ZEBRA - ZEEMAN BROADENING ANALYSIS OF MAGNETIC WHITE-DWARFS .1. MAXIMUM-ENTROPY RECONSTRUCTION OF H-ALPHA PROFILES

The detailed field geometries of magnetic white dwarfs (MWDs), whose f ield strengths range up to a billion G, should shed light on the origi n of these intense fields as well as the role of magnetism in angular momentum transfer during the later stages of stellar evolution. Comple x field structures have already been inferred for a few MWDs based on trial-and-error fits to limited time-series spectroscopy and polarimet ry, but the fitting procedure is both subjective and time-intensive. W e introduce a new and more objective method called ZEBRA (for ZEeman B Roadening Analysis) to investigate the field structure of an MWD from series of Stokes I (unpolarised) and V (circularly polarised) spectra which sample the star's rotation cycle. The Zeeman-broadened and -shif ted Balmer line profiles are translated into plots of areal filling fa ctor as a function of longitudinal and transverse field on the visible disk at each rotational phase (ZEBRA plots). The data inversion is pe rformed using maximum entropy reconstruction techniques. We demonstrat e through tests with simulated data that such ZEBRA plots are powerful diagnostics of the field topology. In the case of purely dipole field s, they are sensitive to the polar strength, offset and viewing angle of the dipole structure. They also quantify deviations from simple dip olar geometries and even surface abundance inhomogeneities.