Performance test of RM CLEAN and its evaluation with chi-square value
  RM CLEAN is a standard method to reconstruct the distribution of cosmic
magnetic fields and polarized sources along the line of sight (LOS) from
observed polarization spectrum. This method is similar to the CLEAN algorithm
for aperture synthesis radio telescope images but it is rather unclear in what
cases RM CLEAN works well. In this paper, we evaluate the performance of RM
CLEAN by simulating spectro-polarimetric observations of two compact sources
located in the same LOS, varying the relative initial polarization angle and
Faraday depth systematically. Especially, we focus on if the two polarized
sources can be resolved in the Faraday depth space and how well the source
parameters can be estimated. We confirm the previous studies that two sources
cannot be resolved when they are closely located in the Faraday depth space for
specific values of the relative initial polarization angle. Further, we
calculate the chi-square value for the fit between the mock data of
polarization spectrum and the one from RM CLEAN. Then we find that the
chi-squared value is not always significantly large even when RM CLEAN gives
wrong results.
