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

摘要

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 determination 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 polarisation vector between the two wavelength ranges; these ambiguities are not resolved because the extent of wavelengths2 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 np ambiguity (±180° times an integer) at predictable polarization 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 wavelength ranges and the appearance of “Faraday ghosts”.