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 mo dels have been developed to explain these extreme properties, several of wh ich have invoked the action of microlensing by substellar mass objects in a foreground galaxy; this not only introduces variability, but also amplifie s an otherwise normal quasar source. Here we present recent spectroscopy an d photometry of the southern BL Lac PKS 0537-441; with an inferred redshift of z similar to 0.9, it represents one of the most distant and most lumino us 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 accou nt for PKS 0537-441's extreme properties. While several observations have f ailed to detect any extended emission in PKS 0537-441, the Hubble Space Tel escope (HST) imaging data presented here indicate the presence of a galacti c component, although we fail to identify any absorption features that reve al the redshift of the emission. We also note that PKS 0537-441 is accompan ied 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 gravitati onally lensed images of more distant sources.