Microlensing of multiply-imaged compact radio sources - Evidence for compact halo objects in the disk galaxy of B1600+434

We present the first unambiguous case of external variability of a radio gr avitational lens, CLASS B1600+434. The Very Lai-ge Array (VLA) 8.5-GHz diff erence light curve of the lensed images, taking the proper time-delay into account, shows the presence of external variability with 14.6-sigma confide nce. We investigate two plausible causes of this external variability: scatterin g by the ionized component of the Galactic interstellar medium and microlen sing by massive compact objects in the bulge/disk and halo of the lens gala xy. Based on the tight relation between the modulation-index (fractional rm s variability) and variability time scale and the quantitative difference b etween the light curves of both lensed images, we conclude that the observe d short-term variability characteristics of the lensed images are incompati ble with scintillation in our Galaxy. This conclusion is strongly supported by multi-frequency Westerbork Synthesis Radio Telescope (WSRT) observation s at 1.4 and 5 Gi;z, which are in disagreement with predictions based on th e scintillation hypothesis. Several arguments against scintillation might n eed to be reevaluated if evidence is found for significant scatter-broadeni ng of lensed image B seen through the lens galaxy. However, the frequency-d ependence and time scale of variability from image A are not affected by th is and remain strong arguments against scintillation. On the other hand, a single superluminal jet-component in the source, havin g an apparent velocity 9 less than or similar to(nu(app)/c)less than or sim ilar to 26, a radius of 2-5 mu as and containing 5-11 % of the observed 8.5 -GHz source flux density, can reproduce the observed modulation-indices and variability time scale at 8.5 GHz, when it is microlensed by compact objec ts in the lens galaxy. It also reproduces the frequency-dependence of the m odulation-indices, determined from the independent WSRT 1.4 and 5-GHz obser vations. The difference between the modulation-indices of the lensed images (i.e. 2.8% and 1.6% at 8.5 GHz in 1998 for images A and B, respectively), if not affected by scatter-broadening of image B by the ionized ISM of the lens galaxy, can be explained through a different mass-function for the com pact objects in the bulge/disk and halo of the lens galaxy. Comparing the o bservations with microlensing simulations, we place a tentative lower limit of greater than or similar to 0.5 M. on the average mass of compact object s in the halo line-of-sight. The above-mentioned set of mass-function and s ource parameters is consistent, although not unique, and should only be reg arded as indicative. The only conclusion fully consistent with the data gathered thus far is tha t we have indeed detected radio microlensing. The far reaching consequence of this statement is that a significant fraction of the mass in the dark-ma tter halo at similar to 6kpc (h=0.65) above the lens-galaxy disk in B1600+4 34 consists of massive compact objects.