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Blazars (BL Lacertae objects)

A class of active galactic nuclei with nearly featureless spectra and dramatic variability — named for BL Lacertae in Lacerta, the Lizard.

History

There is a class of objects that, unlike quasars which have emission lines in their spectra, have continuous spectra and which vary in the same way that quasars do. In many cases they have been identified with radio sources. They are called Lacertids, in honor of the first one discovered, BL Lacertae, located in the constellation Lacerta (the Lizard).

The object BL Lacertae (BL Lac for short) had been in the catalogue of variable stars for some time. It was originally discovered by Cuno Hoffmeister in 1929. In 1965 Arp and Burbidge found that 3C 120, a radio source detected at Cambridge in 1959, is a Seyfert galaxy. In 1968 Penston noticed that the object was located at the same position as the variable star BW Tau in Taurus. Also in 1968, J. Schmitt noticed that there was a variable radio source located at the same position as BL Lac — VRO 42.22.01, detected at the Vermilion River Observatory by MacLeod et al. in 1965.

BL Lac was not a periodic variable; its intensity varied irregularly. When spectra were taken, they were featureless — no emission lines as from quasars, no absorption lines as found in most stars. By 1972 Peter Strittmatter and others had identified four other objects similar to BL Lac.

In 1974, James E. Gunn and J. B. Oke determined that BL Lac was located in a normal elliptical galaxy by blocking light from the central part of the object. Surrounding light showed absorption lines indicating a redshift of 0.07 — at the time, about 420 megaparsecs away — implying a core luminosity at brightest of 1046 ergs/sec.

Ed Spiegel jokingly referred to these objects as blazars, and the name stuck.

Current theories

BL Lacertae objects are now classified as a type of Active Galactic Nuclei (AGN).

Working definitions

  • BL Lacertae objects (BL Lac) — AGN which display dramatic variances in brightness over a short period of time.
  • Blazars — AGN with high radio brightness and/or dramatic optical variability over a short period; appear star-like (optical point source); smooth, flat spectra; partially polarized visible light; output at all wavelengths varies rapidly and more than a classic quasar; flare-like behavior.

Adapted from the original JensenWorld astronomy notes.