Modeling Local Bubble analogs II: Synthetic Faraday rotation maps

Abstract

Faraday rotation describes the change of the linear polarization angle of radiation passing through a magnetized plasma and it is quantified by the rotation measure (RM), which is related to the line-of-sight (LOS) magnetic field component and the thermal electron density traversed by light along its path toward the observer. However, it is challenging to disentangle the signal from different LOS portions and separate the contribution from the local ISM. This is particularly relevant since the Sun is located within the Local Bubble (LB), a low-density and hot cavity formed by past SN events, making it essential to investigate how this environment may influence the observed RM values. The present study investigates the imprint of the local environment on the synthetic RM signal, as measured by an observer within a LB-like cavity. The RM derived from diffuse polarized synchrotron radiation produced by CR electrons at decimeter wavelengths is also analyzed. We produce synthetic RM maps for an observer placed inside a LB candidate, selected from a MHD simulation that resembles the properties of the ISM in the Solar vicinity. Using the capabilities of the radiative transfer code POLARIS, we study the imprint of the cavity walls on the RM signal. As the MHD simulation does not account for CR diffusion, we develop a CR toy-model to study the Faraday rotation of the diffuse polarized synchrotron radiation. We find that (i) the imprint of local structures, such as the walls of the LB candidate and the edges of other supernovae blown cavities, is of fundamental importance for interpreting the global Faraday sky; (ii) the LB has a non negligible contribution to the sinusoidal patterns of RM as a function of Galactic longitude seen in observations; and (iii) the RM signal from diffuse synchrotron emission shows a strong correspondence with the RM signal generated by the LB candidate walls.