Rotation measure variations in Galactic Centre pulsars

Abstract

We report the results of an observational campaign using the Effelsberg 100-m telescope of the pulsars J1746$-$2849, J1746$-$2850, J1746$-$2856 and J1745$-$2912 located in the Central Molecular Zone (CMZ) close to the Galactic centre in order to study rotation measure (RM) variations. We report for the first time the RM value of PSR J1746$-$2850 to be $-12234 \pm 181$ rad m$^{-2}$. This pulsar shows significant variations of RM of $300-400$ rad m$^{-2}$ over the course of months to years that suggest a strongly magnetized environment. The structure function analysis of the RM of PSR J1746$-$2850 revealed a steep power-law index of $1.87_{-0.3}^{+0.4}$ comparable to the value expected for isotropic turbulence. This pulsar also showed large dispersion measure (DM) variation of $\sim 50$ pc cm$^{-3}$ in an event lasting a few months where the RM increased by $\sim 200$ rad m$^{-2}$. The large difference in RM between PSR J1746$-$2849 and PSR J1746$-$2850 despite the small angular separation reveals the presence of a magnetic field of at least 70 $\mu$G in the CMZ and can explain the lack of polarization in the radio images of the region. These results contribute to our understanding of the magnetic field in the CMZ and show similarities between the RM behaviours of these pulsars and some fast radio bursts (FRBs).