G27.06+0.04. The RMs presented in the right-most panel typically have uncertainties of 30–40 rad m⁻². The black contours on the rotation measure map show the regions for which the TIFR GMRT Sky Survey-NRAO VLA Sky Survey (TGSS-NVSS) spectral index was measured. The eastern region (on the arc) and the western region (overlapping with two H II regions) have a similar spectral index of ~ − 0.5. For this andall subsequent figures, the directions of the electric field vector position angles (after accounting for Faraday rotation) are plotted with black lines on the polarization map.