◽  
Maximum Elevation ◽  
Merger Event

ABSTRACT We present low-radio-frequency follow-up observations of AT 2017gfo, the electromagnetic counterpart of GW170817, which was the first binary neutron star merger to be detected by Advanced LIGO–Virgo. These data, with a central frequency of 144 MHz, were obtained with LOFAR, the Low-Frequency Array. The maximum elevation of the target is just 13${_{.}^{\circ}}$7 when observed with LOFAR, making our observations particularly challenging to calibrate and significantly limiting the achievable sensitivity. On time-scales of 130–138 and 371–374 d after the merger event, we obtain 3σ upper limits for the afterglow component of 6.6 and 19.5 mJy beam−1, respectively. Using our best upper limit and previously published, contemporaneous higher frequency radio data, we place a limit on any potential steepening of the radio spectrum between 610 and 144 MHz: the two-point spectral index $\alpha ^{610}_{144} \gtrsim$ −2.5. We also show that LOFAR can detect the afterglows of future binary neutron star merger events occurring at more favourable elevations.

scholarly journals The first power spectrum limit on the 21-cm signal of neutral hydrogen during the Cosmic Dawn at z = 20–25 from LOFAR

2019 ◽  
Vol 488 (3) ◽  
pp. 4271-4287 ◽  
Author(s):  
B K Gehlot ◽  
F G Mertens ◽  
L V E Koopmans ◽  
M A Brentjens ◽  
S Zaroubi ◽  
...  
Keyword(s):  
Power Spectrum ◽  
Neutral Hydrogen ◽  
Hyperfine Line ◽  
The North ◽  
Upper Limits ◽  
Gain Errors ◽  
The Universe ◽  
Early Phases ◽  
Additional Variance ◽  
Spectrum Limit

ABSTRACT Observations of the redshifted 21-cm hyperfine line of neutral hydrogen from early phases of the Universe such as Cosmic Dawn and the Epoch of Reionization promise to open a new window onto the early formation of stars and galaxies. We present the first upper limits on the power spectrum of redshifted 21-cm brightness temperature fluctuations in the redshift range z = 19.8–25.2 (54–68 MHz frequency range) using 14 h of data obtained with the LOFAR-Low Band Antenna (LBA) array. We also demonstrate the application of a multiple pointing calibration technique to calibrate the LOFAR-LBA dual-pointing observations centred on the North Celestial Pole and the radio galaxy 3C220.3. We observe an unexplained excess of $\sim 30\!-\!50{{\ \rm per\ cent}}$ in Stokes / noise compared to Stokes V for the two observed fields, which decorrelates on ≳12 s and might have a physical origin. We show that enforcing smoothness of gain errors along frequency direction during calibration reduces the additional variance in Stokes I compared Stokes V introduced by the calibration on sub-band level. After subtraction of smooth foregrounds, we achieve a 2σ upper limit on the 21-cm power spectrum of $\Delta _{21}^2 \lt (14561\, \text{mK})^2$ at $k\sim 0.038\, h\, \text{cMpc}^{-1}$ and $\Delta _{21}^2 \lt (14886\, \text{mK})^2$ at $k\sim 0.038 \, h\, \text{cMpc}^{-1}$ for the 3C220 and NCP fields respectively and both upper limits are consistent with each other. The upper limits for the two fields are still dominated by systematics on most k modes.

scholarly journals Gaussian process foreground subtraction and power spectrum estimation for 21 cm cosmology

2020 ◽  
Vol 501 (1) ◽  
pp. 1463-1480
Author(s):  
Nicholas S Kern ◽  
Adrian Liu
Keyword(s):  
Power Spectrum ◽  
Gaussian Process ◽  
Signal To Noise Ratio ◽  
Low Frequency ◽  
Gaussian Process Regression ◽  
Spectrum Estimation ◽  
Signal Loss ◽  
Intensity Mapping ◽  
Quadratic Estimator ◽  
Window Functions

ABSTRACT One of the primary challenges in enabling the scientific potential of 21 cm intensity mapping at the epoch of reionization (EoR) is the separation of astrophysical foreground contamination. Recent works have claimed that Gaussian process regression (GPR) can robustly perform this separation, particularly at low Fourier k wavenumbers where the EoR signal reaches its peak signal-to-noise ratio. We revisit this topic by casting GPR foreground subtraction (GPR-FS) into the quadratic estimator formalism, thereby putting its statistical properties on stronger theoretical footing. We find that GPR-FS can distort the window functions at these low k modes, which, without proper decorrelation, make it difficult to probe the EoR power spectrum. Incidentally, we also show that GPR-FS is in fact closely related to the widely studied inverse covariance weighting of the optimal quadratic estimator. As a case study, we look at recent power spectrum upper limits from the Low-Frequency Array (LOFAR) that utilized GPR-FS. We pay close attention to their normalization scheme, showing that it is particularly sensitive to signal loss when the EoR covariance is misestimated. This has possible ramifications for recent astrophysical interpretations of the LOFAR limits, because many of the EoR models ruled out do not fall within the bounds of the covariance models explored by LOFAR. Being more robust to this bias, we conclude that the quadratic estimator is a more natural framework for implementing GPR-FS and computing the 21 cm power spectrum.

