Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

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


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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

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.


  1. 1.

    Gaensler, B. M., Beck, R. & Feretti, L. The origin and evolution of cosmic magnetism. New Astron. Rev. 48, 1003–1012 (2004).

    ADS  Google Scholar 

  2. 2.

    Widrow, L. M. Origin of galactic and extragalactic magnetic fields. Rev. Mod. Phys. 74, 775–823 (2002).

    ADS  Google Scholar 

  3. 3.

    Haverkorn, M., Brown, J. C., Gaensler, B. M. & McClure-Griffiths, N. M. The outer scale of turbulence in the magnetoionized galactic interstellar medium. Astrophys. J. 680, 362–370 (2008).

    ADS  Google Scholar 

  4. 4.

    Haverkorn, M. & Heesen, V. Magnetic fields in galactic Haloes. Space Sci. Rev. 166, 133–144 (2012).

    ADS  Google Scholar 

  5. 5.

    Han, J. Magnetic fields in our Galaxy: how much do we know? (II) Halo fields and the global field structure. In AIP Conference Proc. 609 Astrophysical Polarized Backgrounds (eds Cecchini, S. et al.) 96 (AIP, 2002).

  6. 6.

    Ferrière, K. & Terral, P. Analytical models of X-shape magnetic fields in galactic halos. Astron. Astrophys. 561, A100 (2014).

    ADS  Google Scholar 

  7. 7.

    Shukurov, A. et al. A physical approach to modelling large-scale galactic magnetic fields. Astron. Astrophys. 623, A113 (2019).

    Google Scholar 

  8. 8.

    Jansson, R. & Farrar, G. R. A new model of the galactic magnetic field. Astrophys. J. 757, 14 (2012).

    ADS  Google Scholar 

  9. 9.

    Terral, P. & Ferrière, K. Constraints from Faraday rotation on the magnetic field structure in the galactic halo. Astron. Astrophys. 600, A29 (2017).

    ADS  Google Scholar 

  10. 10.

    Unger, M. & Farrar, G. R. Uncertainties in the magnetic field of the Milky Way. In 35th International Cosmic Ray Conference 301, 558 (Proceedings of Science, 2017).

  11. 11.

    Tüllmann, R., Dettmar, R. J., Soida, M., Urbanik, M. & Rossa, J. The thermal and non-thermal gaseous halo of NGC 5775. Astron. Astrophys. 364, L36–L41 (2000).

    ADS  Google Scholar 

  12. 12.

    Carretti, E. et al. Giant magnetized outflows from the centre of the Milky Way. Nature 493, 66–69 (2013).

    ADS  Google Scholar 

  13. 13.

    Everett, J. E. et al. The Milky Way’s kiloparsec-scale wind: a hybrid cosmic-ray and thermally driven outflow. Astrophys. J. 674, 258–270 (2008).

    ADS  Google Scholar 

  14. 14.

    Manchester, R. N. Pulsar rotation and dispersion measures and the galactic magnetic field. Astrophys. J. 172, 43–52 (1972).

    ADS  Google Scholar 

  15. 15.

    Han, J. L., Manchester, R. N., Lyne, A. G., Qiao, G. J. & van Straten, W. Pulsar rotation measures and the large-scale structure of the galactic magnetic field. Astrophys. J. 642, 868–881 (2006).

    ADS  Google Scholar 

  16. 16.

    Noutsos, A., Johnston, S., Kramer, M. & Karastergiou, A. New pulsar rotation measures and the Galactic magnetic field. Mon. Not. R. Astron. Soc. 386, 1881–1896 (2008).

    ADS  Google Scholar 

  17. 17.

    Sobey, C. et al. Low-frequency Faraday rotation measures towards pulsars using LOFAR: probing the 3D Galactic halo magnetic field. Mon. Not. R. Astron. Soc. 484, 3646–3664 (2019).

    ADS  Google Scholar 

  18. 18.

    Lorimer, D. & Kramer, M. Handbook of Pulsar Astronomy (Cambridge Univ. Press, 2005).

  19. 19.

    Mao, S. A. et al. A survey of extragalactic Faraday rotation at high galactic latitude: the vertical magnetic field of the Milky Way toward the galactic poles. Astrophys. J. 714, 1170–1186 (2010).

    ADS  Google Scholar 

  20. 20.

    Schnitzeler, D. H. F. M. The latitude dependence of the rotation measures of NVSS sources. Mon. Not. R. Astron. Soc. 409, L99–L103 (2010).

