Probing the spin of the central black hole in the galactic centre with
  secondary images
  This paper explores the possibility of determining the spin of the
supermassive black hole (SMBH) in Sgr A*, by using secondary images of stars
orbiting the SMBH. The photons propagate close to the SMBH and their
trajectories probe the space time in a region where the spin of the SMBH is
important. We find the appearance of spikes in the secondary image, which
depends on the angular momentum and spin axis of the SMBH and study the
specific case of the star S2 in detail. The spikes has a magnitude of $\sim 29$
in the K-band and the required angular resolution is of order 15-20 $\mu$as.
The combination of these two requirements poses an extreme observational
challenge, but might be possible with interferometric observations in the
sub-mm regime. The next possible time frame for observing this effect on the
star S2 is in the late 2017 and then it repeats with the period of the star.
