用ORBCOMM原位测量MWA初级束流变化
In situ measurement of MWA primary beam variation using ORBCOMM
摘要：
我们提供了第一个在原地测量的天线元件（瓦）波束形状的默奇森宽场阵列（MWA），低射频干涉仪和SKA前体。大多数当前MWA处理管线使用假定的波束形状，其中的误差可导致绝对和相对通量密度误差以及偏振“泄漏”。这使得理解原束至关重要，特别是对于敏感的实验，如测量再电离时期（EoR）的21厘米线。测量EoR 21 cm线的校准要求非常严格，以至于瓦片之间的光束变化可能会影响我们进行检测的能力。单独从vibrant测量初级束流形状是具有挑战性的，因为多种仪器，大气和天体物理因素会导致数据的不确定性。基于Neben等人（2015）的方法，我们在信号的任何数字化或相关之前直接接入MWA的接收元件。使用ORBCOMM卫星通道，我们能够为XX偏振中的4个单独的瓦片生成全天图。我们发现与Sokolowski等人（2017）的尖端“完全”嵌入式元件（FEE）模型非常一致，并观察到MWA波束形成器在我们的波束图的可靠区域中始终将波束形状重建到约1dB以内。我们还清楚地观察到一个失踪的偶极子从我们的一个波束图瓦片的影响，并表明，FEE模型是能够重现这种修改后的波束形状。最后，我们通过激励和概述额外的现场实验，以进一步约束原光束的行为。
英文摘要：
We provide the first in situ measurements of antenna element (tile) beam shapes of the Murchison Widefield Array (MWA), a low radio-frequency interferometer and an SKA precursor. Most current MWA processing pipelines use an assumed beam shape, errors in which can cause absolute and relative flux density errors, as well as polarisation 'leakage'. This makes understanding the primary beam of paramount importance, especially for sensitive experiments such as a measurement of the 21 cm line from the epoch of reionisation (EoR). The calibration requirements for measuring the EoR 21 cm line are so extreme that tile to tile beam variations may affect our ability to make a detection. Measuring the primary beam shape from visibilities alone is challenging, as multiple instrumental, atmospheric, and astrophysical factors contribute to uncertainties in the data. Building on the methods of Neben et al. (2015), we tap directly into the receiving elements of the MWA before any digitisation or correlation of the signal. Using ORBCOMM satellite passes we are able to produce all-sky maps for 4 separate tiles in the XX polarisation. We find good agreement with the cutting-edge 'fully' embedded element (FEE) model of Sokolowski et al. (2017), and observe that the MWA beamformers consistently recreate beam shapes to within ~1dB in the reliable areas of our beam maps. We also clearly observe the effects of a missing dipole from a tile in one of our beam maps, and show that the FEE model is able to reproduce this modified beam shape. We end by motivating and outlining additional onsite experiments to further constrain the primary beam behaviour.
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