TITLE:
Creating a “Faraday Ghost” inside the Rotation Measure Synthesis Technique, through a Wide Observational “Gap” in Wavelength Coverage
AUTHORS:
Jacques P. Vallée
KEYWORDS:
Magnetic Fields; Radio Polarization; Galactic Magnetic Fields; Spiral Galaxies
JOURNAL NAME:
International Journal of Astronomy and Astrophysics,
Vol.3 No.2,
May
31,
2013
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 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”.