HI intensity mapping with MeerKAT: Calibration pipeline for multi-dish autocorrelation observations

Authors:
  • JWJingying Wang
    Jingying Wang
  • MSMario G. Santos
    Mario G. Santos
  • PBPhilip Bull
    Philip Bull
  • KGKeith Grainge
    Keith Grainge
  • SCSteven Cunnington
    Steven Cunnington
  • JFJose Fonseca
    Jose Fonseca
  • MIMelis O. Irfan
    Melis O. Irfan
  • YLYichao Li
    Yichao Li
  • APAlkistis Pourtsidou
    Alkistis Pourtsidou
  • PSPaula S. Soares
    Paula S. Soares
  • MSMarta Spinelli
    Marta Spinelli
  • GBGianni Bernardi
    Gianni Bernardi

Abstract
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.