Magnetic fields of the W4 superbubble
  Superbubbles and supershells are the channels for transferring mass and
energy from the Galactic disk to the halo. Magnetic fields are believed to play
a vital role in their evolution. We study the radio continuum and polarized
emission properties of the W4 superbubble to determine its magnetic field
strength. New sensitive radio continuum observations were made at 6 cm, 11 cm,
and 21 cm. The total intensity measurements were used to derive the radio
spectrum of the W4 superbubble. The linear polarization data were analysed to
determine the magnetic field properties within the bubble shells. The
observations show a multi-shell structure of the W4 superbubble. A flat radio
continuum spectrum that stems from optically thin thermal emission is derived
from 1.4 GHz to 4.8 GHz. By fitting a passive Faraday screen model and
considering the filling factor fne , we obtain the thermal electron density ne
= 1.0/\sqrt{fne} (\pm5%) cm^-3 and the strength of the line-of-sight component
of the magnetic field B// = -5.0/\sqrt{fne} (\pm10%) {\mu}G (i.e. pointing away
from us) within the western shell of the W4 superbubble. When the known tilted
geometry of the W4 superbubble is considered, the total magnetic field Btot in
its western shell is greater than 12 {\mu}G. The electron density and the
magnetic field are lower and weaker in the high-latitude parts of the
superbubble. The rotation measure is found to be positive in the eastern shell
but negative in the western shell of the W4 superbubble, which is consistent
with the case that the magnetic field in the Perseus arm is lifted up from the
plane towards high latitudes. The magnetic field strength and the electron
density we derived for the W4 superbubble are important parameters for
evolution models of superbubbles breaking out of the Galactic plane.
