Apercal—The Apertif calibration pipeline

Adebahr, B.; Schulz, R.; Dijkema, T. J.; Moss, V. A.; Offringa, A. R.; Kutkin, A. M.; Hulst, J. M. van der; Frank, B. S.; Vilchez, N.; Verstappen, J.; Adams, Everette; Blok, W. J. G. de; Dénes, H.; Hess, Kelley M.; Lucero, D. M.; Morganti, R.; Oosterloo, Tom; Pisano, D. J.; Ivashina, Marianna; Cappellen, W. A. van; Connor, Liam; Coolen, A. H. W. M.; Damstra, S.; Loose, G. M.; Maan, Yogesh; Maccagni, F. M.; Mika, Á.; Mulder, H.; Oostrum, L. C.; Orrù, E.; Smits, R.; Schuur, D. van der; Leeuwen, J. van; Vohl, Dany; Wijnholds, Stefan J.; Ziemke, J.

doi:10.1016/j.ascom.2021.100514

Cited by 9 publications

(9 citation statements)

References 41 publications

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“…During the SVC period, the imaging data were automatically run through the Apertif imaging pipeline (Apercal; Adebahr et al 2022). In the following, we want to describe the main calibration and imaging steps performed by Apercal which are relevant for the generation of the continuum and polarisation data products Notes.…”

Section: Data Reductionmentioning

confidence: 99%

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The Apertif science verification campaign

Adebahr

Berger

Adams

et al. 2022

A&A

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Context. The characteristics of the polarised radio sky are a key ingredient in constraining evolutionary models of magnetic fields in the Universe and their role in feedback processes. The origin of the polarised emission and the characteristics of the intergalactic medium on the line of sight can be investigated using large samples of polarised sources. Ancillary infrared (IR) and optical data can be used to study the nature of the emitting objects. Aims. We analyse five early science datasets from the APERture Tile in Focus (Apertif) phased array feed system to verify the polarisation capabilities of Apertif in view of future larger data releases. We aim to characterise the source population of the polarised sky in the L-Band using polarised source information in combination with IR and optical data. Methods. We use automatic routines to generate full field-of-view Q- and U-cubes and perform rotation measure (RM)-Synthesis, source finding, and cross-matching with published radio, optical, and IR data to generate polarised source catalogues. All sources were inspected individually by eye for verification of their IR and optical counterparts. Spectral energy distribution (SED)-fitting routines were used to determine photometric redshifts, star-formation rates, and galaxy masses. IR colour information was used to classify sources as active galactic nuclei (AGN) or star-forming-dominated and early- or late-type. Results. We surveyed an area of 56 deg2 and detected 1357 polarised source components in 1170 sources. The fraction of polarised sources is 10.57% with a median fractional polarisation of 4.70 ± 0.14%. We confirmed the reliability of the Apertif measurements by comparing them with polarised cross-identified NVSS sources. Average RMs of the individual fields lie within the error of the best Milky Way foreground measurements. All of our polarised sources were found to be dominated by AGN activity in the radio regime with most of them being radio-loud (79%) and of the Fanaroff-Riley (FR)II class (87%). The host galaxies of our polarised source sample are dominated by intermediate disc and star-forming disc galaxies. The contribution of star formation to the radio emission is on the order of a few percent for ≈10% of the polarised sources while for ≈90% it is completely dominated by the AGN. We do not see any change in fractional polarisation for different star-formation rates of the AGN host galaxies. Conclusions. The Apertif system is suitable for large-area high-sensitivity polarised sky surveys. The data products of the polarisation analysis pipeline can be used to investigate the Milky Way magnetic field on projected scales of several arcminutes as well as the origin of the polarised emission in AGN and the properties of their host galaxies.

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Section: Data Reductionmentioning

confidence: 99%

“…For each self-calibration cycle, the solution interval was decreased and the (u, v)-range of data to include was increased. The specific details of the self-calibraton routines are described in Adebahr et al (2022).…”

Section: Data Reductionmentioning

confidence: 99%

The Apertif science verification campaign

Adebahr

Berger

Adams

et al. 2022

A&A

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“…Survey observations are automatically processed by Apercal 3 , the Apertif imaging and calibration pipeline, via an automated trigger, autocal. Full details of this pipeline are provided in Adebahr et al (2022b). A brief description of the processing is provided below.…”

Section: Apercal Processingmentioning

confidence: 99%

First release of Apertif imaging survey data

Adams

Adebahr

Blok

et al. 2022

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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 1.8×10 20 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.

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“…In a single observation, Apertif covers forty adjacent CBs with a total area of ∼ 10 square degrees. The raw data for every CB is processed independently with the automatic pipeline which includes flagging, calibration and imaging (Adebahr et al 2022). For ADR1, the processed frequency range covers the 1292.5-1430 MHz interval.…”

Section: The Datamentioning

confidence: 99%

Continuum source catalog for the first APERTIF data release

Kutkin

Oosterloo

Morganti

et al. 2022

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The first data release of Apertif survey contains 3074 radio continuum images covering a thousand square degrees of the sky. The observations were performed during August 2019 to July 2020. The continuum images were produced at a central frequency 1355 MHz with the bandwidth of ∼ 150 MHz and angular resolution reaching 10 . In this work we introduce and apply a new method to obtain a primary beam model using a machine learning approach, Gaussian process regression. The primary beam models obtained with this method are published along with the data products for the first Apertif data release. We apply the method to the continuum images, mosaic them and extract the source catalog. The catalog contains 249672 radio sources many of which are detected for the first time at these frequencies. We cross-match the coordinates with the NVSS, LOFAR/DR1/value-added and LOFAR/DR2 catalogs resulting in 44523 , 22825 and 152824 common sources respectively. The first sample provides a unique opportunity to detect long term transient sources which have significantly changed their flux density for the last 25 years. The second and the third ones combined together provide information about spectral properties of the sources as well as the redshift estimates.

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Apercal—The Apertif calibration pipeline

Cited by 9 publications

References 41 publications

The Apertif science verification campaign

The Apertif science verification campaign

First release of Apertif imaging survey data

Continuum source catalog for the first APERTIF data release

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