The coherent magnetic field of the Milky Way halo, the Local Bubble, and the Fan region

¹ Université Libre de Bruxelles, CP225 Boulevard du Triomphe, 1050 Brussels, Belgium
² Université de Paris Cité, CNRS, Astroparticule et Cosmologie, 75013 Paris, France

^★ Corresponding author; alexander.korochkin@ulb.be

Received: 9 July 2024
Accepted: 1 November 2024

Abstract

Context. A recent catalogue of the Faraday rotation measures (RMs) of extragalactic sources, together with the synchrotron polarisation data from WMAP and Planck, provide us with a wealth of information on the magnetic fields of the Galaxy. However, the integral character of these observables, together with our position inside the Galaxy, make the inference of the coherent Galactic magnetic field (GMF) complicated and ambiguous.

Aims. We combine several phenomenological components of the GMF – the spiral arms, the toroidal halo, the X-shaped field, and the field of the Local Bubble – to construct a new model of the regular GMF outside the thin disc.

Methods. We use the binned χ² approach to fit the parameters of the model to the data. To have control over the relative contributions of the RM and polarisation data to the fit, we pay special attention to the estimation of errors in data bins. To this end, we developed a systematic method that is uniformly applicable to different data sets. This method takes into account individual measurement errors, the variance in the bin, and fluctuations in the data at angular scales that are larger than the bin size. This leads to a decrease in the errors and, as a result, to better sensitivity of the data to the model content. We cross checked the stability of our method with the new LOFAR data, which have very small errors on the measurements of individual sources.

Results. We find that the four components listed above are sufficient to fit both the RM and polarisation data over the whole sky with only a small fraction masked out. Moreover, we have achieved several important improvements compared to previous approaches. Due to our location inside of the Local Bubble, our model does not require the introduction of striated fields. For the first time, we show that the Fan region can be modelled as a Galactic-scale feature. The pitch angle of the magnetic field in our fit converges to a value of around 20 degrees. Interestingly, this value is very close to the direction of the spiral arms inferred recently from Gaia data on upper-main sequence stars.