Probing interstellar scattering towards the Galactic centre with pulsar
  VLBI
  Temporal scatter-broadening can seriously affect our ability to find pulsars
orbiting the central mass in our Galaxy. Many of these invaluable probes of
geometry around the black hole are expected, but none have been found in close
orbits so far, possibly as result of strong scattering. The magnetar PSR
J1745-2900 discovered in 2013 at a separation of < 3 arcsec is not the optimal
type of pulsar for studies of general relativity, but it can be used to
investigate the scattering properties so that search strategies can be adapted
accordingly. This contribution presents an observation of this magnetar using
short baselines between VLBI stations in Europe in a non-standard
interferometry mode. The most important goal is determining the distance of the
scattering screen, or the distribution of scattering material if not confined
to one screen. The analysis is based on phase-binned visibilities that allow
measuring the shape of the scattering disk and how it grows with increasing
delay over the scattering tail of the pulse profile. Narrow rings growing with
the square root of delay are expected for a single thin scattering screen and
the preliminary results are indeed consistent with this expectation. This means
that most of the angular and temporal broadening is caused by the same and
relatively thin scattering screen and that, in contrast to standard models of
the interstellar scattering behaviour near the Galactic centre, this screen is
located about halfway between the centre and us.
