High Sensitivity Imaging at Low Radio Frequencies

Abstract

The Giant Metrewave Radio Telescope (GMRT)[1] currently is the most sensitive low frequency radio telescope in operation. However, there are many problems associated with low frequency radio observations(chap 2). Low fre- quency radio observation is seriously a ect by man-made radio frequency interference and dynamics of the ionosphere. This requires long observation time with high sampling rate in order to obtain good sensitivity. Therefore the data size becomes very large, even as big as 50-200 GB. The huge size of les and recursive way of calibration requires a lot of computing resources. Visual inspection, which is the traditional way of identifying bad data, is to- tally impractical with these large data sizes.We have implemented a hybrid scheme (Fig.2.7) wherein we carry out most of the analysis on a decimated data set obtained by averaging over several time intervals and channels (sec- tion 2.1.1). The key to this scheme is the ability to transfer the results of data processing from the decimated le to the original le(section 2.1.2). With this we save computation time without losing the sensitivity a orded by the full data. We have also implemented algorithm to detect, quantify and correct deviations in residual data if possible else ag them (section 2.1.3). Altogether, we have developed a software tool(Fig.2.7) which handles large data set e ciently and remove many kind of corrupted data by analysing residual UV data le, which improves sensitivity. We planned to look at two problems which require high sensitivity imag- ing at low radio frequencies: Feedback from Active Galactic nuclei(AGN)into the cluster medium (section 3.1) and O -pulse emission of Pulsars(section 3.2).