| Description: | BL Lac objects compose an especially exotic subclass of AGN explained in the standard AGN unification paradigm as low-luminosity radio galaxies with a relativistic jet pointed toward the observer. Beamed jet emission often dominates their spectral energy distributions, rendering their optical spectra devoid of strong spectral features. We have assembled the largest uniform BL Lac sample to date (~700 objects) from the Sloan Digital Sky Survey (SDSS), and our selection is complemented by information from the FIRST/NVSS radio surveys and the RASS X-ray survey. I will discuss our sample's statistical properties, including a peculiar subset of ~100 radio-quiet BL Lac candidates. Some of these objects may indeed populate the radio-shy tail of the much larger radio-loud BL Lac distribution, while others may constitute a new population of AGN with intrinsically weak emission lines. I will conclude by briefly discussing recent attempts to unify our BL Lac sample with other classes of radio-loud AGN and X-ray binaries.
Image above: Sloan Digital Sky Survey (SDSS) image and spectrum of BL Lac object SDSS J115034.75+415440.1, one of ~700 BL Lac candidates recovered by the SDSS thus far. This source is also a strong radio and X-ray emitter, fitting the standard model where BL Lacs are low luminosity radio galaxies viewed nearly along the axis of a relativistic jet. Its optical spectrum is so dominated by non-thermal jet emission that it appears devoid of any emission lines. |