scholarly journals Search for Primordial Perturbations of the Universe: Observations with Ratan-600 Radio Telescope

1978 ◽  
Vol 79 ◽  
pp. 315-316
Author(s):  
Y. N. Parijskij
Keyword(s):  
Radio Telescope ◽  
Background Radiation ◽  
Low Frequency ◽  
Coma Cluster ◽  
Microwave Background ◽  
Atmospheric Noise ◽  
Upper Limit ◽  
Upper Limits ◽  
Microwave Background Radiation ◽  
The Universe

All kinds of primeval perturbations of the Universe should result in fluctuations of the microwave background radio emission. Here we report our latest upper limits to these fluctuations on scales 5′ to 3°. Using the new 600-m Soviet Radio Telescope we obtained a mean temperature profile of the region from 08h to 15h in R.A., centred at the declination of the Coma Cluster. 20 good records of this region were used in the final reduction of the data. After “normalization” of these data by filtering out low-frequency atmospheric noise and “bursts” which exceed the 4σ level we calculated an upper limit to the fluctuations of the microwave background radiation.

scholarly journals Detailed study of ELAIS N1 field with the uGMRT – II. Source properties and spectral variation of foreground power spectrum from 300–500 MHz observations

2019 ◽  
Vol 490 (1) ◽  
pp. 243-259 ◽  
Author(s):  
Arnab Chakraborty ◽  
Nirupam Roy ◽  
Abhirup Datta ◽  
Samir Choudhuri ◽  
Kanan K Datta ◽  
...  
Keyword(s):  
Power Spectrum ◽  
Spectral Characteristics ◽  
Low Frequency ◽  
Neutral Hydrogen ◽  
Galactic Nuclei ◽  
Detailed Comparison ◽  
Spectral Variation ◽  
Angular Power Spectrum ◽  
Star Forming ◽  
First Time

ABSTRACT Understanding the low-frequency radio sky in depth is necessary to subtract foregrounds in order to detect the redshifted 21 cm signal of neutral hydrogen from the cosmic dawn, the epoch of reionization and the post-reionization era. In this second paper of the series, we present the upgraded Giant Metrewave Radio Telescope (uGMRT) observation of the ELAIS N1 field made at 300–500 MHz. The image covers an area of ∼1.8 deg2 and has a central background rms noise of ∼ 15 μJy beam−1. We present a radio source catalogue containing 2528 sources (with flux densities > 100 μJy) and normalized source counts derived from that. A detailed comparison of detected sources with previous radio observations is shown. We discuss flux-scale accuracy, positional offsets, spectral index distribution and correction factors in source counts. The normalized source counts are in agreement with previous observations of the same field, as well as model source counts from the Square Kilometre Array Design Study simulation. It shows a flattening below ∼1 mJy that corresponds to a rise in populations of star-forming galaxies and radio-quiet active galactic nuclei. For the first time, we estimate the spectral characteristics of the angular power spectrum or multi-frequency angular power spectrum of diffuse Galactic synchrotron emission over a wide frequency bandwidth of 300–500 MHz from radio interferometric observations. This work demonstrates the improved capabilities of the uGMRT.

scholarly journals Gaussian Process Regression for foreground removal in HI intensity mapping experiments

2021 ◽  
Author(s):  
Paula S Soares ◽  
Catherine A Watkinson ◽  
Steven Cunnington ◽  
Alkistis Pourtsidou
Keyword(s):  
Power Spectrum ◽  
Gaussian Process ◽  
Low Frequency ◽  
Gaussian Process Regression ◽  
Principal Component ◽  
Frequency Range ◽  
Intensity Mapping ◽  
Removal Technique ◽  
First Time ◽  
Foreground Removal

Abstract We apply for the first time Gaussian Process Regression (GPR) as a foreground removal technique in the context of single-dish, low redshift H i intensity mapping, and present an open-source python toolkit for doing so. We use MeerKAT and SKA1-MID-like simulations of 21cm foregrounds (including polarisation leakage), H i cosmological signal and instrumental noise. We find that it is possible to use GPR as a foreground removal technique in this context, and that it is better suited in some cases to recover the H i power spectrum than Principal Component Analysis (PCA), especially on small scales. GPR is especially good at recovering the radial power spectrum, outperforming PCA when considering the full bandwidth of our data. Both methods are worse at recovering the transverse power spectrum, since they rely on frequency-only covariance information. When halving our data along frequency, we find that GPR performs better in the low frequency range, where foregrounds are brighter. It performs worse than PCA when frequency channels are missing, to emulate RFI flagging. We conclude that GPR is an excellent foreground removal option for the case of single-dish, low redshift H i intensity mapping in the absence of missing frequency channels. Our python toolkit gpr4im and the data used in this analysis are publicly available on GitHub.

