CHANG-ES XIX: Galaxy NGC 4013 -- a diffusion-dominated radio halo with
  plane-parallel disk and vertical halo magnetic fields
  Using the Continuum HAloes in Nearby Galaxies - an EVLA Survey (CHANG-ES)
radio continuum data from the Karl G. Jansky Very Large Array (VLA) in two
frequency bands (C-band, L-band), we analyzed the radio properties, including
polarization and the transport processes of the CR electrons (CREs), in the
edge-on spiral galaxy NGC 4013. Supplementary LOw-Frequency ARray (LOFAR) data
at 150MHz are used to study the low-frequency properties of this galaxy and
X-ray (Chandra, XMM-Newton) data are used to investigate the central region.
The central point source dominates the radio continuum in both CHANG-ES bands,
but no clear AGN classification is possible at this time. The scale height
analysis shows that Gaussian fits, with halo scale heights of 1.2 kpc in
C-band, 2.0 kpc in L-band, and 3.1 kpc at 150 MHz, better represent the
intensity profiles than do exponential fits. The radio continuum halo of NGC
4013 in C-band is rather small, while the low-frequency LOFAR data reveal a
large halo. The polarization data reveal plane-parallel, regular magnetic
fields within the entire disk and vertical halo components out to heights of
about 6 kpc indicating the presence of an axisymmetric field having a radial
component pointing outwards. The mean magnetic field strength of the disk of
NGC 4013 of 6.6 $\mu$G (using the revised equipartition formula) is rather
small. The interaction and the low star formation rate (SFR) across the disk of
NGC 4013 probably influence the appearance of its radio continuum. Several
observable quantities give consistent evidence that the CR transport in the
halo of NGC 4013 is diffusive: the frequency dependence of the synchrotron
scale height, the disk/halo flux density ratio, the vertical profile of the
synchrotron spectral index, the small propagation speed measured modeled with
spinnaker, and the low temperature of the X-ray emitting hot gas.
