A highly magnetized environment in a pulsar binary system

Spider pulsars are millisecond pulsars in short-period (≲12-h) orbits with low-mass (~0.01-0.4 M_⊙) companion stars. The pulsars ablate plasma from the companion star, causing time delays and eclipses of the radio emission from the pulsar. The magnetic field of the companion has been proposed to strongly influence both the evolution of the binary system¹ and the eclipse properties of the pulsar emission². Changes in the rotation measure (RM) have been seen in a spider system, implying that there is an increase in the magnetic field near the eclipse³. Here we report a diverse range of evidence for a highly magnetized environment in the spider system PSR B1744 - 24A⁴, located in the globular cluster Terzan 5. We observe semi-regular profile changes to the circular polarization, V, when the pulsar emission passes close to the companion. This suggests that there is Faraday conversion where the radio wave tracks a reversal in the parallel magnetic field and constrains the companion magnetic field, B (> 10 G). We also see irregular, fast changes in the RM at random orbital phases, implying that the magnetic strength of the stellar wind, B, is greater than 10 mG. There are similarities between the unusual polarization behaviour of PSR B1744 - 24A and some repeating fast radio bursts (FRBs)^5-7. Together with the possible binary-produced long-term periodicity of two active repeating FRBs^8,9, and the discovery of a nearby FRB in a globular cluster¹⁰, where pulsar binaries are common, these similarities suggest that a proportion of FRBs have binary companions.