Probing the magneto-ionic medium of the Milky Way using pulsars

Abstract

Magnetic fields are fundamental to the dynamics of the interstellar medium (ISM) in spiral galaxies and are often separated into large-scale, regular ($\mathbf{B}$) and small-scale, random ($\mathbf{b}$) components. The thermal electron density, $n_{\mathrm{e}}$, can also be divided into large-scale, diffuse, $⟨n_{\mathrm{e}}⟩$, and small-scale, clumpy, $\hat{I}´n_{\mathrm{e}}$, components. Estimating the properties of $b$ and $\hat{I}´n_{\mathrm{e}}$ from observations, even within the Milky Way, has long been challenging. This work addresses the challenge using pulsars, which probe the Milky Way's magneto-ionic medium. Using data of more than 1200 pulsars from the Australia Telescope National Facility pulsar catalogue, we combine dispersion ($\mathrm{DM}$) and rotation ($\mathrm{RM}$) measures with theoretical models to estimate both small- and large-scale properties of the Galactic magnetic field and thermal electron density. We find no significant correlation between the average parallel magnetic field strength, $⟨B_{\parallel }⟩[\hat{I}¼\mathrm{G}]=1.232\mathrm{RM}[\mathrm{rad}{\mathrm{m}}^{-2}]/\mathrm{DM}[\mathrm{pc}\mathrm{c}{\mathrm{m}}^{-3}]$, and pulsar distance. For pulsars within $20\mathrm{kpc}$, we estimate $|B|\approx 1.2\hat{I}¼\mathrm{G}$ and $⟨n_{\mathrm{e}}⟩\approx 0.05{\mathrm{cm}}^{-3}$. More importantly, we determine correlation lengths of small-scale components, ${\ell }_b\approx 20$ -- $30\mathrm{pc}$ and ${\ell }_{\hat{I}´n_{\mathrm{e}}}\approx 250$ -- $300\mathrm{pc}$. At smaller distances, $B$ remains roughly constant, while $⟨n_{\mathrm{e}}⟩$ increases and both length scales decrease. These results refine our understanding of fundamental scales in the magneto-ionic medium, aiding the interpretation of extragalactic $\mathrm{RM}$s and providing insights into the role of magnetic fields in galaxies.

16 pages, 6 figures, accepted in MNRAS