Polarized sources in ELAIS-N1 LOFAR deep field

The European Large Area ISO Survey-North 1 (ELAIS-N1) field, one of the deepest of the LOFAR Two-Metre Sky Survey (LoTSS) Deep Fields so far, was selected for a polarimetric study at 114.9-177.4MHz. A total area of 25 deg^2^ was imaged at 6"-resolution in the Stokes Q and U parameters. Alignment of polarization angles was done both in frequency and in Faraday space before stacking datasets from 19 eight-hour-long epochs taken in two different LOFAR observing cycles.

A search for polarized sources was carried out in the final, stacked dataset, and the properties of the detected sources were examined. The depolarization level of sources known to be polarized at 1.4GHz was quantified. A one-sigma noise level, {sigma}_QU_, of 19uJy/beam was reached in the central part of the field after stacking. 25 polarized sources were detected above 8{sigma}_QU_, five of which had not been detected in polarization at any other radio frequencies before.

Seven additional polarized components were found by lowering the threshold to 6{sigma}_QU_ at positions corresponding to sources known to be polarized at 1.4GHz. In two radio galaxies, polarization was detected from both radio lobes, so the final number of associated radio continuum sources is 31. The detected sources are weakly polarized, with a median degree of polarization of 1.75% for the sample of sources detected in polarized emission.

For the 10 polarized sources previously identified in a pilot LOFAR study of the ELAIS-N1 field at 20"-resolution, the RM values are consistent but the degrees of polarization are higher in the 6"-resolution data. The sources previously detected in polarization at 1.4GHz are significantly depolarized at 150MHz. The catalog is used to derive the polarized source counts at 150MHz.

This is the deepest and highest-resolution polarization study at 150 MHz to date. A full characterization of the sources and an analysis of the catalog will be presented in Paper II.