The knowledge of receiver beam shapes is essential for accurate radio interferometric imaging. Traditionally, this information is obtained by holographic techniques or by numerical simulation. However, such methods are not feasible for an observation with time varying beams, such as the beams produced by a phased array radio interferometer. We propose the use of the observed data itself for the estimation of the beam shapes. We use the directional gains obtained along multiple sources across the sky for the construction of a time varying beam model. The construction of this model is an ill posed non linear optimization problem. Therefore, we propose to use Riemannian optimization, where we consider the constraints imposed as a manifold. We compare the performance of the proposed approach with traditional unconstrained optimization and give results to show the superiority of the proposed approach.
Aims: This paper aims to derive an expression for the sensitivity of a polarimetric radio interferometer that is valid for all-sky observations of arbitrarily polarized sources, with neither a restriction on FoV nor with any a priori assumption regarding the polarization state of the source. We verify the resulting formula with an all-sky observation using the Murchison Widefield Array (MWA) telescope. Methods: The sensitivity expression is developed from first principles by applying the concept of System Equivalent Flux Density (SEFD) to a polarimetric radio interferometer not by computing $A…
Radio interferometer arrays with non-homogeneous element patterns are more difficult to calibrate compared to the more common homogeneous array. In particular, the non-homogeneity of the patterns has significant implications on the computational tractability of evaluating the calibration solutions. We apply the A-stacking technique to this problem and explore the trade-off to be made between the calibration accuracy and computational complexity. Through simulations, we show that this technique can be favourably applied in the context of an SKA-Low station. We show that the minimum accuracy req…
NOIRE Study Report: Towards a Low Frequency Radio Interferometer in Space
地面低频无线电干涉仪是在过去十年中研制的，为科学界提供了高质量的观测。相反，目前空间射电天文学仪器的单点观测系统的角分辨率很差。提高低频范围（几kHz至100 MHz）的观测能力需要进入太空，并建立一个可用作干涉仪的天基天线网络。我们介绍了NOIRE（Nanosatellites pour un Observatoire Interférométrique Radio dans l 'Espace/ Nanosatellites for a Radio Interferometer Observatory in Space）研究的结果，该研究在法国国家空间研究中心PASO（轨道系统架构平台是法国国家空间研究中心负责早期任务和概念研究的跨学科团队）的帮助下评估了一群专用于低频无线电观测台的纳米卫星的可行性。在这样的平台上，航天系统工程和仪器研制必须作为一个整体来研究：每个节点都是一个传感器，所有传感器必须一起使用才能获得一个测量。研究的主题如下：系统原理和概念（群、节点同质性）;空间和时间管理（测距、时钟同步）;轨道学（月球轨道、拉格朗日点选项）;电信（节点之间和与地面之间）和联网;测量和处理;推进;动力;电磁兼容性。在初步研究中没有发现强有力的表演障碍，尽管这个概念还没有准备好。确定了几项进一步的研究和里程碑。NOIRE团队将与国际团队合作，尝试建造下一代航天系统。
Radio halos are diffuse, extended sources of radio emission detected primarily in massive, merging galaxy clusters. In smaller and/or relaxed clusters, where no halos are detected, one can instead place upper limits to a possible radio emission. Detections and upper limits are both crucial to constrain theoretical models for the generation of radio halos. The upper limits are model dependent for radio interferometers and thus the process of obtaining these is tedious to perform manually. In this paper, we present a Python based tool to automate this process of estimating the upper limits. The …
In the past two decades, a rebirth of interest in low-frequency radio astronomy for 21 cm tomography of the Epoch of Reionization, has given rise to a new class of radio interferometers with $N \gg 100$ antennas. The availability of low-noise receivers that do not require cryogenic cooling has driven down the cost of antennas, making it affordable to build sensitivity with numerous small antennas rather than large dish structures. However, the computational- and storage-costs of such radio arrays, determined by the $\mathcal{O}(N^2)$ scaling of visibility products required for calibration and …
