BL Lac objects are named after the prototype object which was first believed to be a variable star in our galaxy. However, because of its similarities to AGN, BL Lac is now believed to be an extragalactic object. Because of their occasional wild variability, these objects are sometimes referred to as “blazars.” This name also alludes to the similarities these objects have with quasars.

Possibly the most famous sub-class of the blazars are the BL Lacerta objects, or BL Lacs. These are named after their progenitor BL Lacerta which was originally classified as a blue variable star, and found to be the optical counterpart to the radio source VRO 42.22.01 (Schmitt 1968). Their defining feature is the lack of strong emission lines in their optical spectra. The commonly used definition (Stocke et al. 1991) is that the equivalent width satisfies W_⅄¸ ‹ 5 Å. Additionally, if the Ca ii “HandK” break (also termed the “4000ºA break”) due to the host galaxy is present, then the contrast must be ±25%. A (1996) paper modified this definition slightly to include sources with contrasts of up to 40%, with a relaxed restriction on the equivalent widths. The cause of the lack of emission lines in BL Lacs has been the subject of much debate for the past twenty to thirty years. The most commonly used explanation (Blandford and Rees 1978, surely the most cited conference proceedings paper ever!) is that BL Lacs are viewed close to the axis of a relativistic jet. The (synchrotron) emission from this jet is Doppler boosted, increasing its intensity so that it swamps the continuum and line emission that would otherwise be visible.

Approximately 200 are currently known. These objects are highly variable radio and optical emitter which exhibit a high degree of polarization and exhibit apparent superluminal velocities. These objects are dividing into BL Lacertae objects and optically violent variables. They are distant quasar-like objects with no spectral lines which are often highly variable, by up to a factor of 100 over a few months. They exhibit high intensity emission from radio to gamma ray wavelengths. An example is PKS 2155-304, one of the brightest X-ray emitter. Most theorists believe that observations of BL Lacs are detecting a jet of material moving at close to the speed of light towards the observer. However, BL Lacs may be caused by microlensing of individual stars in intervening galaxies.

The question of how exactly to define a BL Lac was complicated somewhat by the (1996) of a broad Hα line in the progenitor of the class, BL Lacerta. This observation was made at a time when the overall flux of the object was at a relatively low level. These observations raise the prospect of there being a Seyfert-like accretion disk in the centre of BL Lacerta, although present data is not quite suitable for discrimination between an accretion disk model or photoionization by the synchrotron jet.

For many years, BL Lacs were divided into two categories: radio-selected (RBLs) and X-ray selected (XBLs). The two categories had very different broad-band spectral energy distributions (SEDs): the RBLs peaked in the IR to optical part of the spectrum, and were generally brighter at radio frequencies than XBLs; while the XBLs peaked at higher energies – into the X-ray part of the spectrum. The division into two categories was supported by a bi-modal distribution of the ratio of radio to X-ray flux. However, recent deeper surveys (such as the RGB, or ROSAT – Green Bank survey have filled in the gap between the two parts of the distribution, and the RBL/XBL terminology has made way for one that is less governed by the manner of selection. Instead, BL Lacs are more commonly referred to as LBLs or HBLs (low- or high-peaking BL Lac objects respectively) – referring to the location of the peak of the broad-band SED. Generally speaking (although by no means exclusively), sources that were known as RBLs are LBLs, and similarly for XBLs and HBLs. This categorization has been used as part of attempts at BL Lac unification schemes.

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 similar 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.

BL Lacertae objects have virtually featureless spectra, making even their redshifts difficult to measure unless the surrounding galaxy can be detected, or emission lines show up when the nucleus is temporarily much fainter than usual. The QSO and BL Lacertae object spectra have good data only in the bluer range, so that they are plotted only from 3500-6000 Angstroms, rather than 3500-7000 as for the other kinds of object.

