Exploring the magnetic field structure of the Milky Way with pulsars in the SKA era

TL;DR

This work addresses the poorly constrained, large-scale magnetic field structure of the Milky Way, especially in far-disk and halo regions, by proposing a pulsar-based approach using the SKA. The authors advocate wide-band, high-sensitivity RM and DM measurements across thousands of pulsars, leveraging RM synthesis to produce Faraday-depth maps and 3D tomography of the magneto-ionic medium, with ⟨B_{||}⟩ estimated from RM and DM via $RM = 0.812 \int n_e \mathbf{B} \cdot d\mathbf{l}$, $DM = \int n_e dl$, and $\langle B_{||} \rangle = 1.232 \frac{RM}{DM}$. They forecast substantial gains in pulsar yields (e.g., ~13,000 normal pulsars and ~1,000 MSPs for SKA1-AA4; ~10,000 normal pulsars and ~800 MSPs for AA*; potentially 20–30k pulsars with SKA2) and thousands of RM measurements along diverse sightlines, enabling detailed mapping of both disk and halo fields. The study highlights disk fields aligned with spiral arms and halo toroidal structures, and emphasizes SKA1-Low's role in detecting faint halo pulsars and improving RM grids, which will advance our understanding of Galactic dynamos and cosmic-ray propagation. Overall, the SKA era is poised to deliver a transformative, high-resolution view of Galactic magnetism through pulsar RM/DM tomography across the Milky Way.

Abstract

The magnetic field structure of the Milky Way can offer critical insights into the origin of galactic magnetic fields. Measurements of magnetic structures of the Milky Way are still sparse in far regions of the Galactic disk and halo. Pulsars are the best probes for the three-dimensional structure of the Galactic magnetic field, primarily owing to their highly polarized short-duration radio pulses, negligible intrinsic Faraday rotation compared to the contribution from the medium in front, and their widespread distribution throughout the Galaxy across the thin disk, spiral arms, and extended halo. In this article, we give an overview of Galactic magnetic field investigation using pulsars. The sensitive SKA1 design baseline (AA4) will increase the number of known pulsars by a factor of around three, and the initial staged delivery array (AA*) will probably double the total number of the current pulsar population. Polarization observations of pulsars with the AA* telescopes will give rotation measures along several thousand lines of sight, enabling detailed exploration of the magnetic structure of both the Galactic disk and the Galactic halo.

Figures (6)

• Figure 1: Distance distribution of known pulsars in the Galactic disk and in the halo ($|b|\geq 8^{\circ}$). The magenta histograms represent pulsars with available rotation measures (RMs).
• Figure 2: The distributions of 1.4 GHz flux densities of known pulsars currently without RM values at $\delta<30^{\circ}$. The sensitivity curves for different DMs of the SKA1-Mid AA4 and AA* are given by adopting an SNR of 50, an integration time of 60 minutes, a sampling time of 50 $\mu$s and a spin-period-dependent pulse duty cycle (with a typical value of 0.1 for $P$$<$ 10 ms but declining with $P^{-1/2}$ when $P$$>$ 10 ms).
• Figure 3: RM distribution of pulsars located with $|b|<8^{\circ}$ projected onto the Galactic plane. The magenta and cyan symbols denote the positive and negative RMs from the FAST GPPS related projects whx+23. New FAST measurements afterwards are indicated by red crosses and blue circles for positive and negative values (Xu et al. in preparation). The approximate locations of spiral arms hh14 are indicated in gray shadow. Only a small portion, denoted by yellow, is invisible to the SKA.
• Figure 4: Large-scale magnetic field directions in the Galactic disk hmvd18.
• Figure 5: The sky distribution of all available RMs of pulsars (top) and extragalactic radio sources (bottom). At present, the total number of pulsar RM is more than 2000, including a collection of new RMs from MeerKAT observations pkj+23 and new FAST observations. As same as in Figure \ref{['DiskDistri']}, the magenta and cyan symbols are from the FAST GPPS related projects xhwy22whx+23. New FAST measurements are indicated by red crosses and blue circles for positive and negative values in the top panel (Xu et al. in preparation).