BL Lacertae

Contents

Introduction

BL Lacertae (BL Lac or S4 2200+420) was known to be variable in the visible part of the electromagnetic spectrum from as early as 1929, and because of its stellar appearance it was originally thought to be a VARIABLE STAR. It is located in the constellation Lacerta, the lizard (RA (J2000) = 22h 02m 43.29s, DEC (J2000) = +42^o 16’ 39.98”). Observations in the late 1960s showed it to have highly variable radio emission as well.

BL Lac was found to be an extragalactic object with the measurement of its redshift (z = 0.069, based on the detection of very weak emission lines) in the early 1970s. During the same period, it was also understood to be an unusual type of ACTIVE GALACTIC NUCLEUS (AGN). Thus, BL Lacertae is the prototype of the class of AGN known as BL LACERTAE OBJECTS (also called BL Lacs). Collectively with some quasars, such as 3C273 and 3C279, they are known as ‘Blazars’. These objects are characterized as having bright nuclei with strongly polarized optical emission and large variability in all wavelengths. Their nonthermal radio-to-gamma-ray continua are thought to be emitted by a relativistic jet oriented close to the line-of-sight. BL Lac objects make up a small subset of AGNs, with about 350 known at present.

BL Lac is an important member of the class because it is relatively nearby, such that details can be well studied. The galaxy surrounding the active nucleus, or host galaxy, is a giant ELLIPTICAL GALAXY, typical of the host galaxies around other BL Lac objects.

Radio observations show BL Lac to exhibit SUPERLUMINAL MOTION, indicative of material being ejected at relativistic velocities from the nucleus. Observations by the Compton Gamma-Ray Observatory showed BL Lac to be a strong gamma-ray source, together with many other blazers. The rapid variability timescales and high luminosities observed at these energies indicate that the gamma rays are produced in a very compact region of the jet.

The nuclear emission in BL Lac is nonthermal and produces a continuum spectrum consisting of two components. The radio-to-ultraviolet spectrum is produced by synchrotron emission with peak power in the infrared-optical region, while the x-ray-to-gamma-ray spectrum is produced by inverse Compton emission. These two components are present in all BL Lacs objects.

BL Lac objects, as a class, are characterized by optical spectra which are featureless or which have extremely weak lines (less than 5Å equivalent width). This property makes the determination of their distances difficult. BL Lac itself is no exception. In the mid-1990s, however, strong, broad and variable emission lines (e.g. H-alpha, equivalent width = 7.3Å luminosity ~2x10⁴¹ erg cm^-2 s^-1) appeared in the spectrum of BL Lac. Current thinking is that the lines appear when the variable continuum emission is low. Furthermore, the presence of emission lines indicates the presence of a radiation field external to the jet, which may play an important role in the jet energetics (providing seed photons for the inverse Compton emission responsible for the gamma-rays).

Bibliography

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• Sambruna R M, Ghisellini G, Hooper E, Koolgaard R I, Pesce J E and Urry C M 1999, ASCA and contemporaneous ground-based observations of the BL Lacertae objects 1749+096 and 2200+420 (BL Lac) Astrophys.J. 515 140-52
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• Vermeulen R C, Ogle P M, Tran H D, Browne I W A, Cohen M H, Readhead A C S, Taylor G B and Goodrich R W 1995 When is BL Lac not a BL Lac? Astrophys.J.Lett. 452 L5-8