HERA Mock Observations: Looking for Closure HERA Memorandum Number 13
  We investigate the use of closure phase as a method to detect the HI 21cm
signal from the neutral IGM during cosmic reionzation. Closure quantities have
the unique advantage of being independent of antenna-based calibration terms.
We employ realistic, large area sky models from Sims et al. (2016). These
include an estimate of the HI 21cm signal generated using 21cm FAST, plus
continuum models of both the diffuse Galactic synchrotron emission and the
extragalactic point sources. We employ the CASA simulator and adopt the
Dillon-Parsons HERA configuration to generate a uv measurement set. We then use
AIPS to calculate the closure phases as a function of frequency ('closure
spectra'), and python scripts for subsequent analysis. We find that the closure
spectra for the HI signal show dramatic structure in frequency, and based on
thermal noise alone, the redundant HERA-331 array should detect these
fluctuations easily. Comparatively, the frequency structure in the continuum
closure spectra is much smoother than that seen in the HI closure spectra.
Unfortunately, when the line and continuum signals are combined, the continuum
dominates the visibilities at the level of 10^3 to 10^4, and the line signal is
lost. We have investigated fitting and removing smooth curves in frequency to
the line plus continuum closure spectra, and find that the continuum itself
shows enough structure in frequency in the closure spectra to preclude
separation of the continuum and line based on such a process. We have also
considered the subtraction of the continuum from the visibilities using a sky
model, prior to calculation of the closure spectra. TRUNCATED.
