HI intensity mapping with MeerKAT: Calibration pipeline for multi-dish
  autocorrelation observations
  While most purpose-built 21cm intensity mapping experiments are close-packed
interferometer arrays, general-purpose dish arrays should also be capable of
measuring the cosmological 21cm signal. This can be achieved most efficiently
if the array is used as a collection of scanning autocorrelation dishes rather
than as an interferometer. As a first step towards demonstrating the
feasibility of this observing strategy, we show that we are able to
successfully calibrate dual-polarisation autocorrelation data from 64 MeerKAT
dishes in the L-band (856-1712 MHz, 4096 channels), with 10.5 hours of data
retained from six nights of observing. We describe our calibration pipeline,
which is based on multi-level RFI flagging, periodic noise diode injection to
stabilise gain drifts and an absolute calibration based on a multi-component
sky model. We show that it is sufficiently accurate to recover maps of diffuse
celestial emission and point sources over a 10 deg x 30 deg patch of the sky
overlapping with the WiggleZ 11hr field. The reconstructed maps have a good
level of consistency between per-dish maps and external datasets, with the
estimated thermal noise limited to 1.4 x the theoretical noise level (~ 2 mK).
The residual maps have rms amplitudes below 0.1 K, corresponding to <1% of the
model temperature. The reconstructed Galactic HI intensity map shows excellent
agreement with the Effelsberg-Bonn HI Survey, and the flux of the radio galaxy
4C+03.18 is recovered to within 3.6%, which demonstrates that the
autocorrelation can be successfully calibrated to give the zero-spacing flux
and potentially help in the imaging of MeerKAT interferometric data. Our
results provide a positive indication towards the feasibility of using MeerKAT
and the future SKA to measure the HI intensity mapping signal and probe
cosmology on degree scales and above.
