LTE spectrum synthesis in magnetic stellar atmospheres - The interagreement of three independent polarised radiative transfer codes

With the aim of establishing a benchmark for the detailed calculation of the polarised line profiles of magnetic stars, we describe an intercomparison of LTE Stokes profiles calculated using three independent, state-of-the-art magnetic spectrum synthesis codes: Cossam, Invers10 and Zeeman2. We find, upon establishing a homogeneous basis for the calculations (identical definitions of the Stokes parameters and the magnetic and stellar reference frames, identical input model stellar atmosphere, identical input atomic data, and identical chemical element abundances and magnetic field distributions), that local and disc-integrated Stokes IQUV profiles of Fe II lambda 4923.9 calculated using the three codes agree very well. For the illustrative case of disc-integrated profiles calculated for abundance log n(Fe)/n(tot) = 4.60, dipole magnetic field intensity B-d = 5 kG, and projected rotational velocity v(e) sin i = 20 km s(-1), Stokes I profiles (depth similar to 40% of the continuum flux I-c) agree to within about 0.05% rms of I-c, Stokes V profiles (full amplitude similar to 10%) to within about 0.02% rms of I-c, and Stokes Q and U profiles (full amplitudes similar to2%) at the sub-0.01% rms level. These differences are sufficiently small so as to allow for congruent interpretation of the best spectropolarimetric data available, as well as for any data likely to become available during the near future. This indicates that uncertainties in modeling Stokes profiles result overwhelmingly from uncertainties in input atomic and physical data, especially the state and structure of model stellar atmospheres.