THE LINEAR-POLARIZATION OF 3C-345 IN THE ULTRAVIOLET

The linear polarization of 3C 345, a superluminal radio source and OW quasar, was observed in two bandpasses in the ultraviolet (centered at 2160 Angstrom and 2770 Angstrom) in 1993 April using the High Speed P hotometer on the Hubble Space Telescope. The quasar is significantly p olarized in the W (p > 5%). Ground-based polarimetry was obtained 11 d ays later, but a difference in the position angle between the observat ions in the visible and those in the UV indicate that the magnitude of the polarization of 3C 345 may have changed over that time. If the tw o observation sets represent the same state of spectral polarization, then the large UV flux implies that either the polarization of the syn chrotron continuum must stop decreasing in the UV, or that there is an additional source of polarized flux in the ultraviolet. Only if the U V observations represent a spectral polarization state with the same p osition angle in the visible seen previously in 3C 345 can the polariz ed flux be represented by a single power law consistent with the three -component model of Smith et al. (1986, 1988). This model consists of a polarized synchrotron component, an unpolarized component from the b road-line region, and an unpolarized component attributed to thermal r adiation from an optically thick accretion disk. Additional simultaneo us polarimetry in the UV and visible will be required to further const rain models of the continuum emission processes in 3C 345 and determin e if the UV polarized flux is synchrotron in origin.