de Antonius Bruyn

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.

Post-correlation radio frequency interference classification methods

We describe and compare several post-correlation radio frequency interference (RFI) classification methods. As data sizes of observations grow with new and improved telescopes, the need for completely automated, robust methods for RFI mitigation is pressing. We investigated several classification methods and find that, for the data sets we used, the most accurate among them is the SumThreshold method. This is a new method formed from a combination of existing techniques, including a new way of thresholding. This iterative method estimates the astronomical signal by carrying out a surface fit in the time-frequency plane. With a theoretical accuracy of 95 per cent recognition and an approximately 0.1 per cent false probability rate in simple simulated cases, the method is in practice as good as the human eye in finding RFI. In addition, it is fast, robust, does not need a data model before it can be executed and works in almost all configurations with its default parameters. The method has been compared using simulated data with several other mitigation techniques, including one based upon the singular value decomposition of the time-frequency matrix, and has shown better results than the rest.

The discovery of a microarcsecond quasar : J1819+3845

We report on the discovery of a source which exhibits over 300% amplitude changes in radio flux density on the period of hours. This source, J1819+3845, is the most extremely variable extragalactic source known in the radio sky. We believe these properties are due to interstellar scintillation, and show that the source must emit at least 55% of its flux density within a radius of<16 microarcseconds at 5GHz. The apparent brightness temperature is>5.10^{12} K, and the source may be explained by a relativistically moving source with a Doppler factor ~15. The scattering occurs predominantly in material only a few tens of parsecs from the earth, which explains its unusually rapid variability. If the source PKS 0405-385 (Kedziora-Chudczer et al 1997) is similarly affected by local scattering material, Doppler factors of ~1000 are not required to explain this source. The discovery of a second source whose properties are well modeled by interstellar scintillation strengthens the argument for this as the cause for much of the variations seen in intra-day variables (IDV).We report on the discovery of a source that exhibits over 300% amplitude changes in radio flux density on the period of hours. This source, J1819+3845, is the most extremely variable extragalactic source known in the radio sky. We believe these properties are due to interstellar scintillation and show that the source must emit at least 55% of its flux density within a radius of fewer than 16 µas at 5 GHz. The apparent brightness temperature is greater than 5x1012 K, and the source may be explained by a relativistically moving source with a Doppler factor of approximately 15. The scattering occurs predominantly in material only a few tens of parsecs from the Earth, which explains its unusually rapid variability. If the source PKS 0405-385 is similarly affected by local scattering material, Doppler factors of approximately 1000 are not required to explain this source. The discovery of a second source whose properties are well modeled by interstellar scintillation strengthens the argument for this as the cause for much of the variation seen in intraday variables.

Early Radio and X-Ray Observations of the Youngest nearby Type Ia Supernova PTF 11kly (SN 2011fe)

On 2011 August 24 (UT) the Palomar Transient Factory (PTF) discovered PTF11kly (SN 2011fe), the youngest and most nearby Type Ia supernova (SN Ia) in decades. We followed this event up in the radio (centimeter and millimeter bands) and X-ray bands, starting about a day after the estimated explosion time.We present our analysis of the radio and X-ray observations, yielding the tightest constraints yet placed on the pre-explosion mass-loss rate from the progenitor system of this supernova. We find a robust limit of Ṁ ≾ 10^(−8)(w/100 km s^(−1))M_☉ yr^(−1) from sensitive X-ray non-detections, as well as a similar limit from radio data, which depends, however, on assumptions about microphysical parameters. We discuss our results in the context of single-degenerate models for SNe Ia and find that our observations modestly disfavor symbiotic progenitor models involving a red giant donor, but cannot constrain systems accreting from main-sequence or sub-giant stars, including the popular supersoft channel. In view of the proximity of PTF11kly and the sensitivity of our prompt observations, we would have to wait for a long time (a decade or longer) in order to more meaningfully probe the circumstellar matter of SNe Ia.

Annual modulation in the scattering of J1819+3845 : Peculiar plasma velocity and anisotropy

We present two years of monitoring observations of the extremely variable quasar J1819+3845. We observe large yearly changes in the timescale of the variations (from ∼1 hour to ∼10 hours at 5 GHz). This annual effect can only be explained if the variations are caused by a propagation effect, and thus affected by the Earths relative speed through the projected intensity pattern. To account for this effect, the scattering plasma must have a transverse velocity with respect to the local standard of rest. The velocity calculated from these observations is in good agreement with that obtained from a two telescope delay experiment (Dennett-Thorpe & de Bruyn 2001). We also show that either the source itself is elongated, or that the scattering plasma is anisotropic, with an axial ratio of >6:1. As the source is extended on scales relevant to the scattering phenomenon, it seems plausible that the anisotropy is due to the source itself, but this remains to be investigated. From the scintillation characteristics we find that the scattering material is a very strong, thin scatterer within ∼ten parsecs. We determine a source size at 5 GHz of 100 to 900 microarcsecs, and associated brightness temperatures of 10 10 to 10 12 K.

