First release of Apertif imaging survey data

¹ ASTRON, the Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
e-mail: adams@astron.nl
² Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
³ Astronomisches Institut der Ruhr-Universität Bochum (AIRUB), Universitätsstrasse 150, 44780 Bochum, Germany
⁴ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
⁵ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁶ Astro Space Center of Lebedev Physical Institute, Profsoyuznaya Str. 84/32, 117997 Moscow, Russia
⁷ Department of Physics, Virginia Polytechnic Institute and State University, 50 West Campus Drive, Blacksburg, VA 24061, USA
⁸ CSIRO Astronomy and Space Science, Australia Telescope National Facility, PO Box 76, Epping, NSW 1710, Australia
⁹ Sydney Institute for Astronomy, School of Physics, University of Sydney, Sydney, New South Wales 2006, Australia
¹⁰ Anton Pannekoek Institute, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
¹¹ NIKHEF, National Institute for Subatomic Physics, Science Park 105, 1098 XG Amsterdam, The Netherlands
¹² Cahill Center for Astronomy, California Institute of Technology, Pasadena, CA, USA
¹³ Department of Electrical Engineering, Chalmers University of Technology, Gothenburg, Sweden
¹⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁵ Department of Physics and Electronics, Rhodes University, PO Box 94, Makhanda 6140, South Africa
¹⁶ National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Pune 411007, Maharashtra, India
¹⁷ Netherlands eScience Center, Science Park 402, 1098 XH Amsterdam, The Netherlands
¹⁸ Department of Physics and Astronomy, West Virginia University, White Hall, PO Box 6315, Morgantown, WV 26506, USA
¹⁹ Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, WV 26505, USA
²⁰ Oxford Astrophysics, Denys Wilkinson Building, University of Oxford, Keble Rd, Oxford OX1 3RH, UK
²¹ Tricas Industrial Design & Engineering, Zwolle, The Netherlands
²² University of Oslo Center for Information Technology, PO Box 1059, 0316 Oslo, Norway

Received: 12 May 2022
Accepted: 5 August 2022

Abstract

Context. Apertif is a phased-array feed system for the Westerbork Synthesis Radio Telescope, providing forty instantaneous beams over 300 MHz of bandwidth. A dedicated survey program utilizing this upgrade started on 1 July 2019, with the last observations taken on 28 February 2022. The imaging survey component provides radio continuum, polarization, and spectral line data.

Aims. Public release of data is critical for maximizing the legacy of a survey. Toward that end, we describe the release of data products from the first year of survey operations, through 30 June 2020. In particular, we focus on defining quality control metrics for the processed data products.

Methods. The Apertif imaging pipeline, Apercal, automatically produces non-primary beam corrected continuum images, polarization images and cubes, and uncleaned spectral line and dirty beam cubes for each beam of an Apertif imaging observation. For this release, processed data products are considered on a beam-by-beam basis within an observation. We validate the continuum images by using metrics that identify deviations from Gaussian noise in the residual images. If the continuum image passes validation, we release all processed data products for a given beam. We apply further validation to the polarization and line data products and provide flags indicating the quality of those data products.

Results. We release all raw observational data from the first year of survey observations, for a total of 221 observations of 160 independent target fields, covering approximately one thousand square degrees of sky. Images and cubes are released on a per beam basis, and 3374 beams (of 7640 considered) are released. The median noise in the continuum images is 41.4 uJy beam⁻¹, with a slightly lower median noise of 36.9 uJy beam⁻¹ in the Stokes V polarization image. The median angular resolution is 11.6″/sin δ. The median noise for all line cubes, with a spectral resolution of 36.6 kHz, is 1.6 mJy beam⁻¹, corresponding to a 3-σ H i column density sensitivity of 1.8 × 10²⁰ atoms cm⁻² over 20 km s⁻¹ (for a median angular resolution of 24″ × 15″). Line cubes at lower frequency have slightly higher noise values, consistent with the global RFI environment and overall Apertif system performance. We also provide primary beam images for each individual Apertif compound beam. The data are made accessible using a Virtual Observatory interface and can be queried using a variety of standard tools.