TIFR-GMRT sky survey (TGSS): an alternate approach to data analysis

Abstract

“TIFR GMRT SKY SURVEY (TGSS)” is a continuum survey at 150 MHz conducted between April 2010 and March 2012 using Giant Metre wave Radio Telescope facility at Pune, India for the whole sky visible from the observatory. Its resolution is significant comparable to existing centimetre surveys despite being a meter wavelength survey- in fact it is at present the highest resolution survey below 200 MHz! [25] There are unique chal lenges in processing of high-resolution radio images at low frequencies due to instrumental instabilities, Radio Frequency Interference, Direction De pendent ionospheric delays etc. Often compromises are made for timely release of images only to be reprocessed whenever there are technical im provements in algorithm, knowledge of primary beam etc only to witness significant improvement in dynamic range through better RMS flux and in creased identification of sources. Recently GMRT has been upgraded and there is public release of a upgraded GMRT continuum processing pipeline with hope of promoting open science. It is being called ‘CAPTURE’: A “CAsa Pipeline-cum-Toolkit for Upgraded Giant Metrewave Radio Tele scope data REduction uGMRT-pipeline” [29]. It promises several benefits and it should equally work with legacy GMRT data. We have collected TGSS raw data from GMRT archive accessible at “https://naps.ncra.tifr.res.in/goa/data/search” corresponding to a well known field with multi-wavelength data GAMA G23 [ RA= 23 h, DEC= -32.5 deg search radius= 300’]. Further data was also downloaded so that scans covering all the 29 phase calibrators and also for few phase calibra tors all scans using them in TGSS were downloaded. We had to tweak settings in CAPTURE regarding flagging to image the TGSS data. First using the 9 pointings from GAMA G23 we identified 2 settings that gives best images and they worked with all other scans we downloaded so far. to We have compared flux scale and dynamic range between images we made and those available as TGSS-ADR cutout services. It is important to note that although ADR images have undergone Direc tion Dependent calibration, our images are limited to only rounds of self calibrations (Direction Independent calibration) we can almost match the dynamic range of ADR. Though apparently RMS noise in our image is slightly higher but it is the flux scale accuracy which is most important. Further we can confirm that flux in our analysis is consistent to existing literature and we do identify a few cases of flux discrepancy when working with ADR image mosaic/cutout. Although this may be expected since no pipeline (including SPAM which ADR is based on[24]) is perfect; however it makes this work important and distinguishable from rest- • We intend to establish an accurate flux density scale for TGSS and also inform any existing issues with ADR. Fur ther no previous work has matched the flux of all 29 phase calibrators in TGSS as per own analysis and that available from ADR MOSAIC/cutout to understand flux issues. Also the variations of flux for same phase calibrator but in different scans was investigated for atleast 5 phase calibrators. • Promote Open Science:All want free publicly available pipelines for repetitive but demanding task such as raw interferometric data re duction. Because without pipelines, the science ready images/ prod ucts from raw radio interferometry data will always depend on the choices a group of researchers use to make those for themselves while other researchers will remain skeptical. This work tested the per formance of ‘CAPTURE’ pipeline at 150 Mhz with legacy GMRT data, issues were learnt and some improvements on usage was made, and we hope it will become a popular choice (being reliable, free and publicly available) to be used by all when it comes to working with legacy GMRT and upgraded GMRT continuum survey data in near future. • We made mosaic of GAMA G23 field, from there we have obtained Differential Source Count and spectral index. Further this work may help constrain foregrounds to probe Epoch of Reionization at redshift 8.5 since the frequency 150MHz corresponds to the redshifted H21 cm signal from the epoch.