Imaging pulsar echoes at low frequencies
  Interstellar scattering is known to broaden distant objects spatially and
temporally. The latter aspect is difficult to analyse, unless the signals carry
their own time stamps. Pulsars are so kind to do us this favour. Typically the
signature is a broadened image with little or no substructure and a similarly
smooth exponential scattering tail in the temporal profile. The case of the
pulsar B1508+55 is special: The profile shows additional components that are
moving relative to the main pulse with time. We use low-frequency VLBI with
LOFAR to test the hypothesis that these components are actually such
scattering-induced echoes, by trying to detect the expected angular offset.
Using international stations (plus the Kilpisj\"arvi Atmospheric Imaging
Receiver Array "KAIRA") and the phased-up core of the LOFAR array, we can do
interferometry at high resolution in time and space. This contribution presents
a selection of results from an ongoing large-scale monitoring campaign. We can
not only detect the offset, but even image a full string of echoes, and relate
the positions with delays. What we find is apparently consistent with
scattering by highly aligned components in a single screen at a distance of 120
pc. Further investigations will improve our understanding of the scattering
process as basis of using the scattering-induced subimages as arms of a giant
interstellar interferometer with insanely high resolution.
