Testing the lensing hypothesis for compact double radio sources and other milli-lensing candidates

Abstract

Condensed objects of ≈ 10⁶ M_⊙ could manifest themselves via their lensing action on the radio cores of background quasars, thus giving rise to compact double sources, or more complex small-separation lenses, on the milli-arcsecond scale. Such compact bodies could be dense cores of small galaxies or remnants of pregalactic stars which have been postulated as a candidate for the dark matter in the universe. Search for examples of milli-arcsecond scale lensing, either due to such remnants or dwarf galaxies, forms a central objective of several new VLBI projects which has already revealed a good candidate. Here we point out that the milli-lensing of superluminal cores of quasars/blazars, when it occurs, can lead to substantial changes in the apparent orientation, flux ratio and velocities of the image components over periods as short as a year. This is essentially because the proper motion of the sub-components of such cores is comparable to the image splitting scales. Hence VLBI monitoring programmes can provide a fairly simple test of the milli-lensing hypothesis.