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A lower limit of 50 microgauss for the magnetic field near the Galactic Centre

Abstract

The amplitude of the magnetic field near the Galactic Centre has been uncertain by two orders of magnitude for several decades. On a scale of 100 parsecs (pc), fields of 1,000 microgauss (μG; refs 1–3) have been reported, implying a magnetic energy density more than 10,000 times stronger than typical for the Galaxy. Alternatively, the assumption of pressure equilibrium between the various phases of the Galactic Centre interstellar medium (including turbulent molecular gas, the contested4 ‘very hot’ plasma, and the magnetic field) suggests fields of 100 μG over 400 pc size scales5. Finally, assuming equipartition, fields of only 6 μG have been inferred from radio observations6 for 400 pc scales. Here we report a compilation of previous data that reveals a downward break in the region's non-thermal radio spectrum (attributable to a transition from bremsstrahlung to synchrotron cooling of the in situ cosmic-ray electron population). We show that the spectral break requires that the Galactic Centre field be at least 50 μG on 400 pc scales, lest the synchrotron-emitting electrons produce too much γ-ray emission, given other existing constraints7. Other considerations support a field of 100 μG, implying that over 10% of the Galaxy's magnetic energy is contained in only 0.05% of its volume.

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Figure 1: Total intensity image of the region at 10 GHz.
Figure 2: Spectrum of the region: data and models.
Figure 3: Plot of the χ 2 per degree of freedom as a function of magnetic field amplitude.
Figure 4: Energy density, U X, in phase ‘X’ of the Galactic Centre interstellar medium as a function of magnetic field.

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Acknowledgements

R.M.C. thanks T. Porter for conversations about the Galactic Centre interstellar radiation field. D.I.J. thanks Monash University for hospitality. R.M.C. and D.I.J. thank J. Dickey for advice about radio data analysis. J.O. is a Jansky Fellow at NRAO; R.M.C. is a J. L. William Fellow at Monash University and a Marie Curie Fellow at Max-Planck-Institut für Kernphysik.

Author Contributions R.M.C. led the work and performed the main analysis. D.I.J. performed the analysis of radio data, including development of the Fourier-based technique for background and foreground removal, was responsible for original radio observations, and provided critical scientific discussion. F.M. provided input on theoretical and statistical problems, and critical discussion of scientific interpretation. J.O. supervised the analysis of archival radio data and the taking of original radio data, and provided input on statistics. R.J.P. provided input on thermal and relevant non-thermal processes and critical discussion of scientific interpretation. R.J.P. and R.M.C. provided supervision of D.I.J. as doctoral candidate. All authors discussed the results and commented on the manuscript.

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Correspondence to Roland M. Crocker.

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Crocker, R., Jones, D., Melia, F. et al. A lower limit of 50 microgauss for the magnetic field near the Galactic Centre. Nature 463, 65–67 (2010). https://doi.org/10.1038/nature08635

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