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

Abstract

Recent catalog of Faraday rotation measures (RM) of extragalactic sources together with the synchrotron polarization data from WMAP and Planck provides us with a wealth of information on magnetic fields of the Galaxy. However, the integral character of these observables together with our position inside the Galaxy makes the inference of the coherent Galactic magnetic field (GMF) complicated and ambiguous. 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 disk. To have control over the relative contributions of the RM and polarization data to the fit we pay special attention to the estimation of errors in data bins. To this end we develop a systematic method which is uniformly applicable to different data sets. This method takes into account individual measurement errors, the variance in the bin as well as fluctuations in the data at angular scales larger than the bin size. This leads to decrease of 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. We found that the four components listed above are sufficient to fit both the RM and polarization 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 account of our location inside of the Local Bubble our model does not require introduction of striated fields. For the first time we showed that the Fan Region can be modeled as a Galactic-scale feature. The pitch angle of the magnetic field in our fit converged to the value around 20 degrees. Interestingly, with value is very close to the direction of the arms inferred recently from Gaia data on upper main sequence stars.

v3: 22 pages, 16 figures, version accepted to A&A

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