A sample of ultra steep spectrum sources selected from the Westerbork In the Southern Hemisphere (WISH) survey

The 352 MHz Westerbork In the Southern Hemisphere (WISH) survey is the southern extension of the WENSS, covering 1.60 sr between −9 ◦ <δ< −26 ◦ to a limiting flux density of ∼18 mJy (5σ). Due to the very low elevation of the observations, the survey has a much lower resolution in declination than in right ascension (54 �� × 54 �� cosec δ). A correlation with the 1.4 GHz NVSS shows that the positional accuracy is less constrained in declination than in right ascension, but there is no significant systematic error. We present a source list containing 73570 sources. We correlate this WISH catalogue with the NVSS to construct a sample of faint Ultra Steep Spectrum (USS) sources, which is accessible for follow-up studies with large optical telescopes in the southern hemisphere. This sample is aimed at increasing the number of known high redshift radio galaxies to allow detailed follow-up studies of these massive galaxies and their environments in the early Universe.

EXTRINSIC RADIO VARIABILITY OF JVAS/CLASS GRAVITATIONAL LENSES

We present flux ratio curves of the fold and cusp (i.e., close multiple) images of six Jodrell Bank VLA Astrometric Survey and Cosmic Lens All-Sky Survey (JVAS/CLASS) gravitational lens systems. The data were obtained over a period of 8.5 months in 2001 with the Multi Element Radio- Linked Interferometer Network ( MERLIN) at 5 GHz with 50 mas resolution, as part of a MERLIN Key Project. Even though the time delays between the fold and cusp images are small (less than or similar to1 day) compared to the timescale of intrinsic source variability, all six lens systems show evidence that suggests the presence of extrinsic variability. In particular, the cusp images of B2045+ 265 - regarded as the strongest case of the violation of the cusp relation (i.e., the sum of the magnifications of the three cusp images add to zero) - show extrinsic variations in their flux ratios up to similar to40% peak to peak on timescales of several months. Its low Galactic latitude of b approximate to - 10degrees and a line of sight toward the Cygnus superbubble region suggest that Galactic scintillation is the most likely cause. The cusp images of B1422+ 231 at b approximate to +69degrees do not show strong extrinsic variability. Galactic scintillation can therefore cause significant scatter in the cusp and fold relations of some radio lens systems ( up to 10% rms), even though these relations remain violated when averaged over a less than or similar to 1 yr time baseline.

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.

A new sample of giant radio galaxies from the WENSS survey - II. A multi-frequency radio study of a complete sample: Properties of the radio lobes and their environment

We have formed a complete sample of 26 low redshift (z less than or similar to 0.3) giant radio galaxies (GRGs) from the WENSS survey, selected at flux densities above 1 Jy at 325 MHz. We present 10.5-GHz observations with the 100-m Effelsberg telescope of 18 sources in this sample. These observations, together with similar data of the remaining eight sources, are combined with data from the WENSS, NVSS and GB6 surveys to study the radio properties of the lobes of these sources at arcminute resolution. We investigate radio source asymmetries, equipartition energy densities in the lobes, the presence of lobe pressure evolution with redshift, the spectral age and the density of the environments of these sources. We iind that the arm length asymmetries of GRGs are slightly larger than those of smaller sized 3CR radio galaxies and that these are difficult to explain as arising from orientation effects only. We also find indications that the lobes of the GRGs, despite their large sizes, are still overpressured with respect to their environment. Further, we argue that any evolution of lobe pressure with redshift in these large sources (e.g. Cotter 1998) is due to selection effects. For sources which could be used in a spectral ageing analysis, we find spectral ages which are large, typically a few times 10(7) Sr. This is comparable to earlier studies of some giant sources and indicates that such large spectral ages are common for this class of radio source. The advance velocities of the radio lobes are typically a few percent of the speed of light; which is higher than those found for smaller, low power (<10(26.5) W Hz(-1) at 178 MHz) radio sources, and more comparable to higher power radio sources. This suggests that the GRGs in our sample are the oldest members of the group of relatively high power radio sources whose radio powers have evolved to their currently observed lower values (cf. Kaiser et al. 1997).