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BL Lac objects are named after the protype object which was first believed
to be a variable star in our galaxy. However, because of its similiarities
to AGNs, BL Lac is now believed to be an extragalactic object. Because
of their occasional wild variability, these objects are sometimes refered
to as "blazars." This name also alludes to the similarities these objects
have with quasars.
BL Lac objects have most of the following characteristics:
- Great and rapid variability in the radio, IR, and visible regions Optical
variability of up to 4 magnitudes is not uncommon. This amounts to a change
in brightness of nearly a factor of 20, and a few BL Lac objects have been
reported to undergo changes in brightness of a factor of 100. While variations
of 10 to 30% have been noted from night-to-night for some objects, the larger
variations usually take place over months or years.
- No emission or absorption lines... continuous spectrum only
- Compact radio source with nonthermal continuous spectrum extending into
the IR and visible
- Stellar appearing optical source with virtually no structure Some BL Lac
objects reveal "fuzz" when observed with the largest telescopes. This "fuzz,"
or faint nebulosity surrounds a point-like stellar appearing source.
- Strong and rapidly varying polarization
Approximately 40 BL Lac objects have been identified. Because of the
virtual absence of emission or absorption lines, redshifts are virtually
unknown. They are believed to be extragalactic because of their radio properties
and because of the "fuzz" observed surrounding some objects. The "fuzz,"
when observed, seems to have a spectrum similiar to that of an elliptical
galaxy. A number of BL Lac objects are known in the vicinity of clusters
of galaxies so this provides indirect evidence in support of their extragalactic
nature.
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