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 (1- ) excess variance and a best 2- upper limit of 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.
Abstract
This paper presents a detailed analysis of the applicability and benefits of baseline-dependent averaging (BDA) in modern radio interferometers and in particular the Square Kilometre Array. ...We demonstrate that BDA does not affect the information content of the data other than a well-defined decorrelation loss for which closed form expressions are readily available. We verify these theoretical findings using simulations. We therefore conclude that BDA can be used reliably in modern radio interferometry allowing a reduction of visibility data volume (and hence processing costs for handling visibility data) by more than 80 per cent.
Calibration artefacts, like the self-calibration bias, usually emerge when data are calibrated using an incomplete sky model. In the first paper of this series, in which we analysed calibration ...artefacts in data from the Westerbork Synthesis Radio Telescope, we showed that these artefacts take the form of spurious positive and negative sources, which we refer to as ghosts or ghost sources. We also developed a mathematical framework with which we could predict the ghost pattern of an east–west interferometer for a simple two-source test case. In this paper, we extend our analysis to more general array layouts. This provides us with a useful method for the analysis of ghosts that we refer to as extrapolation. Combining extrapolation with a perturbation analysis, we are able to (1) analyse the ghost pattern for a two-source test case with one modelled and one unmodelled source for an arbitrary array layout, (2) explain why some ghosts are brighter than others, (3) define a taxonomy allowing us to classify the different ghosts, (4) derive closed form expressions for the fluxes and positions of the brightest ghosts, and (5) explain the strange two-peak structure with which some ghosts manifest during imaging. We illustrate our mathematical predictions using simulations of the KAT-7 (seven-dish Karoo Array Telescope) array. These results show the explanatory power of our mathematical model. The insights gained in this paper provide a solid foundation to study calibration artefacts in arbitrary, i.e. more complicated than the two-source example discussed here, incomplete sky models or full synthesis observations including direction-dependent effects.
LOFAR is the LOw‐Frequency Radio interferometer ARray located at midlatitude (52°53′N). Here we present results on ionospheric structures derived from 29 LOFAR nighttime observations during the ...winters of 2012/2013 and 2013/2014. We show that LOFAR is able to determine differential ionospheric total electron content values with an accuracy better than 0.001 total electron content unit = 1016m−2 over distances ranging between 1 and 100 km. For all observations the power law behavior of the phase structure function is confirmed over a long range of baseline lengths, between 1 and 80 km, with a slope that is, in general, larger than the 5/3 expected for pure Kolmogorov turbulence. The measured average slope is 1.89 with a one standard deviation spread of 0.1. The diffractive scale, i.e., the length scale where the phase variance is 1rad2, is shown to be an easily obtained single number that represents the ionospheric quality of a radio interferometric observation. A small diffractive scale is equivalent to high phase variability over the field of view as well as a short time coherence of the signal, which limits calibration and imaging quality. For the studied observations the diffractive scales at 150 MHz vary between 3.5 and 30 km. A diffractive scale above 5 km, pertinent to about 90% of the observations, is considered sufficient for the high dynamic range imaging needed for the LOFAR epoch of reionization project. For most nights the ionospheric irregularities were anisotropic, with the structures being aligned with the Earth magnetic field in about 60% of the observations.
Key Points
LOFAR is able to measure differential TEC values with an accuracy better than 1 mTECU
The diffractive scale is an easily obtained single number indicating the ionospheric quality of a radio interferometric observation
The ionospheric phase structure functions of most nights show a spatial anisotropy that is in many cases Earth magnetic field aligned
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 Array's (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.
Fourier-domain dedispersion Bassa, C. G.; Romein, J. W.; Veenboer, B. ...
Astronomy & astrophysics,
01/2022, Letnik:
657
Journal Article
Recenzirano
Odprti dostop
We present and implement the concept of the Fourier-domain dedispersion (FDD) algorithm, a brute-force incoherent dedispersion algorithm. This algorithm corrects the frequency-dependent dispersion ...delays in the arrival time of radio emission from sources such as radio pulsars and fast radio bursts. Where traditional time-domain dedispersion algorithms correct time delays using time shifts, the FDD algorithm performs these shifts by applying phase rotations to the Fourier-transformed time-series data. Incoherent dedispersion to many trial dispersion measures (DMs) is compute-, memory-bandwidth-, and input-output-intensive, and dedispersion algorithms have been implemented on graphics processing units (GPUs) to achieve high computational performance. However, time-domain dedispersion algorithms have low arithmetic intensity and are therefore often memory-bandwidth-limited. The FDD algorithm avoids this limitation and is compute-limited, providing a path to exploit the potential of current and upcoming generations of GPUs. We implement the FDD algorithm as an extension of the
DEDISP
time-domain dedispersion software. We compare the performance and energy-to-completion of the FDD implementation using an NVIDIA Titan RTX GPU against both the standard version and an optimized version of
DEDISP
. The optimized implementation already provides a factor of 1.5 to 2 speedup at only 66% of the energy utilization compared to the original algorithm. We find that the FDD algorithm outperforms the optimized time-domain dedispersion algorithm by another 20% in performance and 5% in energy-to-completion when a large number of DMs (≳512) are required. The FDD algorithm provides additional performance improvements for fast-Fourier-transform-based periodicity surveys of radio pulsars, as the Fourier transform back to the time domain can be omitted. We expect that this computational performance gain will further improve in the future since the Fourier-domain dedispersion algorithm better matches the trends in technological advancements of GPU development.
