Stefan J. Wijnholds

Detection and imaging of atmospheric radio flashes from cosmic ray air showers.

The nature of ultrahigh-energy cosmic rays (UHECRs) at energies >1020 eV remains a mystery. They are likely to be of extragalactic origin, but should be absorbed within ∼50 Mpc through interactions with the cosmic microwave background. As there are no sufficiently powerful accelerators within this distance from the Galaxy, explanations for UHECRs range from unusual astrophysical sources to exotic string physics. Also unclear is whether UHECRs consist of protons, heavy nuclei, neutrinos or γ-rays. To resolve these questions, larger detectors with higher duty cycles and which combine multiple detection techniques are needed. Radio emission from UHECRs, on the other hand, is unaffected by attenuation, has a high duty cycle, gives calorimetric measurements and provides high directional accuracy. Here we report the detection of radio flashes from cosmic-ray air showers using low-cost digital radio receivers. We show that the radiation can be understood in terms of the geosynchrotron effect. Our results show that it should be possible to determine the nature and composition of UHECRs with combined radio and particle detectors, and to detect the ultrahigh-energy neutrinos expected from flavour mixing.

EMBRACE: A Multi-Beam 20,000-Element Radio Astronomical Phased Array Antenna Demonstrator

We present the design and development of the electronic multi-beam radio astronomy concept (EMBRACE), a demonstrator that is part of the European contribution towards the square kilometre array, which is currently being designed by the global radio astronomical community. One of the design goals of EMBRACE is to demonstrate the applicability of phased array technology for use in future radio telescopes. The EMBRACE system will ultimately consist of two stations, the largest of which comprises over 20,000 elements and has a physical area of about 160 m2 . The antenna system, covering the 500-1500 MHz frequency range, is designed as a dual polarized system, however only the signals for one polarization are processed. To obtain a cost effective design, RF analog beam-forming is performed on tile level close to the radiators. The demonstrator is designed to provide two independent beams such that different parts of the sky can be observed simultaneously. First results from part of the array are presented and discussed. The results show that the complete data path is functional. Since the design resembles a large regular contiguous array, all coupling can be taken into account in the embedded element patterns. The array factor therefore suffices to describe the scanning of the array reducing significantly calibration complexity compared to, e.g. sparse, random or more irregular arrays. This is confirmed by the first array factor measurements, that were done using a novel technique that does not require calibration of the array. The first measurements on an astronomical source, the Sun, indicate that the system noise temperature lies between 104 and 118 K, which is reassuringly close to the design target of 100 K.

Multisource Self-Calibration for Sensor Arrays

Calibration of a sensor array is more involved if the antennas have direction dependent gains and multiple calibrator sources are simultaneously present. We study this case for a sensor array with arbitrary geometry but identical elements, i.e., elements with the same direction dependent gain pattern. A weighted alternating least squares (WALS) algorithm is derived that iteratively solves for the direction independent complex gains of the array elements, their noise powers and their gains in the direction of the calibrator sources. An extension of the problem is the case where the apparent calibrator source locations are unknown, e.g., due to refractive propagation paths. For this case, the WALS method is supplemented with weighted subspace fitting (WSF) direction finding techniques. Using Monte Carlo simulations we demonstrate that both methods are asymptotically statistically efficient and converge within two iterations even in cases of low SNR.

Calibration challenges for future radio telescopes

Instruments for radio astronomical observations have come a long way. While the first telescopes were based on very large dishes and two-antenna interferometers, current Instruments consist of dozens of steerable dishes, whereas future instruments will be even larger distributed sensor arrays with a hierarchy of phased array elements. For such arrays to provide meaningful output (images), accurate calibration is of critical importance. Calibration must solve for the unknown antenna gains and phases as well as the unknown atmospheric and ionospheric disturbances. Future telescopes will have a large number of elements and a large field of view (FOV). In this case, the parameters are strongly direction-dependent, resulting in a large number of unknown parameters, even if appropriately constrained physical or phenomenological descriptions are used. This makes calibration a daunting parameter-estimation task.

Calibrating high-precision Faraday rotation measurements for LOFAR and the next generation of low-frequency radio telescopes

Faraday rotation measurements using the current and next generation of low-frequency radio telescopes will provide a powerful probe of astronomical magnetic fields. However, achieving the full potential of these measurements requires accurate removal of the time-variable ionospheric Faraday rotation contribution. We present ionFR, a code that calculates the amount of ionospheric Faraday rotation for a specific epoch, geographic location, and line-of-sight. ionFR uses a number of publicly available, GPS-derived total electron content maps and the most recent release of the International Geomagnetic Reference Field. We describe applications of this code for the calibration of radio polarimetric observations, and demonstrate the high accuracy of its modeled ionospheric Faraday rotations using LOFAR pulsar observations. These show that we can accurately determine some of the highest-precision pulsar rotation measures ever achieved. Precision rotation measures can be used to monitor rotation measure variations - either intrinsic or due to the changing line-of-sight through the interstellar medium. This calibration is particularly important for nearby sources, where the ionosphere can contribute a significant fraction of the observed rotation measure. We also discuss planned improvements to ionFR, as well as the importance of ionospheric Faraday rotation calibration for the emerging generation of low-frequency radio telescopes, such as the SKA and its pathfinders.

