Performance test of RM CLEAN and its evaluation with chi-square value

Abstract

RM CLEAN is a standard method to reconstruct the distribution of cosmic magnetic fields and polarized sources along the line of sight (LOS) from the observed polarization spectrum. This method is similar to the CLEAN algorithm for aperture synthesis of 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. In particular, we focus on whether 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 polarization spectrum data and the one from RM CLEAN. We find that the chi-square value is not always significantly large even when RM CLEAN gives wrong results.