Locating the VHE source in the Galactic Centre with milli-arcsecond
  accuracy
  Very high-energy gamma-rays (VHE; E>100 GeV) have been detected from the
direction of the Galactic Centre up to energies E>10 TeV. Up to now, the origin
of this emission is unknown due to the limited positional accuracy of the
observing instruments. One of the counterpart candidates is the super-massive
black hole (SMBH) Sgr A*. If the VHE emission is produced within ~10^{15} cm
~1000 r_G (r_G=G M/c^2 is the Schwarzschild radius) of the SMBH, a decrease of
the VHE photon flux in the energy range 100--300 GeV is expected whenever an
early type or giant star approaches the line of sight within ~ milli-arcseconds
(mas). The dimming of the flux is due to absorption by pair-production of the
VHE photons in the soft photon field of the star, an effect we refer to as
pair-production eclipse (PPE). Based upon the currently known orbits of stars
in the inner arcsecond of the Galaxy we find that PPEs lead to a systematic
dimming in the 100--300 GeV band at the level of a few per cent and lasts for
several weeks. Since the PPE affects only a narrow energy band and is well
correlated with the passage of the star, it can be clearly discriminated
against other systematic or even source-intrinsic effects. While the effect is
too small to be observable with the current generation of VHE detectors,
upcoming high count-rate experiments like the Cherenkov telescope array (CTA)
will be sufficiently sensitive. Measuring the temporal signature of the PPE
bears the potential to locate the position and size of the VHE emitting region
within the inner 1000 r_G or in the case of a non-detection exclude the
immediate environment of the SMBH as the site of gamma-ray production
altogether.
