BL Lacertae

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 AGN, 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
Image of BL Lac, the prototypical “BL Lac”
object. Credit AAVSO.

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
generally 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.