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The wavelength dependence of optical linear polarization of the highly polarized quasar (HPQ) PKS~1546+027 is investigated for the first time. Both spectropolarimetry and broad-band filter polarimetry are presented for a period (1993 March--April) when the object was undergoing a strong outburst in flux and polarization. The rapid polarization variations observed argues that an optically thin synchrotron-emitting region, similar in polarization properties to BL~Lacertae objects and optically violent variable (OVV) quasars, is the major source of polarized flux from this object. The continuum polarization increases dramatically with wavelength and is about twice as high at 8000~\AA\ than at 4000~\AA . Spectropolarimetry acquired using the KPNO 4~m telescope reveals that the emission-line flux from PKS~1546+027 dilutes the continuum polarization and is probably unpolarized. The polarized flux spectrum is well-represented by a steep power-law with a spectral index of $-$2.2, supporting the view that the polarized flux has a nonthermal origin. The observed polarization properties of PKS~1546+027 are consistent with a model that has been proposed to explain the optical to ultraviolet continua of several OVVs. In this model the optical to near-UV spectral flux distribution is a combination of light from a polarized synchrotron component plus an unpolarized component with spectral properties typical of low-polarization quasars. PKS~1546+027 is compared to other HPQs and the optical synchrotron spectral energy distributions of these quasars are compared to the nonthermal continua of BL~Lac objects.
This research is supported by NASA grants NAG~5--1630, HF--1045.01--93A and NSF grant AST~91--14087.
