  We present the first unambiguous case of external variability of a radio
gravitational lens, CLASS B1600+434. The 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-sigma 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 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 WSRT observations at 1.4 and 5 GHz, which
are in strong disagreement with predictions based on the scintillation
hypothesis. ... On the other hand, a single superluminal jet-component, having
an apparent velocity 9<=(v_app/c)<=26, a radius of 2-5 micro-arcsec 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 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 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 ~6 kpc (h=0.65) above the lens-galaxy
disk in B1600+434 consists of massive compact objects. [abridged]

Astrophysics