  Episodically accreting black holes are thought to produce flares when a chunk
of particles is accelerated to high velocity near the black hole horizon. This
also seems to be the case of Sagittarius A* in the Galactic Center, where the
broad-band radiation is produced, likely via the synchrotron self-Compton
mechanism. It has been proposed that strong-field gravitational lensing
magnifies the flares. The effect of lensing is generally weak and requires a
fine-tuned geometrical arrangement, which occurs with only a low probability.
However, there are several aspects that make Sagittarius A* a promising target
to reveal strong gravity effects. Unlike type II (obscured) active galaxies,
chances are that a flare is detected at high inclination, which would be
favourable for lensing. Time delays can then significantly influence the
observed flare duration and the form of light-curve profiles. Here we discuss
an idea that the impact of lensing amplification should be considerably
enhanced when the shape of the flaring clump is appropriately elongated in the
form of a spiral wave or a narrow filament, rather than a simple (circular)
spot which we employed previously within the phenomenological `orbiting spot
model'. By parameterizing the emission region in terms of the spiral shape and
contrast, we are able to extend the spot model to more complicated sources. In
the case of spirals, we notice a possibility that more photons reach a distant
observer at the same moment because of interplay between lensing and
light-travel time. The effect is not symmetrical with respect to leading versus
trailing spirals, so in principle the source geometry can be constrained. In
spite of this, the spot model seems to provide entirely adequate framework to
study the currently available data.
