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Journal Article

The LOFAR Two-meter Sky Survey: Deep Fields Data Release 1 II. The ELAIS-N1 LOFAR deep field


Ciardi,  B.
Computational Structure Formation, MPI for Astrophysics, Max Planck Society;

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Sabater, J., Best, P. N., Tasse, C., Hardcastle, M. J., Shimwell, T. W., Nisbet, D., et al. (2021). The LOFAR Two-meter Sky Survey: Deep Fields Data Release 1 II. The ELAIS-N1 LOFAR deep field. Astronomy and Astrophysics, 648: A2. doi:10.1051/0004-6361/202038828.

Cite as: https://hdl.handle.net/21.11116/0000-0008-CC3E-E
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 h, it reaches a root mean square noise level of ≲20 μJy beam−1 in the central region (and below 30 μJy beam−1 over 10 square degrees). The resolution is ~6 arcsecs and 84 862 radio sources were detected in the full area (68 square degrees) with 74 127 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 (115–177 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 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 per cent for ELAIS-N1.