HE 0241 - 0155 - Evidence for a large scale homogeneous field in a highly magnetic white dwarf ⋆

In the course of the Hamburg/ESO survey we have discovered a white dwarf whose spectrum exhibits many similarities with the prototype of magnetic white dwarfs Grw+70◦8247. In particular several stationary line components indicative for magnetic fields between about 150 and 400 MG are found in both objects. However, the features between 5000 and 5500 Å in the spectrum of HE0241-0155 cannot be explained by stationary line components and demand a relatively homogeneous magnetic field with clustering around 200 MG. For this reason a pure dipole model failed to reproduce this spectral region. An offset-dipole configuration led to some improvement in the fit but a good agreement was only possible for a geometry – described by an expansion into spherical harmonics – where most of the surface is covered with magnetic field strengths strongly clustered around 200 MG. This may indicate the presence of a large magnetic spot whose presence could be tested with time resolved spectro-polarimetry.