摘要
A great challenge for 21 cm intensity mapping experiments is the strong foreground radiation which is orders of magnitude brighter than the 21 cm signal.Removal of the foreground takes advantage of the fact that its frequency spectrum is smooth while the redshifted 21 cm signal spectrum is stochastic.However,a complication is the nonsmoothness of the instrument response.This paper describes the electromagnetic simulation of the Tianlai cylinder array,a pathfinder for 21 cm intensity mapping experiments.Due to the vast scales involved,a direct simulation requires a large amount of computing resources.We have made the simulation practical by using a combination of methods:first simulate a single feed,then an array of feed units,finally with the feed array and a cylindrical reflector together,obtain the response for a single cylinder.We studied its radiation pattern,bandpass response and the effects of mutual coupling between feed units,and compared the results with observation.Many features seen in the measurement result are reproduced well in the simulation,especially the oscillatory features which are associated with the standing waves on the reflector.The mutual coupling between feed units is quantified with Sparameters,which decrease as the distance between the two feeds increases.Based on the simulated S-parameters,we estimate the correlated noise which has been seen in the visibility data,and the results show very good agreement with the data in both magnitude and frequency structures.These results provide useful insights on the problem of 21 cm signal extraction for real instruments.
基金
supported by the Ministry of Science and Technology(MOST)-BRICS Flagship Project 2018YFE0120800
National SKA Program of China No.2020SKA0110401
the National Key R&D Program 2017YFA0402603
the National Natural Science Foundation of China(NSFC,Grant Nos.11973047,11633004 and U1631118)
the Chinese Academy of Sciences(CAS)Strategic Priority Research Program XDA15020200
the CAS Frontier Science Key Project QYZDJ–SSW–SLH017
the CAS Inter-disciplinary Innovation Team(JCTD-2019-05)
the CAS Key Instruments project ZDKYYQ20200008
the Hebei Key Laboratory of Radio Astronomy Technology(HKLRAT)。