M. A. Brentjens

Faraday rotation measure synthesis

We extend the rotation measure work of Burn ( 1966, MNRAS, 133, 67) to the cases of limited sampling of lambda(2) space and non-constant emission spectra. We introduce the rotation measure transfer function (RMTF), which is an excellent predictor of n pi ambiguity problems with the lambda(2) coverage. Rotation measure synthesis can be implemented very efficiently on modern computers. Because the analysis is easily applied to wide fields, one can conduct very fast RM surveys of weak spatially extended sources. Difficult situations, for example multiple sources along the line of sight, are easily detected and transparently handled. Under certain conditions, it is even possible to recover the emission as a function of Faraday depth within a single cloud of ionized gas. Rotation measure synthesis has already been successful in discovering widespread, weak, polarized emission associated with the Perseus cluster (de Bruyn & Brentjens 2005, A&A, 441, 931). In simple, high signal to noise situations it is as good as traditional linear fits to chi versus lambda(2) plots. However, when the situation is more complex or very weak polarized emission at high rotation measures is expected, it is the only viable option.

Foreground simulations for the LOFAR - Epoch of Reionization Experiment

Future high-redshift 21-cm experiments will suffer from a high degree of contamination, due both to astrophysical foregrounds and to non-astrophysical and instrumental effects. In order to reliably extract the cosmological signal from the observed data, it is essential to understand very well all data components and their influence on the extracted signal. Here we present simulated astrophysical foregrounds data cubes and discuss their possible statistical effects on the data. The foreground maps are produced assuming 5 degrees x 5 degrees windows that match those expected to be observed by the LOFAR epoch of reionization (EoR) key science project. We show that with the expected LOFAR-EoR sky and receiver noise levels, which amount to approximate to 52 mK at 150 MHz after 400 h of total observing time, a simple polynomial fit allows a statistical reconstruction of the signal. We also show that the polynomial fitting will work for maps with realistic yet idealized instrument response, i.e. a response that includes only a uniform uv coverage as a function of frequency and ignores many other uncertainties. Polarized Galactic synchrotron maps that include internal polarization and a number of Faraday screens along the line of sight are also simulated. The importance of these stems from the fact that the LOFAR instrument, in common with all current interferometric EoR experiments, has an instrumentally polarized response.

Power spectrum extraction for redshifted 21-cm Epoch of Reionization experiments: the LOFAR case

One of the aims of the Low Frequency Array (LOFAR) Epoch of Reionization (EoR) project is to measure the power spectrum of variations in the intensity of redshifted 21-cm radiation from the EoR. The sensitivity with which this power spectrum can be estimated depends on the level of thermal noise and sample variance, and also on the systematic errors arising from the extraction process, in particular from the subtraction of foreground contamination. We model the extraction process using realistic simulations of the cosmological signal, the foregrounds and noise, and so estimate the sensitivity of the LOFAR EoR experiment to the redshifted 21-cm power spectrum. Detection of emission from the EoR should be possible within 360 h of observation with a single station beam. Integrating for longer, and synthesizing multiple station beams within the primary (tile) beam, then enables us to extract progressively more accurate estimates of the power at a greater range of scales and redshifts. We discuss different observational strategies which compromise between depth of observation, sky coverage and frequency coverage. A plan in which lower frequencies receive a larger fraction of the time appears to be promising. We also study the nature of the bias which foreground fitting errors induce on the inferred power spectrum and discuss how to reduce and correct for this bias. The angular and line-of-sight power spectra have different merits in this respect, and we suggest considering them separately in the analysis of LOFAR data.

