|Annu. Rev. Astron. Astrophys. 1997. 35:
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6.7. Variability of Emission Lines
Compared to quasars, blazars have generally weak emission lines (Miller et al 1978). This is especially true in BL Lac objects, where by definition the lines are practically absent (W < 5 Å). Because the intrinsic line luminosities of BL Lac objects are systematically low compared to FSRQ (Padovani 1992), BL Lacs cannot be quasars with exceptionally beamed continua. Rather, an astrophysical explanation must be found for the intrinsic weakness of their line emission.
The detected lines in BL Lacs are usually of the narrow/forbidden type, which may suggest some fundamental difference between BL Lacs and strong emission line blazars, motivating a separate classification. However, broad H has been seen in quite a few BL Lac objects (Miller et al 1978, Ulrich 1981, Sitko & Junkkarinen 1985, Moles et al 1987), including BL Lac itself (in which the broad line was clearly absent at earlier epochs; Vermeulen et al 1995, Corbett et al 1996); also, significant variations in the intensity of broad H or Ly have been seen (Ulrich 1981, Kidger et al 1996).
These observations demonstrate that the presence or absence of broad lines can be a transitory phenomenon, underscoring the continuity of properties between BL Lac objects and other blazars and supporting a unified view of the blazar phenomenon (Maraschi & Rovetti 1994, Bicknell 1994, Sambruna et al 1996a).