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”.
Highlights
Studies of the magneto-ionic medium in galaxies make use of the “angular rotation of the plane of polarisation”, ΔPA, of the electric vector in the plane of the incoming electromagnetic wave while travelling in a magnetized medium, known as Faraday rotation
In the linear rotation measures (RM) technique, using the position angle versus wavelength squared plane, this is prevented by doing a set of actions to effectively choose the best RM slope through a standard least-squares fit ([8,9,10])
In the RMS technique, [23] suggested that the 180degree ambiguity could manifest itself in the sidelobes of the RM transfer function. [24] used both the linear RM technique and the RM synthesis technique near 350 MHz, and found that the RM synthesis technique may yield an erroneous Faraday structure in the presence of multiple RM components
Summary
Where ΔPA is in radians, λ is in m, and RM [radians/m2] is the rotation measure. The PA of the electric vector of the photon at λ0 cm wavelength is perpendicular to the magnetic field in the emitting region, in the optically-thin synchrotron emission. In the linear Rotation Measure technique, the n ambiguity ( 180 ̊ times an integer) is tested by adding a different n value to the polarization position angles, and solved by a least-squares fit [lsf] of the slope in a plot of the observed linear polarisation position angle as a function of the square of the observing wavelength. A newer RM synthesis [RMS] technique has been proposed recently, to perform a transformation from the observational position-angle versus 2 space to the computational Faraday depth space, to obtain the characteristic RM. Using the linear RM technique, they found the RM near 1365 and near 1485 MHz (and the proper “n” integer value) If these results differed, they discarded the results; the remaining “well-behaved” sources were processed through the RM synthesis technique. Farnsworth et al [14] studied the RM resulting from different techniques in six WSRT areas: Aries, Coma SW, Coma NW, Coma NE 1259 + 2758, A1453 + 4025 and B 1620 + 6012, and found the RM synthesis technique to give differing results in 1 case out of 7 (their lists 6 to 12)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
