2000A&A...358..793K

We present the first unambiguous case of external variability of a radio gravitational lens, CLASS B1600+434. The Very Large Array (VLA) 8.5-GHz difference light curve of the lensed images, taking the proper time-delay into account, shows the presence of external variability with 14.6-σ confidence. We investigate two plausible causes of this external variability: scattering by the ionized component of the Galactic interstellar medium and microlensing by massive compact objects in the bulge/disk and halo of the lens galaxy. Based on the tight relation between the modulation-index (fractional rms variability) and variability time scale and the quantitative difference between the light curves of both lensed images, we conclude that the observed short-term variability characteristics of the lensed images are incompatible with scintillation in our Galaxy. This conclusion is strongly supported by multi-frequency Westerbork Synthesis Radio Telescope (WSRT) observations at 1.4 and 5GHz, which are in disagreement with predictions based on the scintillation hypothesis. Several arguments against scintillation might need to be reevaluated if evidence is found for significant scatter-broadening of lensed image B seen through the lens galaxy. However, the frequency-dependence and time scale of variability from image A are not affected by this and remain strong arguments against scintillation. On the other hand, a single superluminal jet-component in the source, having an apparent velocity 9≲(v_app/c)≲26, a radius of 2-5µas and containing 5-11% of the observed 8.5-GHz source flux density, can reproduce the observed modulation-indices and variability time scale at 8.5GHz, when it is microlensed by compact objects in the lens galaxy. It also reproduces the frequency-dependence of the modulation-indices, determined from the independent WSRT 1.4 and 5-GHz observations. The difference between the modulation-indices of the lensed images (i.e. 2.8% and 1.6% at 8.5GHz 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 compact objects in the bulge/disk and halo of the lens galaxy. Comparing the observations with microlensing simulations, we place a tentative lower limit of >0.5M_☉ on the average mass of compact objects in the halo line-of-sight. The above-mentioned set of mass-function and source parameters is consistent, although not unique, and should only be regarded as indicative. The only conclusion fully consistent with the data gathered thus far is that we have indeed detected radio microlensing. The far reaching consequence of this statement is that a significant fraction of the mass in the dark-matter halo at ∼6kpc (h=0.65) above the lens-galaxy disk in B1600+434 consists of massive compact objects.