Stokes imaging of the AM Her system ST LMi

The Stokes imaging technique of Potter, Hakala & Cropper is applied to the polarized emission from the AM Her system ST LMi. For the first time, the c yclotron emission region on the surface of the white dwarf is mapped in ter ms of optical depth/density in an analytical and objective manner. The regi on is found to consist of a less dense region leading a higher density regi on in orbital phase. It is demonstrated that the emission region needs to h ave a multi-temperature structure in order to explain the spectral slope an d the general morphology of the cyclotron humps observed in the IR during t he bright phase in ST LMi. Furthermore, it is shown that a secondary emission region, fed by the same magnetic field lines that feed the main emission region, could be responsib le for the positive circular polarization in the IR, the position angle var iation and the excess flux during the faint phase of the white light observ ations which cannot be accounted for with a single emission region.