1997A&A...325..109S

The continuum and emission lines properties of a sample of 73 blazars is studied, investigating differences and similarities among normal low-polarization quasars (LPQ), radio-loud high polarized quasars (HPQ), and BL Lacertae objects (BLL). We found that at variance with LPQ, most HPQ have power law optical continua (f_ν∝ν^α ) 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 optical continuum of blazars (α_O=-1.00 ±0.09) is significantly steeper than that of LPQ (α_O~-0.5). On the other hand the average spectral index for the HPQ and BLL subsamples is rather similar (<α_HPQ≥-1.01±0.17 and <α_BLL≥-0.99±0.11). In the α_O-α_RX plane HPQ do not split in two distinct 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 is found between the maximum percentage of optical polarization and both α_O and α_RX , the less polarized object having the steeper spectral index. The comparison of emission line luminosities shows that LPQ have on average stronger lines (∼ 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 of smaller equivalent width as directly induced by the e.w.<5Å definition criteria. This leads us to conclude that from the point of view of optical spectral properties HPQ and RBL are quite similar objects while XBL may be different.