The BL Lacertae Objects

Abstract

Although discovered relatively recently, the class of objects of which BL Lac is the prototype is currently the subject of very active study. The peculiar character of these objects became apparent after the identification (Schmitt 1968) of the variable star BL Lacertae with the unusual radio source VRO 42.22.01 (MacLeod & Andrew 1968) and subsequent detections of rapid variations in the radio spectral flux (Biraud & Veron 1968, Andrew et a1. 1969, Gower 1969) and linear polarization (Olsen 1969). In addition the optical radiation was shown to exhibit a continuous spectrum with neither emission nor absorption lines (Oke, Neugebauer & Becklin 1969; DuPuy et a1. 1969) and a relatively high degree of linear polarization (Visvanathan 1969). Subsequent optical, infrared, and radio observations by several investigators led Strittmatter et al. (1972) to suggest that objects similar to BL Lac comprise a class of quasi-stellar objects (QSOs) with some combination of the following characteristics: (a) absence of emission lines in the core source; (b) rapid variability at radio, infrared, and visual wavelengths; (c) nonthermal continuum with most of the luminosity radiated at infrared wavelengths; and (Ii) strong and rapidly varying polarization. Additional examples ofthis class investigated by Strittmatter et a1. (1972) were OJ 287 (VRO 20.08.01), ON 231 (W Com), ON 325 (B2 1 215+30), and PKS 1 514-24 (AP Lib). The number of BL Lac objects known now exceeds 30 and is increasing rapidly (see Table 1; see also Kinman 1976). Research on BL Lac objects has been concerned mainly with (a) documenting their properties, (b) investigating their relationship to QSOs and galaxies, (e) determining their distances in the absence of emission lines, and (d) accounting for the continuum emission within the constraints set by their rapid variability. A tacit hope has been that the apparently more rapid activity observed in BL Lac objects would lead to an understanding not only of these objects but also of QSOs. In this review we discuss progress made in this area up to approximately September 1975.