    ADS  Google Scholar 

  21. 21.

    Manchester, R. N., Lyne, A. G., D’Amico, N., Johnston, S. & Lim, J. A 5.75-millisecond pulsar in the globular cluster 47 Tucanae. Nature 345, 598–600 (1990).

    ADS  Google Scholar 

  22. 22.

    Manchester, R. N. et al. Discovery of ten millisecond pulsars in the globular cluster 47 Tucanae. Nature 352, 219–221 (1991).

    ADS  Google Scholar 

  23. 23.

    Robinson, C. et al. Millisecond pulsars in the globular cluster 47 Tucanae. Mon. Not. R. Astron. Soc. 274, 547–554 (1995).

    ADS  Google Scholar 

  24. 24.

    Camilo, F., Lorimer, D. R., Freire, P. C. C., Lyne, A. G. & Manchester, R. N. Observations of 20 millisecond pulsars in 47 Tucanae at 20 centimeters. Astrophys. J. 535, 975–990 (2000).

    ADS  Google Scholar 

  25. 25.

    Pan, Z. et al. Discovery of two new pulsars in 47 Tucanae (NGC 104). Mon. Not. R. Astron. Soc. 459, L26–L30 (2016).

    ADS  Google Scholar 

  26. 26.

    Ridolfi, A. et al. Long-term observations of the pulsars in 47 Tucanae—I. A study of four elusive binary systems. Mon. Not. R. Astron. Soc. 462, 2918–2933 (2016).

    ADS  Google Scholar 

  27. 27.

    Freire, P. C. C. et al. Long-term observations of the pulsars in 47 Tucanae—II. Proper motions, accelerations and jerks. Mon. Not. R. Astron. Soc. 471, 857–876 (2017).

    ADS  Google Scholar 

  28. 28.

    Freire, P. C. C. et al. Detection of ionized gas in the globular cluster 47 Tucanae. Astrophys. J. 557, L105–L108 (2001).

    ADS  Google Scholar 

  29. 29.

    Abbate, F. et al. Internal gas models and central black hole in 47 Tucanae using millisecond pulsars. Mon. Not. R. Astron. Soc. 481, 627–638 (2018).

    ADS  Google Scholar 

  30. 30.

    McDonald, I. & Zijlstra, A. A. Globular cluster interstellar media: ionized and ejected by white dwarfs. Mon. Not. R. Astron. Soc. 446, 2226–2242 (2015).

    ADS  Google Scholar 

  31. 31.

    Staveley-Smith, L. et al. The Parkes 21 CM multibeam receiver. Publ. Astron. Soc. Aus. 13, 243–248 (1996).

    ADS  Google Scholar 

  32. 32.

    Armstrong, J. W., Rickett, B. J. & Spangler, S. R. Electron density power spectrum in the local interstellar medium. Astrophys. J. 443, 209–221 (1995).

    ADS  Google Scholar 

  33. 33.

    Rybicki, G. B. & Lightman, A. P. Radiative Processes in Astrophysics (Wiley-Interscience, 1979)

  34. 34.

    Baumgardt, H., Hilker, M., Sollima, A. & Bellini, A. Mean proper motions, space orbits, and velocity dispersion profiles of galactic globular clusters derived from Gaia DR2 data. Mon. Not. R. Astron. Soc. 482, 5138–5155 (2019).

    ADS  Google Scholar 

  35. 35.

    Mao, S. A. et al. Radio and optical polarization study of the magnetic field in the small magellanic cloud. Astrophys. J. 688, 1029–1049 (2008).

    ADS  Google Scholar 

  36. 36.

    Oppermann, N. et al. Estimating extragalactic Faraday rotation. Astron. Astrophys. 575, A118 (2015).

    Google Scholar 

  37. 37.

    Bailes, M. et al. MeerTime—the MeerKAT key science program on pulsar timing. In MeerKAT Science: On the Pathway to the SKA 277, 11 (Proceedings of Science, 2016)

  38. 38.

    Hotan, A. W., van Straten, W. & Manchester, R. N. PSRCHIVE and PSRFITS: an open approach to radio pulsar data storage and analysis. Publ. Astron. Soc. Aus. 21, 302–309 (2004).

    ADS  Google Scholar 

  39. 39.

    van Straten, W., Demorest, P. & Oslowski, S. Pulsar data analysis with PSRCHIVE. Astron. Res. Technol. 9, 237–256 (2012).

