ASKAP reveals giant radio halos in two merging SPT galaxy clusters --
  Making the case for a direction-dependent pipeline --
  Early science observations from the Australian Square Kilometre Array
Pathfinder (ASKAP) have revealed clear signals of diffuse radio emission
associated with two clusters detected by the South Pole Telescope via their
Sunyaev Zel'dovich signal. SPT CLJ0553-3342 (MACSJ0553.4-3342) and SPT
CLJ0638-5358 (Abell S0592) are both high-mass lensing clusters that have
undergone major mergers. To improve the data products of these ASKAP early
science observations and create science-fidelity images of the galaxy clusters,
we performed direction-dependent (DD) calibration and imaging using
state-of-the-art software {\sc killMS} and {\sc DDFacet}. We find that
artefacts in the ASKAP images are greatly reduced after directional
calibration. Here we present our DD calibrated ASKAP radio images of both
clusters showing unambiguous giant radio halos with largest linear scales of
$\sim1$~Mpc. The halo in MACSJ0553.4-3342 was previously detected with GMRT
observations at 323 MHz, but appears more extended in our ASKAP image. Although
there is a shock detected in the thermal X-ray emission of this cluster, we
find that the particle number density in the shocked region is too low to allow
for the generation of a radio shock. The radio halo in Abell S0592 is a new
discovery, and the Southwest border of the halo coincides with a shock detected
in X-rays. We discuss the origins of these halos considering both the hadronic
and turbulent re-acceleration models as well as sources of \textit{seed}
electrons. This work gives a positive indication of the potential of ASKAP's
Evolutionary Map of the Universe (EMU) survey in detecting intracluster medium
radio sources, and showcases the improvement in data products after utilising
third-generation calibration techniques.
