An Investigation of Gravitational Lensing in the Southern BL Lac PKS 0537–441

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.