Large magnetic field variations towards the Galactic Centre magnetar, PSR J1745-2900

TL;DR

This study reveals large, rapid variations in the magnetic field near the Galactic Centre by analyzing the Faraday rotation measure changes of the magnetar PSR J1745-2900 over four years.

Contribution

It demonstrates that magnetic field fluctuations dominate the observed RM changes, with a minimum fluctuation scale of about 2 au, providing new insights into the magnetic environment near the Galactic Centre.

Findings

RM variations up to 3500 rad m$^{-2}$ observed over four years.

Magnetic field fluctuations, not electron density, drive RM changes.

Magnetic fluctuation scale estimated at ~2 au.

Abstract

Polarised radio emission from PSR J1745-2900 has already been used to investigate the strength of the magnetic field in the Galactic Centre, close to Sagittarius A*. Here we report how persistent radio emission from this magnetar, for over four years since its discovery, has revealed large changes in the observed Faraday rotation measure, by up to 3500 rad m$^{-2}$ (a five per cent fractional change). From simultaneous analysis of the dispersion measure, we determine that these fluctuations are dominated by variations in the projected magnetic field, rather than the integrated free electron density, along the changing line of sight to the rapidly moving magnetar. From a structure function analysis of rotation measure variations, and a recent epoch of rapid change of rotation measure, we determine a minimum scale of magnetic fluctuations of size ~ 2 au at the Galactic Centre distance, inferring PSR J1745-2900 is just ~ 0.1 pc behind an additional scattering screen.