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.

In Situ Antenna Performance Evaluation of the LOFAR Phased Array Radio Telescope

The low frequency array (LOFAR) is a phased array radio telescope that is currently being built in The Netherlands with extensions throughout Europe. It was officially opened on June 12, 2010 and is an important pathfinder for the square kilometre array. The Dutch LOFAR system will consist of 36 stations covering the 10-250 MHz frequency range. In this paper we discuss the sky noise limited design of the antenna system and present a novel technique to obtain the ratio of effective area and system temperature directly from the calibration results, despite the presence of multiple sources within the 2π sr field of view of the antennas. This ratio is the key sensitivity parameter for radio telescopes. The presented technique allows in situ performance evaluation using astronomical calibration sources, i.e., without the use of reference sources, a controlled environment or lab equipment. We use this technique to evaluate the performance of some of the already available LOFAR hardware and demonstrate that LOFAR has the desired sky noise dominated performance.

The LOFAR radio environment

Aims: This paper discusses the spectral occupancy for performing radio astronomy with the Low-Frequency Array (LOFAR), with a focus on imaging observations. Methods: We have analysed the radio-frequency interference (RFI) situation in two 24-h surveys with Dutch LOFAR stations, covering 30-78 MHz with low-band antennas and 115-163 MHz with high-band antennas. This is a subset of the full frequency range of LOFAR. The surveys have been observed with a 0.76 kHz / 1 s resolution. Results: We measured the RFI occupancy in the low and high frequency sets to be 1.8% and 3.2% respectively. These values are found to be representative values for the LOFAR radio environment. Between day and night, there is no significant difference in the radio environment. We find that lowering the current observational time and frequency resolutions of LOFAR results in a slight loss of flagging accuracy. At LOFARs nominal resolution of 0.76 kHz and 1 s, the false-positives rate is about 0.5%. This rate increases approximately linearly when decreasing the data frequency resolution. Conclusions: Currently, by using an automated RFI detection strategy, the LOFAR radio environment poses no perceivable problems for sensitive observing. It remains to be seen if this is still true for very deep observations that integrate over tens of nights, but the situation looks promising. Reasons for the low impact of RFI are the high spectral and time resolution of LOFAR; accurate detection methods; strong filters and high receiver linearity; and the proximity of the antennas to the ground. We discuss some strategies that can be used once low-level RFI starts to become apparent. It is important that the frequency range of LOFAR remains free of broadband interference, such as DAB stations and windmills.

Constraining the epoch of reionization with the variance statistic: simulations of the LOFAR case

The Epoch of Reionization (EoR) is the epoch in which most of the neutral gas in the Universe was re-ionized by the radiation from the first stars and galaxies. Many projects are underway to detect the redshifted 21 cm signal of the neutral hydrogen from the EoR with the low frequency radio telescopes. These experiments aim for a statistical detection of the signal due to its very low signal-to-noise ratio. We study extraction of the sample variance of the signal to constrain the global properties of the EoR. We show that the LOw Frequency ARray (LOFAR) should be able to detect the signal with a significance of 3 standard deviations in 600 hours of integration. Additionally, it should be able to constrain the timing and duration of reionization with uncertainties of 0.4 and 1 redshifts, respectively, with 95 percent confidence. We also show the upper limits on the signal detection obtained from the analysis of 115 hours of the observed data with LOFAR. In the observed redshift range of 7 to 11, our current best upper limits are 25 mK at 1 MHz, 12 arcmin spectral and spatial resolutions, respectively.

Chromatic effects in the 21 cm global signal from the cosmic dawn

The redshifted 21 cm brightness distribution from neutral hydrogen is a promising probe into the cosmic dark ages, cosmic dawn and re-ionization. Low Frequency Arrays (LOFAR) Low Band Antennas (LBA) may be used in the frequency range 45 to 85 MHz (30 > z > 16) to measure the sky-averaged redshifted 21 cm brightness temperature as a function of frequency, or equivalently, cosmic redshift. These low frequencies are affected by strong Galactic foreground emission that is observed through frequency-dependent ionospheric and antenna beam distortions which lead to chromatic mixing of spatial structure into spectral structure. Using simple models, we show that (i) the additional antenna temperature due to ionospheric refraction and absorption are at an ˜1 per cent level - two-to-three orders of magnitude higher than the expected 21 cm signal, and have an approximate ν-2 dependence, (ii) ionospheric refraction leads to a knee-like modulation on the sky spectrum at ν ≈ 4 times plasma frequency. Using more realistic simulations, we show that in the measured sky spectrum, more than 50 per cent of the 21 cm signal variance can be lost to confusion from foregrounds and chromatic effects. To mitigate this confusion, we recommend modelling of chromatic effects using additional priors and interferometric visibilities rather than subtracting them as generic functions of frequency as previously proposed.