ABSTRACT
We report the detection of a bright fast radio burst, FRB 191108, with Apertif on the Westerbork Synthesis Radio Telescope. The interferometer allows us to localize the FRB to a narrow 5 ...arcsec × 7 arcmin ellipse by employing both multibeam information within the Apertif phased-array feed beam pattern, and across different tied-array beams. The resulting sightline passes close to Local Group galaxy M33, with an impact parameter of only 18 kpc with respect to the core. It also traverses the much larger circumgalactic medium (CGM) of M31, the Andromeda Galaxy. We find that the shared plasma of the Local Group galaxies could contribute ∼10 per cent of its dispersion measure of 588 pc cm−3. FRB 191108 has a Faraday rotation measure (RM) of +474 $\pm \, 3$ rad m−2, which is too large to be explained by either the Milky Way or the intergalactic medium. Based on the more moderate RMs of other extragalactic sources that traverse the halo of M33, we conclude that the dense magnetized plasma resides in the host galaxy. The FRB exhibits frequency structure on two scales, one that is consistent with quenched Galactic scintillation and broader spectral structure with Δν ≈ 40 MHz. If the latter is due to scattering in the shared M33/M31 CGM, our results constrain the Local Group plasma environment. We found no accompanying persistent radio sources in the Apertif imaging survey data.
This is the third installment in a series of papers in which we investigate calibration artefacts. Calibration artefacts (also known as ghosts or spurious sources) are created when we calibrate with ...an incomplete model. In the first two papers of this series, we developed a mathematical framework which enabled us to study the ghosting mechanism itself. An interesting concomitant of the second paper was that ghosts appear in symmetrical pairs. This could possibly account for spurious symmetrization. Spurious symmetrization refers to the appearance of a spurious source (the antighost) symmetrically opposite an unmodelled source around a modelled source. The analysis in the first two papers indicates that the antighost is usually very faint, in particular, when a large number of antennas are used. This suggests that spurious symmetrization will mainly occur at an almost undetectable flux level. In this paper, we show that phase-only calibration produces an antighost that is N-times (where N denotes the number of antennas in the array) as bright as the one produced by phase and amplitude calibration and that this already bright ghost can be further amplified by the primary beam correction.
The search for transient phenomena at low radio frequencies is now coming of age with the development of radio sky monitors with a large field of view, which are made feasible by new developments in ...calibration algorithms and computing. However, accurate calibration of such arrays is challenging, especially within the latency requirements of near real-time transient monitors, and is the main cause of limiting their sensitivities. This paper describes a strategy for real-time, wide-field direction-dependent calibration of the Amsterdam-ASTRON Radio Transients Facility and Analysis Center (AARTFAAC) array, which is a sensitive, continuously available all-sky monitor based on the LOw Frequency ARray (LOFAR). The monitor operates in a zenith pointing, snapshot imaging mode for image plane detection of bright radio transients. We show that a tracking calibration approach with solution propagation satisfies our latency, computing, and calibration accuracy constraints. We characterize the instrument and verify the calibration strategy under a variety of observing conditions. This brings out several phenomena, which can bias the calibration. The real-time nature of the application further imposes strict latency and computational constraints. We find that although ionosphere-induced phase errors present a major impediment to accurate calibration, these can be corrected in the direction of the brightest few sources to significantly improve image quality. Our real-time calibration pipeline implementation processes a single spectral channel of a snapshot observation in ~0.2 s on test hardware, which is well within its latency budget. Autonomously calibrating and imaging one second snapshots, our approach leads to a typical image noise of ~10 Jy for a ~90 kHz channel, reaching dynamic ranges of ~2000:1. We also show that difference imaging allows thermal-noise limited transient detection, despite the instrument being confusion-noise limited.
The propagation of radio waves from distant compact radio sources through turbulent interstellar plasma in our Galaxy causes these sources to twinkle, a phenomenon called interstellar scintillation. ...Such scintillations are a unique probe of the micro-arcsecond structure of radio sources as well as of the sub-AU-scale structure of the Galactic interstellar medium. Weak scintillations (i.e. an intensity modulation of a few percent) on timescales of a few days or longer are commonly seen at centimetre wavelengths and are thought to result from the line-of-sight integrated turbulence in the interstellar plasma of the Milky Way. So far, only three sources were known that show more extreme variations, with modulations at the level of some dozen percent on timescales shorter than an hour. This requires propagation through nearby (
d
≲ 10 pc) anomalously dense (
n
e
∼ 10
2
cm
−3
) plasma clouds. Here we report the discovery with Apertif of a source (J1402+5347) showing extreme (∼50%) and rapid variations on a timescale of just 6.5 min in the decimetre band (1.4 GHz). The spatial scintillation pattern is highly anisotropic, with a semi-minor axis of about 20 000 km. The canonical theory of refractive scintillation constrains the scattering plasma to be within the Oort cloud. The sightline to J1402+5347, however, passes unusually close to the B3 star Alkaid (
η
UMa) at a distance of 32 pc. If the scintillations are associated with Alkaid, then the angular size of J1402+5347 along the minor axis of the scintels must be smaller than ≈10
μ
as, yielding an apparent brightness temperature for an isotropic source of ≳10
14
K.
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