Polarization in the ELAIS-N1 LOFAR deep field

Sammanfattning: The physical characteristics of cosmic magnetic fields are encoded in the po- larization properties of the extragalactic radio sources. Linearly polarized radiation undergoes Faraday rotation as it crosses a magneto-ionic medium; the rotation of the polarization angle of the signal is equal to its wavelength squared times the Rotation Measure (RM), that is proportional to the line- of-sight integral of the product of the magnetic field component along the line of sight and the density of thermal electrons. Mapping the values of the RM across the sky provides a mean to constrain cosmic magnetic fields. For this purpose, statistical studies of the properties of polarized radio sources spread over cosmological distances are essential. The advantage of low radio-frequency observations over higher-frequency ones is the better precision on the inferred RM values. On the other hand, at low radio-frequency observations are more affected by depolarization, which affect the detection rate. Indeed, the population of faint polarized extragalac- tic sources at low radio frequencies is still mostly unknown. In this context, the LOw Frequency ARray (LOFAR) plays an important role because of sensitivity, angular resolution and precision on the inferred RM values that can be achieved through low-frequency broad-band polarimetry, allowing us to study the polarized radio emission at frequencies around 150 MHz. In our work, we developed a new method to combine polarimetric obser- vations with slightly different frequency configurations, and we applied this method to the European Large Area ISO Survey-North 1 (ELAIS-N1) deep field, one of the deepest of the LOFAR Two-Meter Sky Survey (LoTSS) Deep Fields so far, at 114.9–177.4 MHz. We imaged an area of 25 deg2 at 6-arcsec of resolution in which, through stacking of 19 8-hour-long epochs, detected 1.28 sources per square degrees, the highest number density of polarized sources ever found at low radio frequencies. We compared our results with other RM catalogs and we quantified the depolarization properties of sources detected also at 1.4 GHz. We also modeled the source counts in polarization from the source counts in total flux density. This work dealt with technical and theoretical challenges inherent to the observation and interpretation of polarimetric data and represents a step in solving complex issues that modern radio astronomy is facing due the large amounts of data generated by new-generation radio-interferometers.