S. Yatawatta
A5075439666- Radio Astronomy Observations and Technology
- Astrophysics and Cosmic Phenomena
- Antenna Design and Optimization
- Soil Moisture and Remote Sensing
- Galaxies: Formation, Evolution, Phenomena
- Radio Wave Propagation Studies
- Superconducting and THz Device Technology
- Advanced Wireless Communication Techniques
- Scientific Research and Discoveries
- Optical measurement and interference techniques
- Advanced MIMO Systems Optimization
- Pulsars and Gravitational Waves Research
- Astronomy and Astrophysical Research
- Wireless Communication Networks Research
- GNSS positioning and interference
- Cooperative Communication and Network Coding
- Gamma-ray bursts and supernovae
- Geophysics and Gravity Measurements
- Astronomical Observations and Instrumentation
- Image and Signal Denoising Methods
- Microwave Engineering and Waveguides
- Blind Source Separation Techniques
- Advanced Measurement and Metrology Techniques
- Direction-of-Arrival Estimation Techniques
- Advanced Wireless Communication Technologies
Netherlands Institute for Radio Astronomy
2015-2024
AstroTec Holding (Netherlands)
2011-2021
University of Groningen
2010-2020
Astronomy and Space
2010-2015
Royal Observatory
2014
University of Edinburgh
2014
Leibniz Institute for Astrophysics Potsdam
2014
Astronomical Institute of the Slovak Academy of Sciences
2008-2013
Laboratory for Atmospheric and Space Physics
2013
University of Colorado Boulder
2013
We present the first limits on Epoch of Reionization 21 cm H i power spectra, in redshift range z = 7.9–10.6, using Low-Frequency Array (LOFAR) High-Band Antenna (HBA). In total, 13.0 hr data were used from observations centered North Celestial Pole. After subtraction sky model and noise bias, we detect a non-zero (1-σ) excess variance best 2-σ upper limit at k 0.053 h cMpc−1 9.6–10.6. The decreases when optimizing smoothness direction- frequency-dependent gain calibration, with increasing...
ABSTRACT A new upper limit on the 21 cm signal power spectrum at a redshift of z ≈ 9.1 is presented, based 141 h data obtained with Low-Frequency Array (LOFAR). The analysis includes significant improvements in spectrally smooth gain-calibration, Gaussian Process Regression (GPR) foreground mitigation and optimally weighted inference. Previously seen ‘excess power’ due to spectral structure gain solutions has markedly reduced but some excess still remains correlation distinct from thermal...
Future high redshift 21-cm experiments will suffer from a degree of contamination, due both to astrophysical foregrounds and non-astrophysical instrumental effects. In order reliably extract the cosmological signal observed data, it is essential understand very well all data components their influence on extracted signal. Here we present simulated datacubes discuss possible statistical effects data. The foreground maps are produced assuming 5 deg x windows that match those expected be by...
Low frequency radio waves, while challenging to observe, are a rich source of information about pulsars. The LOw Frequency ARray (LOFAR) is new interferometer operating in the lowest 4 octaves ionospheric "radio window": 10-240MHz, that will greatly facilitate observing pulsars at low frequencies. Through huge collecting area, long baselines, and flexible digital hardware, it expected LOFAR revolutionize astronomy frequencies visible from Earth. next-generation telescope pathfinder Square...
<i>Context. <i/>The cosmological 21 cm line promises to be a formidable tool for cosmology, allowing the investigation of end so-called dark ages, when first galaxies formed.<i>Aims. <i/>Astrophysical foregrounds are expected about three orders magnitude greater than signal and therefore represent serious contamination line. Detailed knowledge both their intensity polarization structure on relevant angular scale 1–30 arcmin will essential extracting from data.<i>Methods. <i/>We present...
Faraday rotation measurements using the current and next generation of low-frequency radio telescopes will provide a powerful probe astronomical magnetic fields. However, achieving full potential these requires accurate removal time-variable ionospheric contribution. We present ionFR, code that calculates amount for specific epoch, geographic location, line-of-sight. ionFR uses number publicly available, GPS-derived total electron content maps most recent release International Geomagnetic...
Pulsars emit low-frequency radio waves through to high-energy gamma-rays that are generated anywhere from the surface out edges of magnetosphere. Detecting correlated mode changes in multi-wavelength emission is therefore key understanding physical relationship between these sites. Through simultaneous observations, we have detected synchronous switching and X-ray properties PSR B0943+10. When pulsar a sustained 'bright' mode, X-rays show only an un-pulsed, non-thermal component. Conversely,...
The aim of the new generation radio synthesis arrays such as LOw Frequency ARray (LOFAR) and Square Kilometre Array (SKA) is to achieve much higher sensitivity, resolution frequency coverage than what available now, especially at low frequencies. To accomplish this goal, accuracy calibration techniques used considerable importance. Moreover, since these telescopes produce huge amounts data, speed convergence a major bottleneck. errors in are due system noise (sky instrumental) well...
The aim of the LOFAR Epoch Reionization (EoR) project is to detect spectral fluctuations redshifted HI 21cm signal. This signal weaker by several orders magnitude than astrophysical foreground signals and hence, in order achieve this, very long integrations, accurate calibration for stations ionosphere reliable removal are essential. One prospective observing windows EoR will be centered at North Celestial Pole (NCP). We present results from observations NCP window using highband antenna...
