Observational constraints on models for the interstellar magnetic field in the Galactic disk

TL;DR

This study uses pulsar rotation and dispersion measures to test three common models of the Galactic magnetic field, finding that none fit well and suggesting a more complex magnetic field structure in the Galactic disk.

Contribution

It provides the first comprehensive observational constraints on the three main theoretical models of the Galactic magnetic field using pulsar data.

Findings

All three models are inconsistent with pulsar data.

None of the models significantly outperforms the others.

The Galactic magnetic field likely has a more complex pattern.

Abstract

Our purpose is to place firm observational constraints on the three most widely used theoretical models for the spatial configuration of the large-scale interstellar magnetic field in the Galactic disk, namely, the ring, the axisymmetric and the bisymmetric field models. We use the rotation measures (RMs) of low-latitude Galactic pulsars and combine them with their dispersion measures and estimated distances to map out the line-of-sight component of the interstellar magnetic field in the near half of the Galactic disk. We then fit our map of the line-of-sight field to the three aforementioned theoretical field models and discuss the acceptability of each fit, in order to determine whether the considered field model is allowed by the pulsar data or not. Strictly speaking, we find that all three field models are ruled out by the pulsar data. Furthermore, none of them appears to perform significantly better than the others. From this we conclude that the large-scale interstellar magnetic field in the Galactic disk has a more complex pattern than just circular, axisymmetric or bisymmetric.