The Apertif science verification campaign - Characteristics of polarised
  radio sources
  We analyse five early science datasets from the APERture Tile in Focus
(Apertif) phased array feed system to verify the polarisation capabilities of
Apertif in view of future larger data releases. We aim to characterise the
source population of the polarised sky in the L-Band using polarised source
information in combination with IR and optical data. We use automatic routines
to generate full field-of-view Q- and U-cubes and perform RM-Synthesis, source
finding, and cross-matching with published radio, optical, and IR data to
generate polarised source catalogues. SED-fitting routines were used to
determine photometric redshifts, star-formation rates, and galaxy masses. IR
colour information was used to classify sources as AGN or
star-forming-dominated and early- or late-type. We surveyed an area of
56deg$^2$ and detected 1357 polarised source components in 1170 sources. The
fraction of polarised sources is 10.57% with a median fractional polarisation
of 4.70$\pm$0.14%. We confirmed the reliability of the Apertif measurements by
comparing them with polarised cross-identified NVSS sources. Average RMs of the
individual fields lie within the error of the best Milky Way foreground
measurements. All of our polarised sources were found to be dominated by AGN
activity in the radio regime with most of them being radio-loud (79%) and of
the FRII class (87%). The host galaxies of our polarised source sample are
dominated by intermediate disc and star-forming disc galaxies. The contribution
of star formation to the radio emission is on the order of a few percent for
$\approx$10% of the polarised sources while for $\approx$90% it is completely
dominated by the AGN. We do not see any change in fractional polarisation for
different star-formation rates of the AGN host galaxies.