    Google Scholar 

  40. 40.

    van Straten, W. High-fidelity radio astronomical polarimetry using a millisecond pulsar as a polarized reference source. Astrophys. J. Suppl. 204, 13 (2013).

    ADS  Google Scholar 

  41. 41.

    Tiburzi, C. et al. The high time resolution universe survey—IX. Polarimetry of long-period pulsars. Mon. Not. R. Astron. Soc. 436, 3557–3572 (2013).

    ADS  Google Scholar 

  42. 42.

    Wardle, J. F. C. & Kronberg, P. P. The linear polarization of quasi-stellar radio sources at 3.71 and 11.1 centimeters. Astrophys. J. 194, 249–255 (1974).

    ADS  Google Scholar 

  43. 43.

    Simmons, J. F. L. & Stewart, B. G. Point and interval estimation of the true unbiased degree of linear polarization in the presence of low signal-to-noise ratios. Astron. Astrophys. 142, 100–106 (1985).

    ADS  Google Scholar 

  44. 44.

    Everett, J. E. & Weisberg, J. M. Emission beam geometry of selected pulsars derived from average pulse polarization data. Astrophys. J. 553, 341–357 (2001).

    ADS  Google Scholar 

  45. 45.

    Naghizadeh-Khouei, J. & Clarke, D. On the statistical behaviour of the position angle of linear polarization. Astron. Astrophys. 274, 968 (1993).

    ADS  Google Scholar 

  46. 46.

    Noutsos, A., Karastergiou, A., Kramer, M., Johnston, S. & Stappers, B. Phase-resolved Faraday rotation in pulsars. Mon. Not. R. Astron. Soc. 396, 1559–1572 (2009).

    ADS  Google Scholar 

  47. 47.

    Dai, S. et al. A study of multifrequency polarization pulse profiles of millisecond pulsars. Mon. Not. R. Astron. Soc. 449, 3223–3262 (2015).

    ADS  Google Scholar 

  48. 48.

    Brentjens, M. A. & de Bruyn, A. G. Faraday rotation measure synthesis. Astron. Astrophys. 441, 1217–1228 (2005).

    ADS  Google Scholar 

  49. 49.

    Sun, X. H. et al. Comparison of algorithms for determination of rotation measure and Faraday structure. I. 1100-1400 MHz. Astron. J. 149, 60 (2015).

    ADS  Google Scholar 

  50. 50.

    Tinbergen, J. Astronomical Polarimetry (Cambridge Univ. Press, 1996).

  51. 51.

    Yan, W. M. et al. Polarization observations of 20 millisecond pulsars. Mon. Not. R. Astron. Soc. 414, 2087–2100 (2011).

    ADS  Google Scholar 

  52. 52.

    Foreman-Mackey, D., Hogg, D. W., Lang, D. & Goodman, J. emcee: The MCMC Hammer. Publ. Astron. Soc. Pac. 125, 306 (2013).

    ADS  Google Scholar 

  53. 53.

    The Astropy Collaboration et al. Astropy: A community Python package for astronomy. Astron. Astrophys. 558, A33 (2013).

  54. 54.

    The Astropy Collaboration et al.The Astropy project: building an open-science project and status of the v2.0 core package. Astron. J. 156, 123 (2018).

    ADS  Google Scholar 

  55. 55.

    Minter, A. H. & Spangler, S. R. Observation of turbulent fluctuations in the interstellar plasma density and magnetic field on spatial scales of 0.01 to 100 parsecs. Astrophys. J. 458, 194 (1996).

    ADS  Google Scholar 

  56. 56.

    Chiuderi, C. & Velli, M. Basics of Plasma Astrophysics (Springer, 2015).

  57. 57.

    McLaughlin, D. E. et al. Hubble space telescope proper motions and stellar dynamics in the core of the globular cluster 47 Tucanae. Astrophys. J. Suppl. Ser. 166, 259–297 (2006).

    ADS  Google Scholar 

Download references


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.

Author information




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.

Corresponding author

Correspondence to Federico Abbate.

Ethics declarations

Competing interests

Authors declare no competing interests.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Abbate, F., Possenti, A., Tiburzi, C. et al. Constraints on the magnetic field in the Galactic halo from globular cluster pulsars . Nat Astron 4, 704–710 (2020).

Download citation


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing