Magnetic fields of the W4 superbubble

Context. 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. Aims. We study the radio continuum and polarized emission properties of the W4 superbubble to determine its magnetic field strength. Methods. New sensitive radio continuum observations were made at lambda 6 cm, lambda 11 cm, and lambda 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. Results. 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 f(ne), we obtain the thermal electron density n(e) = 1.0/root fn(e) (+/-5%) cm(-3) and the strength of the line-of-sight component of the magnetic field B-// = -5.0/root fn(e) (+/-10%) 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 B-tot 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 where the magnetic field in the Perseus arm is lifted up from the plane towards high latitudes. Conclusions. 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.