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We apply two methods to estimate the 21-cm bispectrum from data taken within the Epoch of Reionisation (EoR) project of the Murchison Widefield Array (MWA). Using data acquired with the Phase II compact array allows a direct bispectrum estimate to be undertaken on the multiple redundantly spaced triangles of antenna tiles, as well as an estimate based on data gridded to the uv-plane. The direct and gridded bispectrum estimators are applied to 21 h of high-band (167–197 MHz; z = 6.2–7.5) data from the 2016 and 2017 observing seasons. Analytic predictions for the bispectrum bias and variance for point-source foregrounds are derived. We compare the output of these approaches, the foreground contribution to the signal, and future prospects for measuring the bispectra with redundant and non-redundant arrays. We find that some triangle configurations yield bispectrum estimates that are consistent with the expected noise level after 10 h, while equilateral configurations are strongly foreground-dominated. Careful choice of triangle configurations may be made to reduce foreground bias that hinders power spectrum estimators, and the 21-cm bispectrum may be accessible in less time than the 21-cm power spectrum for some wave modes, with detections in hundreds of hours.
We provide the first in situ measurements of antenna element beam shapes of the Murchison Widefield Array. Most current processing pipelines use an assumed beam shape, which can cause absolute and relative flux density errors and polarisation ‘leakage’. Understanding the primary beam is then of paramount importance, especially for sensitive experiments such as a measurement of the 21-cm line from the epoch of reionisation, where the calibration requirements are so extreme that tile to tile beam variations may affect our ability to make a detection. Measuring the primary beam shape from visibilities is challenging, as multiple instrumental, atmospheric, and astrophysical factors contribute to uncertainties in the data. Building on the methods of Neben et al. [Radio Sci., 50, 614], we tap directly into the receiving elements of the telescope before any digitisation or correlation of the signal. Using ORBCOMM satellite passes we are able to produce all-sky maps for four separate tiles in the XX polarisation. We find good agreement with the beam model of Sokolowski et al. [2017, PASA, 34, e062], and clearly observe the effects of a missing dipole from a tile in one of our beam maps. We end by motivating and outlining additional on-site experiments.
We describe the motivation and design details of the ‘Phase II’ upgrade of the Murchison Widefield Array radio telescope. The expansion doubles to 256 the number of antenna tiles deployed in the array. The new antenna tiles enhance the capabilities of the Murchison Widefield Array in several key science areas. Seventy-two of the new tiles are deployed in a regular configuration near the existing array core. These new tiles enhance the surface brightness sensitivity of the array and will improve the ability of the Murchison Widefield Array to estimate the slope of the Epoch of Reionisation power spectrum by a factor of ∼3.5. The remaining 56 tiles are deployed on long baselines, doubling the maximum baseline of the array and improving the array u, v coverage. The improved imaging capabilities will provide an order of magnitude improvement in the noise floor of Murchison Widefield Array continuum images. The upgrade retains all of the features that have underpinned the Murchison Widefield Array’s success (large field of view, snapshot image quality, and pointing agility) and boosts the scientific potential with enhanced imaging capabilities and by enabling new calibration strategies.
In an Effort to extend and accelerate regional cooperation and coordination in the “War on Drugs” in the Western Hemisphere, President George Bush hosted a widelypublicized, regional, anti-drug presidential summit in San Antonio (Texas) on 26-27 February 1992. This cumbre was conceived as an expanded sequel to the first “Andean” drug summit held in Cartagena (Colombia) on 15 February 1990. In addition to the original four-country participants in Cartagena I — the United States, Colombia, Peru, and Bolivia — Ecuador, Venezuela, and Mexico attended Cartagena II as well.
In September 1989, President Bush outlined a comprehensive, multi-faceted drug control strategy with both national and international dimensions. The strategy focused on reducing both the demand and supply of illicit drugs. Treatment, prevention/education, research, law enforcement, and international efforts are major components of the strategy. An important goal of the strategy was to reduce the amount of illicit drugs illegally entering the United States by 15% within 2 years and by 60% within 10 years. The president refined the strategy and forwarded it to Congress on 25 January 1990 (US-ONDCP, 1990: 49-52, 120-121). The following year, in February 1991, policymakers modified goals to a 20% reduction by 1993 and a 65% reduction by the year 2001 (US-ONDCP, 1991: 15).
The Flow of drugs from Mexico to the United States has been a source of trouble in US-Mexican relations for at least two decades. The dominant view in Mexico is that the problem arises from the inability of the United States to control its domestic demand for heroin, cocaine, and marijuana. The dominant US view has been that the Mexican government has failed to make effective efforts to control the supply of drugs. At times — in particular after the killing of Enrique Camarena, an agent of the Drug Enforcement Administration (DEA), in 1985 — US government anger at Mexico's alleged failure to maintain the integrity of its anti-drug efforts has been the dominant source of friction between the two nations.
In Just 20 Years Peru has shifted from beacon of hope to basket case. As late as the mid-1970s, Peru's reformist military government (1968-1980) appeared to offer significant possibilities for economic and political development (defined as improved distribution of income and greater participation by the citizenry). From 1940 to 1975, economic growth and low inflation had been the norm. A major agrarian reform during the military docenio (12-year rule) created production cooperatives nation wide; the industrial community gave workers a meaningful management role in the operation of their firms. Both stirred the imagination of many Peruvians and the academic community alike.
Paraguay and its closest neighbors, the Rio Plata Basin from one point of view or the Southern Cone from another, have experienced an increasing challenge from the drug traffic in recent years. Initially, everything linked to drug use and traffic was considered—in general, much oversimplified terms — mainly as the social problem of a rich society, primarily that of the United States. The South American countries, preoccupied with surviving the blows of the “lost decade” while trying, simultaneously, both to throw off authoritarian regimes in terminal crisis and to negotiate transitions from democracy, assumed this problem could not affect them. In any event, that aspect of the drug trade which concerned the countries of South America above all was the growing tragedy of Colombia, which was just beginning to make headlines in the world press.