[Paper Review] The GMRT 150 MHz All-sky Radio Survey: First Alternative Data Release TGSS ADR1

H. T. Intema, P. Jagannathan+2 more|eSpace (Curtin University)|Mar 14, 2016

Radio Astronomy Observations and TechnologyPhysics and Astronomy85 references305 citations

TL;DR

This paper presents the first full public data release of TGSS ADR1, a 150 MHz all-sky radio survey with high angular resolution, produced with direction-dependent ionospheric calibration, yielding 0.62 million sources over 36,900 deg^2 with ~5 mJy/beam noise and ~25 arcsec resolution.

ABSTRACT

We present the first full release of a survey of the 150 MHz radio sky, observed with the Giant Metrewave Radio Telescope between April 2010 and March 2012 as part of the TGSS project. Aimed at producing a reliable compact source survey, our automated data reduction pipeline efficiently processed more than 2000 hours of observations with minimal human interaction. Through application of innovative techniques such as image-based flagging, direction-dependent calibration of ionospheric phase errors, correcting for systematic offsets in antenna pointing, and improving the primary beam model, we created good quality images for over 95 percent of the 5336 pointings. Our data release covers 36,900 square degrees (or 3.6 pi steradians) of the sky between -53 deg and +90 deg DEC, which is 90 percent of the total sky. The majority of pointing images have a background RMS noise below 5 mJy/beam with an approximate resolution of 25" x 25" (or 25" x 25" / cos (DEC - 19 deg) for pointings south of 19 deg DEC). We have produced a catalog of 0.62 Million radio sources derived from an initial, high reliability source extraction at the 7 sigma level. For the bulk of the survey, the measured overall astrometric accuracy is better than 2" in RA and DEC, while the flux density accuracy is estimated at ~10 percent. Within the scope of the TGSS ADR project, the source catalog as well as 5336 mosaic images (5 deg x 5 deg) and an image cutout service, are made publicly available online as a service to the astronomical community. Next to enabling a wide range of different scientific investigations, we anticipate that these survey products provide a solid reference for various new low-frequency radio aperture array telescopes (LOFAR, LWA, MWA, SKA-low), and can play an important role in characterizing the EoR foreground. The TGSS ADR project aims at continuously improving the quality of the survey data products.

Motivation & Objective

• Motivate and enable low-frequency, high-resolution all-sky radio surveys as a reference for current and future facilities (LOFAR, LWA, MWA, SKA-low).
• Develop and demonstrate a robust, automated data processing pipeline capable of direction-dependent calibration to correct ionospheric effects and pointing/beam systematics.
• Produce high-quality imaging products (mosaics) and a compact source catalog suitable for wide scientific use and cross-wavelength studies.

Proposed method

• Reprocessed 150 MHz GMRT data (2010–2012) with a fully automated SPAM pipeline that includes direction-dependent ionospheric calibration.
• Pre-processing transfers calibrations from the best primary calibrator scans and applies sky-temperature based system temperature corrections.
• Direction-independent calibration with wide-field imaging and self-calibration to build an initial sky model and perform phase-only and amplitude calibrations.
• Direction-dependent calibration (peeling) of bright sources to derive ionospheric phase screens and generate facet-based gain tables for wide-field imaging.
• Iterative flagging, ripples detection, and subtraction of residuals, followed by bandpass calibration and final phase-only self-calibration to produce the fourth self-calibrated image.
• Astrometry checks against NVSS/SUMSS/MGPS-2 and flux calibration refinements, with mosaicking of 5°×5° tiles into the final all-sky products.

Experimental results

Research questions

• RQ1 What is the achievable astrometric and flux-density accuracy for a high-resolution, low-frequency all-sky survey after applying direction-dependent ionospheric calibration?
• RQ2 How many pointings and over what sky area can a robust, automated pipeline produce high-quality images and a reliable source catalog at 150 MHz?
• RQ3 What are the properties (noise, resolution, source counts) of the TGSS ADR1 catalog and images, and how do they compare to other low-frequency surveys?
• RQ4 Can the TGSS ADR1 products serve as a reference for current and upcoming low-frequency aperture arrays (LOFAR, LWA, MWA, SKA-low) and EoR foreground studies?
• RQ5 What ongoing improvements are planned to further enhance data quality and sky coverage in subsequent TGSS releases?

Key findings

• Survey covers 36,900 deg^2 (3.6π sr) of the sky between -53° and +90° declination.
• Median RMS noise is 3.5 mJy/beam with an angular resolution of 25″ × 25″ (or 25″ × 25″/cos(DEC−19°) south of −19°).
• Catalog contains 0.62 million radio sources.
• Astrometric accuracy is typically better than 2″ in RA and DEC for most fields; flux-density accuracy is ~10%.
• Image quality and source catalogs were produced for 5336 pointings (96.7% of the grid), with 5452 images across 5336 pointing locations and 5336 mosaic 5°×5° images released publicly.
• About 3.3% of pointings (177) have no image due to data issues; 144 pointings in the lowest DEC range were excluded due to severe calibration challenges.
• Serves as a high-resolution, low-frequency reference catalog for current and future instruments and EoR foreground characterization.