Confirmation of a Faraday Rotation Measure Anomaly in Cygnus

TL;DR

This paper confirms a significant reversal in the Faraday Rotation Measure in Cygnus, linking it to an expanding plasma shell associated with the Cygnus OB1 superbubble, and provides a simple physical model for this phenomenon.

Contribution

It verifies the Faraday Rotation Measure reversal in Cygnus and introduces a straightforward plasma shell model explaining the observed magnetic field and density enhancements.

Findings

Confirmed the RM sign reversal in Cygnus.

Linked the RM feature to an expanding plasma shell.

Model explains the magnitude and scale of the RM change.

Abstract

We confirm the reality of a reversal of the sign of the Faraday Rotation Measure in the Galactic plane in Cygnus (Lazio et al, 1990), possibly associated with the Cygnus OB1 association. The rotation measure changes by several hundred rad/m$^2$ over an angular scale of $2-5^{\circ}$. We show that a simple model of an expanding plasma shell with an enhanced density and magnetic field, consistent with observations of H$\alpha$ emission in this part of sky, and physically associated with a superbubble of the Cygnus OB1 association, can account for the magnitude and angular scale of this feature.