ARE HIGH POLARIZATION QUASARS AND BL LACERTAE OBJECTS REALLY DIFFERENT - A STUDY OF THE OPTICAL SPECTRAL PROPERTIES

The continuum and emission lines properties of a sample of 73 blazars is studied, investigating differences and similarities among normal lo w-polarization quasars (LPQ), radio-loud high polarized quasars (HPQ), and BL Lacertae objects (BLL). We found that at variance with LPQ, mo st HPQ have power law optical continua (f(nu) proportional to nu(alpha )) with no indication of blue bumps or strong FeII and Balmer emission . Comparison of LPQ with blazars, considered as a class including both HPQ and BLL, confirms and put on firm statistical bases that the opti cal continuum of blazars (alpha(O) = -1.00 +/- 0.09) is significantly steeper than that of LPQ (alpha(O) similar to -0.5). On the other hand the average spectral index for the HPQ and BLL subsamples is rather s imilar (< alpha(HPQ) > = - 1.01 +/- 0.17 and < alpha(BLL) > = - 0.99 /- 0.11) In the alpha(O) - alpha(RX) plane HPQ do not split in two dis tinct groups as do BLL, which spilt in Radio (RBL) and X-ray selected (XBL) objects. All HPQ cluster together in the same region occupied by RBL, while XBL are clearly distinguished. A significant correlation i s found between the maximum percentage of optical polarization and bot h alpha(O) and alpha(RX), the less polarized object having the steeper spectral index. The comparison of emission line luminosities shows th at LPQ have on average stronger lines (similar to a factor of 6) with respect to blazars. Comparing HPQ and RBL we show they exhibit similar range of line luminosity, BL Lacs being just the objects with lines o f smaller equivalent width as directly induced by the e.w.< 5 Angstrom definition criteria. This leads us to conclude that from the point of view of optical spectral properties HPQ and RBL are quite similar obj ects while XBL may be different.