Creating a “Faraday Ghost” inside the Rotation Measure Synthesis Technique, through a Wide Observational “Gap” in Wavelength Coverage

Abstract

Several recently published Faraday rotation measures (RM) derived using the novel RM synthesis technique are likely in error. If a set of polarimetric observations contains a large gap in the wavelength coverage, the rotation measure de termination is sometimes ambiguous; this is also true even when two long wavelength ranges are observed but are separated by a wide gap. Essentially, there are 180 ° ambiguities in the observed Position Angle of the electric polarisa tion vector between the two wavelength ranges; these ambiguities are not resolved because the extent of wavelengths 2 covered, within each of the two ranges, is too small to uniquely determine the RM in isolation. We find that unphysical “Faraday ghosts” can be mathematically constructed with a n p ambiguity ( ± 180 ° times an integer) at predictable polari zation position angles when using only two wavelength ranges separated by a gap, as a function of the width of the gap ( Equation ( 4 )) . Our computations suggest an empirical correlation between an observational gap between two wave length ranges and the appearance of “Faraday ghosts”.