2001ApJ...561..653C

We report the results of Hubble Space Telescope Wide Field and Planetary Camera 2 broadband F555W and F702W photometric and F555W polarimetric observations of the ``Cloverleaf'' QSO H1413+1143. This is a four-component gravitationally-lensed broad absorption line (BAL) QSO. Observations were obtained at two epochs in 1999 March and 1999 June separated by ~100 days. The observations were photometrically and polarimetrically calibrated using the standard ``pipeline'' calibration procedures implemented at the Space Telescope Science Institute. The goal of our program was to detect any relative changes among the components and between the two epochs. Over this time baseline we detected an ~0.07 mag dimming in component D of the lensed image, which we interpret as evidence for microlensing. In 1999 March we find significant evidence for a difference in the relative linear polarization of component D in comparison to the other three components; in 1999 June the combined polarization of the Cloverleaf components was lower. In 1999 March the apparently microlensed component D has a rotated polarization position angle and a somewhat higher degree of polarization than the other three components. We suggest that this difference in polarization is due to microlensing magnification of part of a scatter-light (i.e., polarized) continuum-producing region. The results indicate that in the Cloverleaf the size scale of the polarized scattered-light region exceeds ~10¹⁶ cm but lies interior to the region producing the broad emission lines (<10¹⁸ cm).