Constraints on the magnetic field in the Galactic halo from globular cluster pulsars

Abstract

The Galactic magnetic field plays an important role in the evolution of the Galaxy, but its small-scale behaviour is still poorly known. It is not known whether the Galactic field permeates the halo of the Galaxy. By observing pulsars in the halo globular cluster 47 Tucanae, we have probed the Galactic magnetic field at arcsecond scales, discovering an unexpected large gradient in the component of the magnetic field parallel to the line of sight. This gradient is aligned with a direction perpendicular to the Galactic disk and could be explained by magnetic fields amplified to some 60 μG within the globular cluster. Such a scenario supports the existence of a magnetized outflow that extends from the Galactic disk to the halo and interacts with 47 Tucanae.

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Fig. 1: Detected gradient of the RM as a function of the pulsar positions.
Fig. 2: Map showing the gradient of the RM and the distribution of pulsars in 47 Tuc.
Fig. 3: Magnetic field lines in the interaction between a Galactic wind and 47 Tuc.
Fig. 4: Best fit of the RM in the Galactic wind model.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Code availability

The custom codes used for the analysis and described in the Methods are provided as Supplementary Software.

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Acknowledgements

We acknowledge the help of A. Jameson in performing the observations. We thank K. Liu, S. A. Mao and M. Kramer for useful comments. The Parkes radio telescope is part of the Australia Telescope, which is funded by the Commonwealth of Australia for operation as a National Facility managed by the Commonwealth Scientific and Industrial Research Organisation. We are indebted to the communities behind the multiple open-source software packages on which this work depended. This research made use of Astropy, a community-developed core Python package for Astronomy. F.A., A.P. and A.R. acknowledge support from the Ministero degli Affari Esteri della Cooperazione Internazionale–Direzione Generale per la Promozione del Sistema Paese–Progetto di Grande Rilevanza ZA18GR02. Part of this work has also been funded using resources from the research grant ‘iPeska’ (P.I. A.P.) funded under the INAF national call Prin-SKA/CTA approved with the Presidential Decree 70/2016. The authors acknowledge the support of J. Mack (STScI) and G. Piotto (University of Padova) in the creation of Fig. 2.

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Contributions

F.A. calibrated the data, estimated the RM, created the magnetic field models, performed the statistical analysis and compiled the manuscript. A.P. conceived and supervised the project and revised the manuscript. C.T. helped in the calibration and RM estimation process and revised the manuscript. E.B. provided access to the data, pre-analysed the observations and revised the manuscript. W.v.S. provided crucial help in the polarization calibration process and revised the manuscript. A.R. and P.F. shared the latest timing results and revised the manuscript. A.R. also helped in the production of the polarization profiles shown in Supplementary Figs. 1–3.

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Correspondence to Federico Abbate.

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Extended data

Extended Data Fig. 1 Diagram of the shock forming in front of the globular cluster.

The shock is cause by the superalfvenic motion of the cluster in the frame of the Galactic wind. The globular cluster (not to scale) is the dashed circle, the thick black line is the shock front and the blue lines are the magnetic field lines. The quantities denoted with the subscript 1 are the velocity, density and magnetic field of the gas in the upstream region, while the quantities denoted with the subscript 2 are the same in the downstream region. The density of the gas in the cluster is denoted by nGC.

Supplementary information

Supplementary Information

Supplementary Figs. 1–6.

Supplementary Software

Five Python scripts to perform various tasks, as described in the enclosed README.

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Abbate, F., Possenti, A., Tiburzi, C. et al. Constraints on the magnetic field in the Galactic halo from globular cluster pulsars . Nat Astron (2020). https://doi.org/10.1038/s41550-020-1030-6

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