BL Lac Objects

BL Lacertae objects (BL Lac objects or just BL Lacs, for short) exhibit either no emission lines or only very weak ones, but have a strong continuum stretching from RF through X-ray frequencies. They are generally radio-loud AGNs. They also exhibit strong polarization of their emitted light (typically several percent, as compared with 1 percent or less for most other AGNs).

Blazars
As noted in the right panel, BL Lacs are members of a general class of AGNs called blazars. (We shall be somewhat loose in our terminology and use the terms "blazars" and "BL Lacs" interchangeably.) Blazars are relatively rare among AGNs. For example, some 10,000 quasars and more than 1000 Seyfert galaxies are known, but only several hundred blazars have been discovered thus far. Blazars are extremely luminous, with typical ones being 10,000 times more luminous than the Milky Way and therefore some 1000 times more luminous than a Seyfert galaxy.

Host Galaxies
By masking the bright core that is responsible for producing the intense and rather featureless continuum, it is possible to acquire a spectrum of the "fuzz" seen faintly around many BL Lacs. This spectrum, and the variation of the light intensity of the fuzz with distance from the core, indicate that the fuzz is the outer part of a giant elliptical galaxy in more than 90 percent of the cases that have been analyzed. This suggests that most BL Lacs are the active cores of giant elliptical galaxies. From faint spectral lines in the masked spectrum (corresponding primarily to normal starlight from the host galaxy) the redshift of the parent galaxy can be inferred. Most BL Lacs have redshifts less than about 0.1, but the OVV subclass of blazars (see the right frame) typically exhibits redshifts of 0.5 or greater. Thus, blazars often correspond to more distant objects than Seyfert galaxies or radio galaxies, but most are closer than quasars.

Rapid Luminosity Variation
Blazars can exhibit dramatic variability in both their light output and in their polarization. In fact, they change more violently than any other luminous sources known except supernovae. The top right figure shows a light curve at radio wavelengths illustrating the variability in intensity of BL Lac over a period of several years. As for our earlier discussion of quasars and Seyfert galaxies, the period of variability for active galaxies places constraints on the minimum size of the central engine that powers them. The top right figure illustrates the longer term variation of BL Lac, but blazars can change both their intensity and their polarization significantly on timescales of a day or less. The rapid luminosity variation is thought to have two sources:

  • The size of the light emitting region may more cleanly reflect the true size of the central engine in a BL Lac, since in elliptical galaxies there is not as much surrounding dust to absorb and re-emit the primary radiation as in the spiral galaxy host of a Seyfert.
  • Relativistic beaming effects to be discussed further below and later in this chapter may give the illusion of variability that is more rapid than the intrinsic variability of the source.

    • Given our previous model for Seyfert galaxies, it will come as no surprise that we believe blazars to be powered by supermassive black holes. However, blazars differ in many respects from Seyfert galaxies (for example, the strong polarized continuum without emission lines). How can we account for those differences with a black hole model?

    A Model of a Blazar
    The properties of blazars may be approximately accounted for by invoking a central black hole engine model similar to that for Seyfert galaxies. However, unlike for Seyferts where the evidence is that jets from the central engine are relatively weak, the properties of blazars (in particular their very strong nonthermal and polarized continuum, rapid variability, and radio loudness) indicate that there are strong jets coming from the central engine. Furthermore, the preceding properties, and that we do not see radio lobes for blazars like we see for lobe radio galaxies, suggest that the strong blazar jets are oriented almost parallel to our line-of-sight. Thus, one blazar jet points almost directly toward us and the opposite jet (the counterjet) points almost directly away from us. The basic model is illustrated in the lefthand figure.

    Similarities and Differences

    The preceding model for a blazar suggests some similarities with a core-halo radio source, which probably also involves a central black hole with radio jets oriented near our line-of-sight. However, blazars are typically much more luminous at non-radio wavelengths. There is also some similarity with the model that we invoked for a Seyfert 1 active galaxy. However, blazars are usually found in elliptical rather than spiral galaxies, and they appear to have stronger jets than Seyfert galaxies. The continuum synchrotron emission of the jet swamps the broad line emission that characterizes Seyfert 1 spectra.

    Blazar Properties
    As we shall discuss in more detail in the next module, the jets are probably collimated by strong magnetic fields. Electrons are accelerated to near light velocity and emit synchrotron radiation as they spiral in this field. Since we look almost directly down the bore, so to speak, of the synchrotron jet, the light that we see is dominated by the polarized synchrotron continuum. Furthermore, since the jet is relativistic, there are strong relativistic focusing effects that make the blazar appear to be even brighter than it actually is, and to vary in brightness more rapidly than would otherwise be the case.

    These properties explain the blazar's lack of emission lines (they are probably produced, in a manner similar to that for Seyfert galaxies, but are swamped by the beamed synchrotron continuum), the high luminosity (a blazar is a luminous AGN further enhanced by relativistic focusing effects), the polarization (synchrotron radiation is polarized), and the rapid time variability (the rapid variation of the compact black hole source is amplified even further by relativistic effects).