Redundant calibration is a technique in radio astronomy that allows calibration of radio arrays whose antennas lie on a lattice by exploiting the fact that redundant baselines should see the same sky signal. Because the number of measured visibilities scales quadratically with the number of antennas but the number of unknowns describing the individual antenna responses and the available information about the sky scales only linearly with the array size, the problem is always over-constrained as long as the array is big and dense enough. This is true even for non-lattice array configurations. I…
System equivalent flux density (SEFD) is an important figure of merit of a radio telescope. This paper aims to derive a general expression for SEFD of a polarimetric tripole interferometer. The derivation makes only two basic and reasonable assumptions. First, the noise under consideration is zero mean and when expressed in complex phasor domain, has independent and identically distributed (iid) real and imaginary components. Correlated and non-identically distributed noise sources are allowed as long as the real and imaginary components remain iid. Second, the system noise is uncorrelated bet…
We introduce CALAMITY, a precision bandpass calibration method for radio interferometry. CALAMITY can solve for direction independent gains with arbitrary frequency structure to the high precision required for 21 cm cosmology with minimal knowledge of foregrounds or antenna beams and does not require any degree of redundancy (repeated identical measurements of the same baseline). We have achieved this through two key innovations. Firstly, we model the foregrounds on each baseline independently using a flexible and highly efficient set of basis functions that have minimal overlap with 21 cm mod…
GALARIO: a GPU Accelerated Library for Analysing Radio Interferometer Observations
我们提出了GALARIO，一个计算库，利用现代图形处理单元（GPU）的力量，以加速从无线电干涉仪，如ALMA或VLA的观测分析。GALARIO加速了从通用2D模型图像或径向亮度分布（对于轴对称源）计算合成亮度的速度。在GPU上，GALARIO比标准Python快150倍，比CPU上的串行C++代码快10倍。高度模块化，易于使用并在现有代码中采用，GALARIO有两个编译库，一个用于Nvidia GPU，一个用于多核CPU，两者具有相同的功能和相同的接口。GALARIO附带Python绑定，但也可以直接在C或C++中使用。GALARIO的多功能性和速度开辟了新的分析途径，否则将非常耗时，例如拟合大量物体的高分辨率观测，或分子气体发射的整个光谱立方体。它是一种通用工具，可应用于任何使用射电干涉仪观测的领域。源代码可以在https://github.com/mtazzari/galario上获得，并遵循开放源代码GNU Lesser General Public License v3。
Future generations of radio interferometers targeting the 21\,cm signal at cosmological distances with $N\gg 1000$ antennas could face a significant computational challenge in building correlators with the traditional architecture, whose computational resource requirement scales as $\mathcal{O}(N^2)$ with array size. The fundamental output of such correlators is the cross-correlation products of all antenna pairs in the array. The FFT-correlator architecture reduces the computational resources scaling to $\mathcal{O}(N\log{N})$ by computing cross-correlation products through a spatial Fourier …
The radio sky at lower frequencies, particularly below 20 MHz, is expected to be a combination of increasingly bright non-thermal emission and significant absorption from intervening thermal plasma. The sky maps at these frequencies cannot therefore be obtained by simple extrapolation of those at higher frequencies. However, due to severe constraints in ground-based observations, this spectral window still remains greatly unexplored. In this paper, we propose and study, through simulations, a novel minimal configuration for a space interferometer system which would enable imaging of the radio …
Montblanc: GPU accelerated Radio Interferometer Measurement Equations in support of Bayesian Inference for Radio Observations
University of Cape Town（开普敦大学）；University of the Western Cape（西开普大学）；Rhodes University（罗德斯大学）；Observatoire de Paris（巴黎天文台）；CNRS（法国国家科学研究中心）；Université Paris Diderot（巴黎狄德罗大学）；SKA South Africa（SKA南非）
Observation of Ultra-high-energy Cosmic Rays with the ANITA Balloon-borne Radio Interferometer
我们报告了16个平均能量为1.5 x 10^{19} eV的宇宙射线事件的观测结果，这些事件是通过宇宙射线空气簇射与南极地磁场相互作用产生的无线电脉冲进行的，这一过程被称为地球同步辐射。我们提出了第一个超宽带，远场测量的无线电频谱密度的地球同步辐射的范围从300-1000兆赫。发射在垂直于投影地磁场的平面中是100%线性偏振的。我们观察到的事件中有14个被认为是由于无线电波束从冰面反射而发生的相位反转，另外两个事件是直接从地平线上方看到的。
