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.