Polarization and microlensing in the quadruply imaged broad absorption line quasars H1413+117

We have obtained the first spectropolarimetric observations of the four images of the gravitationally lensed broad absorption line (BAL) quasar H1413+117. This gravitationally lensed system is known to be polarized and strongly affected by microlensing. It therefore constitutes a priviledged laboratory to study the effect of gravitational microlensing on polarization. Since microlensing can selectively magnify a small region of the source plane, it may magnify (a part of) the scattering region that emits the polarized light and alter the polarization properties in a way that depends on the geometry of the scattering region. We found that the image D of BAL QSO H1413+117 is microlensed and exhibits a significantly different polarization from the image A which is essentially unaffected by microlensing. We showed this can be consistently interpreted by assuming two competing sources of continuum, roughly perpendicularly polarized.