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

Abstract

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 sub-stellar 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; 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 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. It is also noted that PKS 0537-441 is accompanied by several small, but extended companions, located a few arcseconds from the point-like 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.

Broad-band continuum and line emission of theγ-ray blazar PKS 0537–441

“…On the basis of their measured central wavelengths (see Table 2), the emission lines are consistently identified with Lyα, Si IV, and C IV at z = 0.896 ± 0.001. This is slightly larger than, but still consistent with historical redshift determination (z = 0.894, Peterson et al 1976), based on Mg IIλ2798 emission, but only marginally consistent with z = 0.892 ± 0.001 given by Lewis & Ibata (2000), based on the Balmer line series and O III. The intensities and equivalent widths of the UV emission lines are reported in Table 2.…”

“…2.4.1), assuming H 0 = 65 km s −1 Mpc −1 , Ω m = 0.3, Ω Λ = 0.7, we derive a total line luminosity L BLR ∼ 4.7 × 10 44 erg s −1 (the other observed optical emission lines are sufficiently weak that their contribution to the line luminosity is not significant, see Lewis & Ibata 2000). Using this and the external photon density assumed in our model, U ext = 6 × 10 −3 erg cm −3 , we can evaluate the size of the broad line region, R BLR 7.9×10 17 cm.…”

“…We therefore believe that the extra light reported by Lewis & Ibata (2000) is fully accounted for by the contribution of the scattered light. Moreover, we note that no signature of stellar features has been reported in the optical spectra of PKS 0537-441 to support the presence of a foreground galaxy (Stickel et al 1988;Lewis & Ibata 2000). Therefore, the microlensing hypothesis does not appear tenable (see also Romero et al 2000).…”

“…This source was also imaged by HST using WFPC2 and F702W filter Scarpa et al 2000) and found to be unresolved. However, based on the analysis of the same HST images, Lewis & Ibata (2000) claim the finding of faint non-axisymmetric extended emission. This result was obtained from the subtraction of a scaled PSF constructed with the Tiny Tim software (Krist 1995).…”

“…Search for extended optical emission around PKS 0537-441 has a long and controversial history (e.g., Falomo et al 1992;Lewis & Ibata 2000;Scarpa et al 2000). Clarifying the nature of this emission is of importance both for the study of the properties of the galaxy and environment hosting this very active nucleus and for the alleged possibility of microlensing effects.…”