Context. The LOw Frequency ARray (LOFAR) radio telescope is a giant digital phased array interferometer with multiple antennas distributed in Europe. It provides discrete sets of Fourier components the sky brightness. Recovering original brightness distribution aperture synthesis forms an inverse problem that can be solved by various deconvolution and minimization methods Aims. Recent papers have established clear link between nature interferometry measurement "compressed sensing" (CS)...
ABSTRACT Observations of the redshifted 21-cm hyperfine line neutral hydrogen from early phases Universe such as Cosmic Dawn and Epoch Reionization promise to open a new window onto formation stars galaxies. We present first upper limits on power spectrum brightness temperature fluctuations in redshift range z = 19.8–25.2 (54–68 MHz frequency range) using 14 h data obtained with LOFAR-Low Band Antenna (LBA) array. also demonstrate application multiple pointing calibration technique calibrate...
The redshifted 21 cm line of neutral hydrogen is a promising probe the Epoch Reionization (EoR). However, its detection requires thorough understanding and control systematic errors. We study two biases observed in LOFAR EoR residual data after calibration subtraction bright discrete foreground sources. first effect suppression diffuse foregrounds, which could potentially mean signal. second an excess noise beyond thermal noise. shows fluctuations on small frequency scales, hence it can not...
ABSTRACT We derive constraints on the thermal and ionization states of intergalactic medium (IGM) at redshift ≈ 9.1 using new upper limits 21-cm power spectrum measured by LOFAR radio telescope a prior ionized fraction that estimated from recent cosmic microwave background (CMB) observations. have used results reionization simulation code grizzly Bayesian inference framework to constrain parameters which describe physical state IGM. find that, if gas heating remains negligible, an IGM with...
One of the aims Low Frequency Array (LOFAR) Epoch Reionization (EoR) project is to measure power spectrum variations in intensity redshifted 21-cm radiation from EoR. The sensitivity with which this can be estimated depends on level thermal noise and sample variance, also systematic errors arising extraction process, particular subtraction foreground contamination. We model process using realistic simulations cosmological signal, foregrounds noise, so estimate LOFAR EoR experiment spectrum....
In the coming years a new insight into galaxy formation and thermal history of Universe is expected to come from detection highly redshifted cosmological 21 cm line. The line signal buried under Galactic extragalactic foregrounds which are likely be few orders magnitude brighter. Strategies techniques for effective subtraction these foreground sources require detailed knowledge their structure in both intensity polarization on relevant angular scales 1-30 arcmin. We present results...
The characteristic outer scale of turbulence and the ratio random to ordered components magnetic field are key parameters characterise in interstellar gas, which affects propagation cosmic rays within Galaxy. We provide new constraints those two parameters. use LOw Frequency ARray (LOFAR) image diffuse continuum emission Fan region at (l,b) (137.0,+7.0) 80"x70" resolution range [146,174] MHz. detect multi-scale fluctuations Galactic synchrotron compute their power spectrum. Applying...
Dispersion in the interstellar medium is a well known phenomenon that follows simple relationship, which has been used to predict time delay of dispersed radio pulses since late 1960s. We performed wide-band simultaneous observations four pulsars with LOFAR (at 40-190 MHz), 76-m Lovell Telescope 1400 MHz) and Effelsberg 100-m 8000 test accuracy dispersion law over broad frequency range. In this paper we present results these show accurate better than 1 part 100000 across our observing band....
This study aims to characterize linear polarization structures in LOFAR observations of the interstellar medium (ISM) 3C196 field, one primary fields LOFAR-Epoch Reionization key science project. We have used high band antennas (HBA) image this region and Rotation Measure (RM) synthesis unravel distribution polarized Faraday depth. The brightness temperature detected Galactic emission is $5-15~{\rm K}$ intensity covers range from -3 +8 ${\rm rad~m^{-2}}$ most interesting morphological...
Increasing data volumes delivered by a new generation of radio interferometers require computationally efficient and robust calibration algorithms. In this paper, we propose distributed as way improving both computational cost well robustness in calibration. We exploit the parallelism across frequency that is inherent astronomical observations are recorded multiple channels at different frequencies. Moreover, also smoothness variation parameters frequency. Data enables us to distribute...
One of the most promising ways to study epoch reionization (EoR) is through radio observations redshifted 21-cm line emission from neutral hydrogen. These are complicated by fact that mapping redshifts line-of-sight positions distorted peculiar velocities gas. Such distortions can be a source error if they not properly understood, but also encode information about cosmology and astrophysics. We effects redshift space on power spectrum radiation EoR using large scale $N$-body radiative...
Aims. This paper discusses the spectral occupancy for performing radio astronomy with Low-Frequency Array (LOFAR), a focus on imaging observations.
This study aims to characterise the polarized foreground emission in ELAIS-N1 field and address its possible implications for extraction of cosmological 21-cm signal from Low-Frequency Array - Epoch Reionization (LOFAR-EoR) data. We use high band antennas LOFAR image this region RM-synthesis unravel structures at Galactic latitudes. The brightness temperature detected is on average 4 K intensity covers range -10 +13rad m^-2 Faraday depth. total polarization angle show a wide morphological...
Detection of the 21-cm signal coming from epoch reionization (EoR) is challenging especially because, even after removing foregrounds, residual Stokes I maps contain leakage polarized emission that can mimic signal. Here, we discuss instrumental polarization Low Frequency Array (LOFAR) and present realistic simulations leakages between parameters. From LOFAR observations in 3C196 field, have quantified level caused by nominal model beam LOFAR, compared it with EoR using power spectrum...