Other blazars

As well as BL Lacs, the blazar class contains those quasars variously described as OVVs (Optically Violent Variables), HPQs (High Polarization Quasars) or FSRQs (Flat-Spectrum Radio Quasars). These are quasars that share many of the same properties of BL Lacs (such as high polarization and rapid variability), but with quasar-like emission lines. The different names come from the different classification schemes used: either by optical variability, optical polarization or radio spectral index. The name FSRQ is quite common, as it derives from the important fact that nearly all blazar sources have compact radio structure (i.e. are core-dominated) and tend to have flat radio spectra. A flat spectrum is defined as satisfying α› -0:5, with F_v ∝ V^α

Optical emission in blazars

The optical emission in blazars tends to be highly polarized, with levels of at least a few percent, and often much higher. This gives the first important clue as to the nature of this optical emission. A form of radiation that is naturally quite highly polarized is synchrotron radiation. This is radiation that is emitted when relativistic particles (such as electrons) move through a magnetic field. The particles spiral around the field lines, and as they do so they emit photons. The spectrum of synchrotron radiation covers a very broad range of frequencies { it can extend from radio frequencies (where it is found most often) up to optical and even X-ray energies. In blazars, the synchrotron emission is most likely to come from a relativistic jet that extends out from the center of the source (although this is not always where synchrotron emission comes from – supernova remnants emit most of their radio emission in the form of synchrotron).

From examining the multi-wavelength SEDs of blazars it is found that a continuous, perhaps slightly curved, spectrum connects the radio, IR, and optical data points (and in some cases, the X-ray points as well). This, and the observed polarization of the IR and optical, has led to models that invoke a single synchrotron component to explain the emission over this entire wavelength range. A further argument for synchrotron emission at optical wavelengths is the featureless continuum seen in BL Lacs. A continuum with no emission lines is precisely what you would expect when the emission is dominated by synchrotron. What other components are there in the optical emission of blazars? For many objects, the synchrotron emission accounts very well for the continuum emission. In some, however, there is good evidence for emission from an accretion disk, which, as we shall see, is the dominant emission component in “normal” quasars. One example is a study (1986) on two HPQs: PKS 1510–0895 and PKS 0736+017. In both these cases, emission that is likely to be from an accretion disk was found to make up a quarter to a half of the visual light. There is also the case of the broad Hα line in the spectrum of BL Lacerta, which possibly indicates the presence of an ionizing continuum from a Seyfert-like accretion disk.

This brings us to the topic of emission lines in blazars. BL Lacs, by definition, have little to no emission lines present in their spectra, while HPQs have emission lines of the same strength as “normal” (that is, low polarization) quasars. It was (1997) found that BL Lacs and HPQs had similar distributions of emission line luminosities, and suggested that the difference between the two classes was that BL Lacs had a stronger continuum. However, it is possible instead that BL Lacs have systematically weaker emission lines, due to a relative dearth of emission line gas. A further alternative (which may be the case with BL Lacerta) may be that the accretion disk (if it is present) is weak, and so the ionizing continuum is not sufficient to produce emission lines that are observable over the non-thermal (synchrotron) continuum. In other words, the lines are there, but they are just very weak. This is similar to the beaming hypothesis, but without requiring the stronger continuum.

Finally, a word or two about the higher energy emission from blazars. The Energetic Gamma Ray Experiment Telescope (EGRET), on board the Compton Gamma Ray Observatory, detected gamma rays (at energies of › 100 MeV) from at least 93 blazars (66 blazars were detected with high confidence, and the remaining 27 were low confidence detections). It was found that these “EGRET blazars” emitted more energy in the gamma ray region of their spectrum than any other. (See the figures in Appendix D for examples of these types of sources.) Also, a small number of blazars have been detected at even higher energies (‹ 1 TeV), using•Cerenkov detectors such as Whipple and HEGRA.

The source of this gamma ray emission appears to be inverse Compton radiation, whereby highly relativistic particles interact with photons and “up-scatter” them, increasing the photon energy by a factor of » ±2 (where α is the Lorentz factor of the particle). If there is a significant amount of synchrotron emission, particularly at optical wavelengths, then the radiating particles are sufficiently relativistic to up-scatter any ambient photons to very high energies. The ambient photons can be the synchrotron emitted by the relativistic particles (in which case the up-scattered radiation is termed Synchrotron Self-Compton radiation, or SSC), or they can originate from some other source, such as the accretion disk, the broad line region, or even the cosmic microwave background (in which case it is termed External Compton radiation, or EC). This theory is able to explain quite well the broad-band spectra of blazars in terms of two broad peaks: the synchrotron peak, at low (i.e. IR to optical to soft X-ray) energies; and the Inverse Compton peak, at high (i.e. gamma ray).