Astronomical widefield imaging of interferometric radio data is computationally expensive, especially for the large data volumes created by modern non-coplanar many-element arrays. We present a new widefield interferometric imager that uses the w-stacking algorithm and can make use of the w-snapshot algorithm. The performance dependencies of CASA's wprojection and our new imager are analysed and analytical functions are derived that describe the required computing cost for both imagers. On data from the Murchison Widefield Array, we find our new method to be an order of magnitude faster than w-projection, as well as being capable of full-sky imaging at full resolution and with correct polarisation correction. We predict the computing costs for several other arrays and estimate that our imager is a factor of 2-12 faster, depending on the array configuration. We estimate the computing cost for imaging the low-frequency Square-Kilometre Array observations to be 60 PetaFLOPS with current techniques. We find that combining w-stacking with the w-snapshot algorithm does not significantly improve computing requirements over pure w-stacking. The source code of our new imager is publicly released.
In this paper we explore for the first time the relative magnitudes of three fundamental sources of uncertainty, namely, foreground contamination, thermal noise and sample variance in detecting the Hi power spectrum from the Epoch of Reionization (EoR). We derive limits on the sensitivity of a Fourier synthesis telescope to detect EoR based on its array configuration and a statistical representation of images made by the instrument. We use the Murchison Widefield Array (MWA) configuration for our studies. Using a unified framework for estimating signal and noise components in the Hi power spectrum, we derive an expression for and estimate the contamination from extragalactic point-like sources in three-dimensional k-space. Sensitivity for EoR Hi power spectrum detection is estimated for different observing modes with MWA. With 1000 hours of observing on a single field using the 128tile MWA, EoR detection is feasible (S/N > 1 for k 0.8 Mpc −1 ). Bandpass shaping and refinements to the EoR window are found to be effective in containing foreground contamination, which makes the instrument tolerant to imaging errors. We find that for a given observing time, observing many independent fields of view does not offer an advantage over a single field observation when thermal noise dominates over other uncertainties in the derived power spectrum.
Using the Very Large Array (VLA) the C76α and C53α recombination lines (RLs) have been detected toward the ultra-compact H ii region (UCH ii region) G35.20−1.74. We also obtained upper limits to the carbon RLs at 6 cm (C110α & C111α) and 3.6 cm (C92α) wavelengths with the VLA. In addition, continuum images of the W48A complex ( which includes G35.20−1.74 ) are made with 1 The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under a cooperative agreement -2angular resolutions in the range 14 ′′ to 2 ′′ . Modeling the multi-wavelength line and continuum data has provided the physical properties of the UCH ii region and the photodissociation region (PDR) responsible for the carbon RL emission. The gas pressure in the PDR, estimated using the derived physical properties, is at least four times larger than that in the UCH ii region. The dominance of stimulated emission of carbon RLs near 2 cm, as implied by our models, is used to study the relative motion of the PDR with respect to the molecular cloud and ionized gas. Our results from the kinematical study are consistent with a pressure-confined UCH ii region with the ionizing star moving with respect to the molecular cloud. However, based on the existing data, other models to explain the extended lifetime and morphology of UCH ii regions cannot be ruled out.
We present new low-frequency observations of the nearby radio galaxy Fornax A at 154 MHz with the Murchison Widefield Array, microwave flux-density measurements obtained from WMAP and Planck data, and γ-ray flux densities obtained from Fermi data. We also compile a comprehensive list of previously published images and flux-density measurements at radio, microwave and X-ray energies. A detailed analysis of the spectrum of Fornax A between 154 MHz and 1510 MHz reveals that both radio lobes have a similar spatially-averaged spectral index, and that there exists a steep-spectrum bridge of diffuse emission between the lobes. Taking the spectral index of both lobes to be the same, we model the spectral energy distribution of Fornax A across an energy range spanning eighteen orders of magnitude, to investigate the origin of the X-ray and γ-ray emission. A standard leptonic model for the production of both the X-rays and γ-rays by inverse-Compton scattering does not fit the multi-wavelength observations. Our results best support a scenario where the X-rays are produced by inverse-Compton scattering and the γ-rays are produced primarily by hadronic processes confined to the filamentary structures of the Fornax A lobes.
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