Fast large‐scale reionization simulations

We present an efficient method to generate large simulations of the epoch of reionization without the need for a full three-dimensional radiative transfer code. Large dark-matter-only simulations are post-processed to produce maps of the redshifted 21-cm emission from neutral hydrogen. Dark matter haloes are embedded with sources of radiation whose properties are either based on semi-analytical prescriptions or derived from hydrodynamical simulations. These sources could either be stars or power-law sources with varying spectral indices. Assuming spherical symmetry, ionized bubbles are created around these sources, whose radial ionized fraction and temperature profiles are derived from a catalogue of one-dimensional radiative transfer experiments. In case of overlap of these spheres, photons are conserved by redistributing them around the connected ionized regions corresponding to the spheres. The efficiency with which these maps are created allows us to span the large parameter space typically encountered in reionization simulations. We compare our results with other, more accurate, three-dimensional radiative transfer simulations and find excellent agreement for the redshifts and the spatial scales of interest to upcoming 21-cm experiments. We generate a contiguous observational cube spanning redshift 6 to 12 and use these simulations to study the differences in the reionization histories between stars and quasars. Finally, the signal is convolved with the Low Frequency Array (LOFAR) beam response and its effects are analysed and quantified. Statistics performed on this mock data set shed light on possible observational strategies for LOFAR.

Foreground Removal using FastICA: A Showcase of LOFAR-EoR

We introduce a new implementation of the fastica algorithm on simulated Low Frequency Array Epoch of Reionization data with the aim of accurately removing the foregrounds and extracting the 21-cm reionization signal. We find that the method successfully removes the foregrounds with an average fitting error of 0.5 per cent and that the 2D and 3D power spectra are recovered across the frequency range. We find that for scales above several point spread function scales, the 21-cm variance is successfully recovered though there is evidence of noise leakage into the reconstructed foreground components. We find that this blind independent component analysis technique provides encouraging results without the danger of prior foreground assumptions.

The scale of the problem: recovering images of reionization with Generalized Morphological Component Analysis

The accurate and precise removal of 21-cm foregrounds from Epoch of Reionization redshifted 21-cm emission data is essential if we are to gain insight into an unexplored cosmological era. We apply a non-parametric technique, Generalized Morphological Component Analysis or GMCA, to simulated LOFAR-EoR data and show that it has the ability to clean the foregrounds with high accuracy. We recover the 21-cm 1D, 2D and 3D power spectra with high accuracy across an impressive range of frequencies and scales. We show that GMCA preserves the 21-cm phase information, especially when the smallest spatial scale data is discarded. While it has been shown that LOFAR-EoR image recovery is theoretically possible using image smoothing, we add that wavelet decomposition is an efficient way of recovering 21-cm signal maps to the same or greater order of accuracy with more flexibility. By comparing the GMCA output residual maps (equal to the noise, 21-cm signal and any foreground fitting errors) with the 21-cm maps at one frequency and discarding the smaller wavelet scale information, we find a correlation coefficient of 0.689, compared to 0.588 for the equivalently smoothed image. Considering only the central 50% of the maps, these coefficients improve to 0.905 and 0.605 respectively and we conclude that wavelet decomposition is a significantly more powerful method to denoise reconstructed 21-cm maps than smoothing.

Detection and extraction of signals from the epoch of reionization using higher-order one-point statistics

Detecting redshifted 21-cm emission from neutral hydrogen in the early Universe promises to give direct constraints on the epoch of reionization (EoR). It will, though, be very challenging to extract the cosmological signal (CS) from foregrounds and noise which are orders of magnitude larger. Fortunately, the signal has some characteristics which differentiate it from the foregrounds and noise, and we suggest that using the correct statistics may tease out signatures of reionization. We generate mock data cubes simulating the output of the Low Frequency Array (LOFAR) EoR experiment. These cubes combine realistic models for Galactic and extragalactic foregrounds and the noise with three different simulations of the CS. We fit out the foregrounds, which are smooth in the frequency direction, to produce residual images in each frequency band. We denoise these images and study the skewness of the one-point distribution in the images as a function of frequency. We find that, under sufficiently optimistic assumptions, we can recover the main features of the redshift evolution of the skewness in the 21-cm signal. We argue that some of these features – such as a dip at the onset of reionization, followed by a rise towards its later stages – may be generic, and give us a promising route to a statistical detection of reionization.

