2000ApJ...528..650L

The BL Lac family of active galaxies possess almost featureless spectra and exhibit rapid variability over their entire spectral range. A number of models have been developed to explain these extreme properties, several of which have invoked the action of microlensing by substellar mass objects in a foreground galaxy; this not only introduces variability, but also amplifies an otherwise normal quasar source. Here we present recent spectroscopy and photometry of the southern BL Lac PKS 0537-441; with an inferred redshift of z∼0.9, it represents one of the most distant and most luminous members of the BL Lac family. The goal of the observations was not only to confirm the redshift of PKS 0537-441, but also to determine the redshift of a putative galaxy along the line of sight to the BL Lac object; it has been proposed that this galaxy is the host of microlensing stars that account for PKS 0537-441's extreme properties. While several observations have failed to detect any extended emission in PKS 0537-441, the Hubble Space Telescope (HST) imaging data presented here indicate the presence of a galactic component, although we fail to identify any absorption features that reveal the redshift of the emission. We also note that PKS 0537-441 is accompanied by several small but extended companions, located a few arcseconds from the pointlike BL Lac source. Two possibilities present themselves: either they represent true companions of PKS 0537-441, or are themselves gravitationally lensed images of more distant sources.