The Planck clusters in the LOFAR sky. II. LoTSS-DR2: Recovering diffuse extended emission with LOFAR

Context. Extended radio sources in the sky require a dense sampling of short baselines to be properly imaged by interferometers. This problem arises in many areas of radio astronomy, such as in the study of galaxy clusters, which may host megaparsec-scale diffuse synchrotron sources in the form of radio halos. In clusters where no radio halos are detected, owing to intrinsic absence of emission or extrinsic (instrumental and/or observational) effects, it is possible to determine upper limits.
Aims: We consider a sample of Planck galaxy clusters from the Second Data Release of the LOFAR Two Meter Sky Survey (LoTSS-DR2) where no radio halos are detected. We aim to use this sample to test the capabilities of LOFAR to recover diffuse extended emission and derive upper limits.
Methods: Through the injection technique, we simulated radio halos with various surface brightness profiles. We then predicted the corresponding visibilities and imaged them along with the real visibilities. This method allows us to test the fraction of flux density losses owing to inadequate uv coverage and obtain thresholds at which the mock emission becomes undetectable by visual inspection.
Results: The dense uv coverage of LOFAR at short spacings allows us to recover ≳ 90% of the flux density of targets with sizes up to ~15′. We find a relation that provides upper limits based on the image noise and extent (in terms of number of beams) of the mock halo. This relation can be safely adopted to obtain upper limits without injecting when artefacts introduced by the subtraction of the discrete sources are negligible in the central region of the cluster. Otherwise, the injection process and visual inspection of the images are necessary to determine more reliable limits. Through these methods, we obtain upper limits for 75 clusters to be exploited in ongoing statistical studies.