scholarly journals The AARTFAAC Cosmic Explorer: observations of the 21-cm power spectrum in the EDGES absorption trough

2020 ◽  
Vol 499 (3) ◽  
pp. 4158-4173 ◽  
Author(s):  
B K Gehlot ◽  
F G Mertens ◽  
L V E Koopmans ◽  
A R Offringa ◽  
A Shulevski ◽  
...  
Keyword(s):  
Power Spectrum ◽  
Power Spectra ◽  
Absorption Feature ◽  
Wide Field ◽  
Deep Part ◽  
Processing Pipeline ◽  
Upper Limit ◽  
Upper Limits ◽  
First Results ◽  
Drift Scan

ABSTRACT The 21-cm absorption feature reported by the EDGES collaboration is several times stronger than that predicted by traditional astrophysical models. If genuine, a deeper absorption may lead to stronger fluctuations on the 21-cm signal on degree scales (up to 1 K in rms), allowing these fluctuations to be detectable in nearly 50 times shorter integration times compared to previous predictions. We commenced the ‘AARTFAAC Cosmic Explorer’ (ACE) program, which employs the AARTFAAC wide-field image, to measure or set limits on the power spectrum of the 21-cm fluctuations in the redshift range z = 17.9–18.6 (Δν = 72.36–75.09 MHz) corresponding to the deep part of the EDGES absorption feature. Here, we present first results from two LST bins: 23.5–23.75 and 23.75–24.00 h, each with 2 h of data, recorded in ‘semi drift-scan’ mode. We demonstrate the application of the new ACE data-processing pipeline (adapted from the LOFAR-EoR pipeline) on the AARTFAAC data. We observe that noise estimates from the channel and time-differenced Stokes V visibilities agree with each other. After 2 h of integration and subtraction of bright foregrounds, we obtain 2σ upper limits on the 21-cm power spectrum of $\Delta _{21}^2 \lt (8139~\textrm {mK})^2$ and $\Delta _{21}^2 \lt (8549~\textrm {mK})^2$ at $k = 0.144~h\, \textrm {cMpc}^{-1}$ for the two LST bins. Incoherently averaging the noise bias-corrected power spectra for the two LST bins yields an upper limit of $\Delta _{21}^2 \lt (7388~\textrm {mK})^2$ at $k = 0.144~h\, \textrm {cMpc}^{-1}$. These are the deepest upper limits thus far at these redshifts.

scholarly journals Precision requirements for interferometric gridding in the analysis of a 21 cm power spectrum

2019 ◽  
Vol 631 ◽  
pp. A12 ◽  
Author(s):  
A. R. Offringa ◽  
F. Mertens ◽  
S. van der Tol ◽  
B. Veenboer ◽  
B. K. Gehlot ◽  
...  
Keyword(s):  
Power Spectrum ◽  
Power Spectra ◽  
Low Frequency ◽  
Projection Algorithm ◽  
Wide Field ◽  
Kernel Size ◽  
Image Domain ◽  
Projection Image ◽  
Excess Power ◽  
Convolutional Gridding

Context. Experiments that try to observe the 21 cm redshifted signals from the epoch of reionisation (EoR) using interferometric low-frequency instruments have stringent requirements on the processing accuracy. Aims. We analyse the accuracy of radio interferometric gridding of visibilities with the aim to quantify the power spectrum bias caused by gridding. We do this ultimately to determine the suitability of different imaging algorithms and gridding settings for an analysis of a 21 cm power spectrum. Methods. We simulated realistic Low-Frequency Array (LOFAR) data and constructed power spectra with convolutional gridding and w stacking, w projection, image-domain gridding, and without w correction. These were compared against data that were directly Fourier transformed. The influence of oversampling, kernel size, w-quantization, kernel windowing function, and image padding were quantified. The gridding excess power was measured with a foreground subtraction strategy, for which foregrounds were subtracted using Gaussian progress regression, as well as with a foreground avoidance strategy. Results. Constructing a power spectrum with a significantly lower bias than the expected EoR signals is possible with the methods we tested, but requires a kernel oversampling factor of at least 4000, and when w-correction is used, at least 500 w-quantization levels. These values are higher than typically used values for imaging, but they are computationally feasible. The kernel size and padding factor parameters are less crucial. Of the tested methods, image-domain gridding shows the highest accuracy with the lowest imaging time. Conclusions. LOFAR 21 cm power spectrum results are not affected by gridding. Image-domain gridding is overall the most suitable algorithm for 21 cm EoR power spectrum experiments, including for future analyses of data from the Square Kilometre Array (SKA) EoR. Nevertheless, convolutional gridding with tuned parameters results in sufficient accuracy for interferometric 21 cm EoR experiments. This also holds for w stacking for wide-field imaging. The w-projection algorithm is less suitable because of the requirements for kernel oversampling, and a faceting approach is unsuitable because it causes spatial discontinuities.

