Peering the Milky Way by FAST:
Interstellar magnetic Fields via Faraday rotatation of weak pulsars

Magnetic fields are ubiquitous in the Milky Way as well as distant galaxies, which play important roles in diverse astrophysical processes, for instance, the evolution of molecular clouds and star formation, the transport of cosmic rays, and the propagation of axionlike particles. It has long been a great challenge to reveal the global magnetic structure of the Milky Way Galaxy since we live inside it and close to the disk edge.

Several tracers have been observed for interstellar magnetic fields, such as starlight polarization, polarized thermal emission from aligned dust grain, synchrotron emission from diffuse ISM, Zeeman splitting of spectral lines from clouds, and Faraday rotation of polarized radio sources, only the information for one component of the magnetic fields can be estimated (see the review by Han 2017). Observations of Faraday rotation of a large number of pulsars inside the MilkyWay give key probes to reveal the large-scale Galactic magnetic field structure. However, many pulsars are very weak for measuring their polarization profiles and then deriving the Faraday rotation, no mention of many distant weak pulsars to be discovered.

We measure Faraday rotation for 134 faint pulsars in the Galactic halo (together with significant improvements in basic parameters for 15 of them). Together with another 311 faint pulsars which are either newly discovered in the project of the Galactic Plane Pulsar Snapshot (GPPS) survey or previously known pulsars without RMs, and previously available RMs for pulsars and extragalactic radio sources, we make tomographic analysis of the Galactic magnetic field structure.

Figure: Left: The Galactic distribution of RMs for 1860 pulsars. The sizes of symbols are proportional to the square root of the RM magnitudes. The circles and plus signs represent negative and positive RMs, respectively. Right: RM distribution of pulsars with |b| < 8 degree in the first Galactic quadrant. New measurements are indicated by red crosses and blue circles for positives and negatives.

The large-scale toroidal halo fields are estimated to have a field strength of 2 μG. The scale height of the halo magnetic fields is found to be at least 2.7±0.3 kpc. The RM differentiation of large number of pulsars in the Galactic disk at longitude of 26 to 90 degree gives evidence for the clockwise magnetic fields in two interarm regions inside the Scutum arm and between the Scutum and Sagittarius arm, and the clockwise fields in the Local-Perseus interarm region and field reversals in the Perseus arm and beyond.