Deep Westerbork observations of Abell 2256 at 350 MHz

Deep polarimetric Westerbork observations of the galaxy cluster Abell 2256 are presented, covering a frequency range of 325–377 MHz. The central halo source has a diameter of the order of 1.2 Mpc (18 � ), which is somewhat larger than at 1.4 GHz. With α = −1.61 ± 0.04, the halo spectrum between 1.4 GHz and 22.25 MHz is less steep than previously thought. The centre of the ultra steep spectrum source in the eastern part of the cluster exhibits a spectral break near 400 MHz. It is estimated to be at least 51 million years old, but possibly older than 125 million years. A final measurement requires observations in the 10–150 MHz range. It remains uncertain whether the source is a radio tail of Fabricant galaxy 122, situated in the northeastern tip of the source. Faraday rotation measure synthesis revealed no polarized flux at all in the cluster. The polarization fraction of the brightest parts of the relic area is less than 1%. The RM-synthesis nevertheless revealed 9 polarized sources in the field enabling an accurate measurement of the Galactic Faraday rotation (−33 ± 2r ad m −2 in front of the relic). Based on its depolarization on longer wavelengths, the line-of-sight magnetic field in relic filament G is estimated to be between 0.02 and 2 μG. A value of 0.2 μG appears most reasonable given the currently available data.

Upper Limits on the 21 cm Epoch of Reionization Power Spectrum from One Night with LOFAR

We present the first limits on the Epoch of Reionization 21 cm H I power spectra, in the redshift range z = 7.9–10.6, using the Low-Frequency Array (LOFAR) High-Band Antenna (HBA). In total, 13.0 hr of data were used from observations centered on the North Celestial Pole. After subtraction of the sky model and the noise bias, we detect a non-zero Δ^2_I = (56 ± 13 mK)^2 (1-σ) excess variance and a best 2-σ upper limit of Δ^2_(21) < (79.6 mK)^2 at k = 0.053 h cMpc^(−1) in the range z = 9.6–10.6. The excess variance decreases when optimizing the smoothness of the direction- and frequency-dependent gain calibration, and with increasing the completeness of the sky model. It is likely caused by (i) residual side-lobe noise on calibration baselines, (ii) leverage due to nonlinear effects, (iii) noise and ionosphere-induced gain errors, or a combination thereof. Further analyses of the excess variance will be discussed in forthcoming publications.

First LOFAR observations at very low frequencies of cluster-scale non-thermal emission: the case of Abell 2256

Abell 2256 is one of the best known examples of a galaxy cluster hosting large-scale diffuse radio emission that is unrelated to individual galaxies. It contains both a giant radio halo and a relic, as well as a number of head-tail sources and smaller diffuse steep-spectrum radio sources. The origin of radio halos and relics is still being debated, but over the last years it has become clear that the presence of these radio sources is closely related to galaxy cluster merger events. Here we present the results from the first LOFAR low band antenna (LBA) observations of Abell 2256 between 18 and 67 MHz. To our knowledge, the image presented in this paper at 63 MHz is the deepest ever obtained at frequencies below 100 MHz in general. Both the radio halo and the giant relic are detected in the image at 63 MHz, and the diffuse radio emission remains visible at frequencies as low as 20 MHz. The observations confirm the presence of a previously claimed ultra-steep spectrum source to the west of the cluster center with a spectral index of -2.3 +/- 0.4 between 63 and 153 MHz. The steep spectrum suggests that this source is an old part of a head-tail radio source in the cluster. For the radio relic we find an integrated spectral index of -0.81 +/- 0.03, after removing the flux contribution from the other sources. This is relatively flat which could indicate that the efficiency of particle acceleration at the shock substantially changed in the last similar to 0.1 Gyr due to an increase of the shock Mach number. In an alternative scenario, particles are re-accelerated by some mechanism in the downstream region of the shock, resulting in the relatively flat integrated radio spectrum. In the radio halo region we find indications of low-frequency spectral steepening which may suggest that relativistic particles are accelerated in a rather inhomogeneous turbulent region.