Systematic effects in LOFAR data: A unified calibration strategy

Abstract

Context. New generation low-frequency telescopes are exploring a new parameter space in terms of depth and resolution. The data taken with these interferometers, for example with the LOw Frequency ARray (LOFAR), are often calibrated in a low signal-to-noise ratio regime and the removal of critical systematic effects is challenging. The process requires an understanding of their origin and properties. Aims. In this paper we describe the major systematic effects inherent to next generation low-frequency telescopes, such as LOFAR. With this knowledge, we introduce a data processing pipeline that is able to isolate and correct these systematic effects. The pipeline will be used to calibrate calibrator observations as the first step of a full data reduction process. Methods. We processed two LOFAR observations of the calibrator 3C 196: the first using the Low Band Antenna (LBA) system at 42-66 MHz and the second using the High Band Antenna (HBA) system at 115-189 MHz. Results. We were able to isolate and correct for the effects of clock drift, polarisation misalignment, ionospheric delay, Faraday rotation, ionospheric scintillation, beam shape, and bandpass. The designed calibration strategy produced the deepest image to date at 54 MHz. The image has been used to confirm that the spectral energy distribution of the average radio source population tends to flatten at low frequencies.Conclusions. We prove that LOFAR systematic effects can be described by a relatively small number of parameters. Furthermore, the identification of these parameters is fundamental to reducing the degrees of freedom when the calibration is carried out on fields that are not dominated by a strong calibrator.

Low-frequency Radio Absorption in Tycho’s Supernova Remnant

“…The LBA data were reduced with the LOFAR Low-Frequency Pipeline (de Gasperin et al 2019). The pipeline calibrates the calibrator and transfers the solutions to the target, taking into account the main systematic effects in the LOFAR telescope, such as clock drift, polarisation misalignment, ionospheric delay, Faraday rotation, ionospheric scintillation, beam shape, and bandpass.…”

Particle acceleration in a nearby galaxy cluster pair: the role of cluster dynamics

“…This latest version of the pipeline includes improvements in the calibration and imaging of extended sources and is currently adopted to process LoTSS observations (see Section 5 in Shimwell et al 2019), and will be thoroughly discussed in Tasse et al (in prep.). The data processing exploits prefactor Williams et al 2016;de Gasperin et al 2019), killMS (Tasse 2014a,b;Smirnov & Tasse 2015) and DDFacet (Tasse et al 2018) to perform direction-independent and direction-dependent calibration and imaging of the entire LOFAR field-of-view (FoV). To improve the image quality towards the target field, we subtract out from the uv-data all the sources in the LOFAR FoV using the models derived from the pipeline except those in a 38.3 × 38.3 region containing RXCJ1825 and CIZAJ1824.…”

LOFAR Discovery of a Radio Halo in the High-redshift Galaxy Cluster PSZ2 G099.86+58.45

“…PSZ2G099 is located at ∼ 15 from the center of pointing P214 + 55 (in the region of the HET-DEX Spring Field), acquired on 14 May 2015 (ObsID L343224). Data reduction of this pointing was performed with the pipeline described by Shimwell et al (2019), which performs direction-independent and dependent calibration and imaging of the full LOFAR fieldof-view, using prefactor (van Weeren et al 2016b;Williams et al 2016;de Gasperin et al 2019;Mechev et al 2018), killMS (Tasse 2014a,b;Smirnov & Tasse 2015) and DDFacet (Tasse et al 2018). To improve the image quality in the direction of PSZ2G099, we used the products of the pipeline, subtracted all the sources outside a region of 15 × 15 surrounding the target, and performed extra phase and amplitude self-calibration loops in this sub-field (more details by van Weeren et al in prep.).…”