A Flux Scale for Southern Hemisphere 21cm EoR Experiments
  We present a catalog of spectral measurements covering a 100-200 MHz band for
32 sources, derived from observations with a 64-antenna deployment of the
Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER)
in South Africa. For transit telescopes such as PAPER, calibration of the
primary beam is a difficult endeavor, and errors in this calibration are a
major source of error in the determination of source spectra. In order to
decrease reliance on accurate beam calibration, we focus on calibrating sources
in a narrow declination range from -46d to -40d. Since sources at similar
declinations follow nearly identical paths through the primary beam, this
restriction greatly reduces errors associated with beam calibration, yielding a
dramatic improvement in the accuracy of derived source spectra. Extrapolating
from higher frequency catalogs, we derive the flux scale using a Monte-Carlo
fit across multiple sources that includes uncertainty from both catalog and
measurement errors. Fitting spectral models to catalog data and these new PAPER
measurements, we derive new flux models for Pictor A and 31 other sources at
nearby declinations. 90% of these confirm and refine a power-law model for flux
density. Of note is the new Pictor A flux model, which is accurate to 1.4% and
shows, in contrast to previous models, that between 100 MHz and 2 GHz, the
spectrum of Pictor A is consistent with a single power law given by a flux at
150 MHz of 382+/-5.4 Jy, and a spectral index of -0.76+/-0.01. This accuracy
represents an order of magnitude improvement over previous measurements in this
band, and is limited by the uncertainty in the catalog measurements used to
estimate the absolute flux scale. The simplicity and improved accuracy of
Pictor A's spectrum make it an excellent calibrator for experiments seeking to
measure 21cm emission from the Epoch of Reionization.
