Modeling Faraday structures. I. 1100-1400 MHz

The input models include sources with one Faraday thin component, two Faraday thin components, and one Faraday thick component. The frequency set is similar to POSSUM/GALFACTS with a 300MHz bandwidth from 1.1 to 1.4GHz. We define three figures of merit motivated by the underlying science: (1) an average RM weighted by polarized intensity, RM_wtd_, (2) the separation {Delta}_{phi}_ of two Faraday components, and (3) the reduced chi-squared {Chi}_r_^2^.

Based on the current test data with a signal-to-noise ratio of about 32, we find the following. (1) When only one Faraday thin component is present, most methods perform as expected, with occasional failures where two components are incorrectly found. (2) For two Faraday thin components, QU-fitting routines perform the best, with errors close to the theoretical ones for RM_wtd_ but with significantly higher errors for {Delta}_{phi}_.

All other methods, including standard Faraday synthesis, frequently identify only one component when {Delta}_{phi}_ is below or near the width of the Faraday point-spread function. (3) No methods as currently implemented work well for Faraday thick components due to the narrow bandwidth. (4) There exist combinations of two Faraday components that produce a large range of acceptable fits and hence large uncertainties in the derived single RMs; in these cases, different RMs lead to the same Q, U behavior, so no method can recover a unique input model.

Further exploration of all these issues is required before upcoming surveys will be able to provide reliable results on Faraday structures.