The class of highly polarized quasars - Observations and description

The properties of the class of quasars whose optical continua are highly polarized are presented and discussed. A recent polarization survey of quasars has discovered ten new definite members of this class and eight possible members. There are now 17 highly polarized quasars known and an additional nine quasars which are possibly highly polarized. This represents a dramatic increase in the size of the class and allows a much better description of their properties. In general, highly polarized quasars are compact, flat-spectrum radio sources, have steep optical continua, and exhibit rapid large-amplitude variability. While their continua are very similar to BL Lac objects, their typical luminosities and emission line strengths appear to be similar to those of normal low-polarization quasars. As a link between normal quasars and BL Lac objects, the highly polarized quasars are a key test for theoretical models.