scholarly journals Quantifying density-ionization correlations with the 21-cm power spectrum

2020 ◽  
Vol 498 (1) ◽  
pp. 373-384
Author(s):  
Michael Pagano ◽  
Adrian Liu
Keyword(s):  
Power Spectrum ◽  
Intergalactic Medium ◽  
Neutral Hydrogen ◽  
Upper Limits ◽  
Error Bars ◽  
Rule Out

ABSTRACT The epoch of reionization (EoR) – when neutral hydrogen in the intergalactic medium was systematically ionized – is a period in our Universe’s history that is currently poorly understood. However, a key prediction of most models is a correlation between the density and ionization field during the EoR. This has consequences for the 21-cm power spectrum. Here, we propose a parametrization for the density-ionization correlation and study the dependence of the 21-cm power spectrum on this parametrization. We use this formalism to forecast the ability of current and future observations to constrain these correlations. We find that upper limits on the dimensionless power spectrum at redshifts 7.5 < z < 8.5 using k bins between $0.1 \lt k \lt 0.75\, \textrm {Mpc}^{-1}$ with error bars at the level of ${\sim\! }20\, \textrm {mK}^2$ about our fiducial model would rule out uncorrelated reionization at $99{{\ \rm per\ cent}}$ credibility. Beyond upper limits, we find that at its full sensitivity, the Hydrogen Epoch of Reionization Array (HERA) will be able to place strong constraints on the sign and magnitude of density-ionization correlations.

scholarly journals Comparison of Observing Modes for Statistical Estimation of the 21 cm Signal from the Epoch of Reionisation

2014 ◽  
Vol 31 ◽  
Author(s):  
Cathryn M. Trott
Keyword(s):  
Power Spectrum ◽  
Signal To Noise Ratio ◽  
High Redshift ◽  
Power Spectra ◽  
Low Frequency ◽  
Neutral Hydrogen ◽  
Beam Shape ◽  
Drift Scan ◽  
Murchison Widefield Array ◽  
Aperture Arrays

AbstractNoise considerations for experiments that aim to statistically estimate the 21 cm signal from high redshift neutral hydrogen during the Epoch of Reionisation (EoR) using interferometric data are typically computed assuming a tracked observation, where the telescope pointing centre and instrument phase centre are the same over the observation. Current low frequency interferometers use aperture arrays of fixed dipoles, which are steered electronically on the sky, and have different properties to mechanically-steered single apertures, such as reduced sensitivity away from zenith, and discrete pointing positions on the sky. These properties encourage the use of two additional observing modes: (1) zenith drift, where the pointing centre remains fixed at the zenith, and the phase centre tracks the sky, and (2) drift + shift, a hybrid mode where the telescope uses discrete pointing centres, and the sky drifts during each fixed pointing. These three observing modes view the sky differently, and therefore yield different uncertainties in the power spectrum according to the balance of radiometric noise and cosmic variance. The coherence of measurements made by the instrument in these modes dictates the optimal reduction in thermal noise by combination of coherent modes, and the reduction in cosmic variance by combination of incoherent modes (views of different patches of the sky). Along with calibration and instrument stability considerations, the balance between these noise components provides one measure for the utility of these three modes for measuring a statistical signature of the EoR signal. We provide a general framework for estimating the uncertainty in the power spectrum for a given observing mode, telescope beam shape, and interferometer antenna distribution. We then apply this framework to the Murchison Widefield Array (MWA) using an analysis of the two-dimensional (2D) and one-dimensional (1D) power spectra for 900 hours of observing. We demonstrate that zenith drift scans can yield marginally lower uncertainty in the signal power compared with tracked scans for the MWA EoR experiment, and that moderately higher signal-to-noise ratio (S/N) estimates of the amplitude (3%) and slope (1%) of the 1D power spectrum are accessible, translating directly into a reduction in the required observing time to reach the same estimation precision. We find that the additional sensitivity of pointing at zenith, and the reduction in cosmic variance available with a zenith drift scan, makes this an attractive observing mode for current and future arrays.

Sign in / Sign up

Export Citation Format

Share Document