1995A&A...301..641R

The southern blazar PKS 0537-441 has recently displayed very strong intraday variability at 1.42GHz. Flux density variations with amplitudes of ∼45% on time scales as short as 10⁴s were registered, implicating brightness temperatures higher than 10²¹K in a straightforward intrinsic interpretation. We suggest and discuss the possibility that this variability have an extrinsic origin based on gravitational microlensing by small compact objects (namely brown dwarfs) in a foreground galaxy. In the proposed model the variability arises when a superluminal emission knot in the jet of the blazar is lensed by objects of the dark halo of the intervening galaxy. For an emission region with a superluminal velocity β_app∼10 in the lens plane, the masses of the lenses are in the 10^–4-10^–3M_☉ range. From the event rate we have estimated a dark matter density 0.009≲ρ_o≲0.043hM_☉/pc³ in the center of the foreground galaxy.