The LOFAR Two Metre Sky Survey: Deep Fields II. The ELAIS-N1 LOFAR deep field

J. Sabater, P. N. Best, C Tasse, M. J. Hardcastle, T W Shimwell, David Nisbet, V Jelic, J. R. Callingham, H. J. A. Rottgering, M. Bonato, M Bondi, B. Ciardi, R K Cochrane, M J Jarvis, Rohit Kondapally, Léon V. E. Koopmans, S. P. O'Sullivan, I. Prandoni, Dominik J. Schwarz, D. J. B. SmithL Wang, W. L. Williams, Saleem Zaroubi

Research output: Contribution to journalArticlepeer-review

Abstract

The LOFAR Two-metre Sky Survey (LoTSS) will cover the full northern sky and, additionally, aims to observe the LoTSS Deep Fields to a noise level of ≤ 10 μJy beam −1 over several tens of square degrees in areas that have the most extensive ancillary data. This paper presents the ELAIS-N1 deep field, the deepest of the LoTSS Deep Fields to date. With an effective observing time of 163.7 hours it reaches a RMS noise level of ≤20μ Jy beam −1 in the central region (and below 30μ Jy beam − 1 over 10 square degrees). The resolution
is ∼ 6 arcsecs and 84862 radio sources were detected in the full area (68 square degrees) with 74127 sources in the highest quality area at less than 3 degrees from the pointing centre. The observation reaches a sky density of more than 5000 sources per square degree in the central region (∼5 square degrees). We present the calibration procedure, which addresses the special configuration of some observations and the extended bandwidth covered (115 to 177 MHz; central frequency 146.2 MHz) compared to standard LoTSS. We also describe the methods used to calibrate the flux density scale using cross-matching with sources detected by other radio surveys in the literature. We find the flux density uncertainty related to the flux density scale to be ∼ 6.5 per cent. By studying the variations of the flux density measurements between different epochs we show that relative flux density calibration is reliable out to about 3 degrees radius, but that additional flux density uncertainty is present for all sources at about the 3 per cent level; this is likely to be associated with residual calibration errors, and is shown to be more significant in datasets with poorer ionosphere conditions. We also provide intra-band spectral indices which can be useful to detect sources with unusual spectral properties. The final uncertainty in the flux densities is estimated to be ∼ 10 per cent for ELAIS-N1.
Original languageEnglish
Article numberA2
Pages (from-to)1-20
Number of pages20
JournalAstronomy and Astrophysics
Volume648
DOIs
Publication statusPublished - 7 Apr 2021

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