Constraints from Faraday rotation on the magnetic field structure in the
  Galactic halo
  We examine the constraints imposed by Faraday rotation measures of
extragalactic point sources on the structure of the magnetic field in the halo
of our Galaxy. Guided by radio polarization observations of external spiral
galaxies, we look in particular into the possibility that field lines in the
Galactic halo have an X shape. We employ the analytical models of spiraling,
possibly X-shape magnetic fields derived in a previous paper to generate
synthetic all-sky maps of the Galactic Faraday depth, which we fit to an
observational reference map with the help of Markov Chain Monte Carlo
simulations. We find that the magnetic field in the Galactic halo is slightly
more likely to be bisymmetric (azimuthal wavenumber, $m = 1$) than axisymmetric
($m = 0$). If it is indeed bisymmetric, it must appear as X-shaped in radio
polarization maps of our Galaxy seen edge-on from outside, but if it is
actually axisymmetric, it must instead appear as nearly parallel to the
Galactic plane.
