New rotation measures of distant pulsars in the inner Galaxy and magnetic field reversals

Abstract

We present new rotation measures (RMs) for 27 pulsars, most of which were discovered in searches by Clifton & Lyne and Clifton et al. These pulsars have generally high dispersion measures and low Galactic latitudes, and are located in the first quadrant of the Galaxy. The new RMs, considered together with existing RMs in the same directions, provide further evidence that the large-scale Galactic magnetic field along the Sagittarius arm is opposite in direction from the local field. We have run models used by Rand & Kulkarni, but with a more careful selection of the data, to find that the field changes direction at about 0.4 kpc inside the solar circle, locally points to Galactic longitude |$88^\circ$|⁠, and has a strength of |$1.4\, {\mu}{\rm G}$|⁠. These RMs also provide evidence for a second reversal of the magnetic field direction in the inner Galaxy at a radius of 5.5 kpc, with a much stronger field beyond the reversal. The primary signature of this reversal is a systematic segregation of positive and negative RMs by distance and longitude in the region |$0^\circ\lt l \lt40^\circ,\, 3 \lt D \lt 7 \ {\rm\,kpc}$|⁠. RMs certainly do not increase with pulsar distance in the inner Galaxy, as would be expected if no such reversal existed. The magnetic field strength probably increases smoothly toward the Galactic Centre. RMs of low-latitude extragalactic sources are consistent with this geometry. Small-scale irregularities in the magnetic field, and possibly a more complicated large-scale field structure in the inner Galaxy, preclude more detailed statements. The data are presented in a variety of projections to aid in understanding of the Galactic magnetic field structure.