Pulsars Were Used To Study The Magnetic Field Of The Milky Way Like Never Before

This Hubble Space Telescope image shows a globular cluster known as NGC 104 — or, more commonly, 47 Tucanae. NASA/ESA/HST

Studying the properties of the Milky Way is not often an easy task. A particularly elusive property has been measuring the magnetic field of the galaxy we call home. Researchers have now found a way to study the magnetic field on a scale of just a few light-years.

As reported in Nature Astronomy, the researchers used particle winds emitted by pulsars, a type of pulsating neutron star, to measure the behavior of the magnetic field of the galaxy. In particular, they studied pulsars from globular cluster 47 Tucanae, a collection of stars located in the galactic halo – a spherical region that surrounds the disk of the Milky Way where the Sun resides.

47 Tucanae is located 15,000 light-years from the galactic disk in an area that was believed to be fairly undisturbed. However, the team suggest this might not be the case. The observations showed a strong magnetic field perpendicular to the galactic disk that continues into the halo, suggesting there must be a magnetized outflow of particles extending from the disk through the halo that is interacting with this globular cluster.

The measurements wouldn’t have been possible without pulsars. These peculiar stars are precise clocks, pulsating at a constant rhythm. If astronomers spot unexpected delays from the pulses, the cause is usually external. Free-moving electrons can cause such delays in what's known as a dispersion measure, which has been seen in this cluster. 

The light of pulsars is also highly polarized, meaning its electromagnetic oscillation is happening in a specific direction. Magnetic fields can change this property and the effect quantified by the rotation measure. Combining this with the dispersion measure gives researchers a way to use the pulsar to measure the magnetic field along the line of sight.

“The pulsars in this cluster can give us a unique and unprecedented insight into the large-scale geometry of the magnetic field in the Galactic halo,” lead author Federico Abbate, working at the Max Planck Institute for Radio Astronomy, said in a statement.

Polarization measurements of the electrons themselves can be obtained by studying the system in radio waves, with recent observations of 47 Tucanae confirming the presence of a strong magnetic field. The team believes that next-generation radio telescopes should be able to refine these observations and discover how the magnetic field of the Milky Way is extending so far into intergalactic space.

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