Distance and line-of-sight extent of the outer-Galaxy synchrotron spur

Determine the line-of-sight extent and distance from the observer of the localized outer-Galaxy synchrotron enhancement modeled as a spiral spur, since the available synchrotron data do not constrain either quantity.

Background

The paper introduces a phenomenological outer-Galaxy foreground component to account for extended total-intensity emission that may not be captured by polarization-based foreground templates. Its angular position and widths can be fitted, but the physical placement of the emitting structure along the line of sight is not determined by the available synchrotron maps.

For concreteness, the authors place the spur at a distance of 1 kpc, while emphasizing that this is an assumed location rather than an inference. Resolving the distance and depth would be necessary to determine the associated cosmic-ray electron normalization and to interpret the feature physically rather than phenomenologically.