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, D. Nisbet, V. Jelic, J. R. Callingham, H. J. A. Rottgering, M. Bonato, M. Bondi, B. Ciardi, R. K. Cochrane, M. J. Jarvis, R. Kondapally, L. V. E. Koopmans, S. P. O'Sullivan, I. Prandoni, D. J. Schwarz, D. J. B. SmithL. Wang, W. L. Williams, S. Zaroubi

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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 microJy/bm 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 root mean square (RMS) noise level below 20 microJy/bm in the central region (and below 30 microJy/bm over 10 square degrees). The resolution is 6 arcsecs and 84862 radio sources were detected in the full area (68 sq. deg.) 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 sq. deg. in the central ~5 sq. deg. region. 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%. By studying the variations of the flux density measurements between different epochs, we show that relative flux density calibration is reliable out to about a 3 degree 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% for ELAIS-N1.
Original languageEnglish
Article numberA2
Number of pages20
JournalAstronomy & Astrophysics
Issue numberApril 2021
Early online date7 Apr 2021
Publication statusE-pub ahead of print - 7 Apr 2021


  • astro-ph.GA
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