Milestones in the Observations of Cosmic Magnetic Fields

Magnetic fields are observed everywhere in the universe. In this review, we concentrate on the observational aspects of the magnetic fields of Galactic and extragalactic objects. Readers can follow the milestones in the observations of cosmic magnetic fields obtained from the most important tracers of magnetic fields, namely, the star-light polarization, the Zeeman effect, the rotation measures (RMs, hereafter) of extragalactic radio sources, the pulsar RMs, radio polarization observations, as well as the newly implemented sub-mm and mm polarization capabilities. The magnetic field of the Galaxy was first discovered in 1949 by optical polarization observations. The local magnetic fields within one or two kpc have been well delineated by starlight polarization data. The polarization observations of diffuse Galactic radio background emission in 1962 confirmed unequivocally the existence of a Galactic magnetic field. The bulk of the present information about the magnetic fields in the Galaxy comes from analysis of rotation measures of extragalactic radio sources and pulsars, which can be used to construct the 3-D magnetic field structure in the Galactic halo and Galactic disk. Radio synchrotron spurs in the Galactic center show a poloidal field, and the polarization mapping of dust emission and Zeeman observation in the central molecular zone reveal a toroidal magnetic field parallel to the Galactic plane. For nearby galaxies, both optical polarization and multifrequency radio polarization data clearly show the large-scale magnetic field following the spiral arms or dust lanes. For more distant objects, radio polarization is the only approach available to show the magnetic fields in the jets or lobes of radio galaxies or quasars. Clusters of galaxies also contain widely distributed magnetic fields, which are reflected by radio halos or the RM distribution of background objects. The intergalactic space could have been magnetized by outflows or galactic superwinds even in the early universe. The Zeeman effect and polarization of sub-mm and mm emission can be used for the study of magnetic fields in some Galactic molecular clouds but it is observed only at high intensity. Both approaches together can clearly show the role that magnetic fields play in star formation and cloud structure, which in principle would be analogous to galaxy formation from protogalactic clouds. The origin of the cosmic magnetic fields is an active field of research. A primordial magnetic field has not been as yet directly detected, but itsexistence must be considered to give the seed field necessary for many amplification processes that have been developed. Possibly, the magnetic fields were generated in protogalactic plasma clouds by the dynamo process, and maintained again by the dynamo after galaxies were formed.

Estimate of an environmental magnetic field of fast radio bursts

Fast radio bursts （FRBs） are a type of newly-discovered transient astronomical phenomenon. They have short durations, high dispersion measures and a high event rate. However, due to unknown dis- tances and undetected electromagnetic counterparts at other wavebands, it is difficult to further investigate FRBs. Here we propose a method to study their environmental magnetic field using an indirect method. Starting withdispersion measures and rotation measures （RMs）, we try to obtain the parallel magnetic field component ^-B ││ which is the average value along the line of sight in the host galaxy. Because both RMs and redshifls are now unavailable, we demonstrate the dependence of ^-B ││ on these two separate quantities. This result, if the RM and redshift of an FRB are measured, would be expected to provide a clue towards understanding an environmental magnetic field of an FRB.

Fallback Disk-Involved Spin-Down of Young Radio Pulsars

Disks originating from supernova fallback have been suggested to surround young neutron stars. Interaction between the disk and the magnetic field of the neutron star may considerably influence the evolution of the star through the so called propeller effect. There are many controversies about the efficiency of the propeller mechanism proposed in the literature. We investigate the faUback diskinvolved spin-down of young pulsars. By comparing the simulated and measured results of pulsar evolution, we present some possible constraints on the propeller torques exerted by the disks on neutron stars.

A possible origin of the Galactic Center magnetar SGR 1745–2900

Since there is a large population of massive O/B stars and putative neutron stars （NSs） located in the vicinity of the Galactic Center （GC）, intermediate-mass X-ray binaries （IMXBs） constituted by an NS and a B-type star probably exist there. We investigate the evolutions of accreting NSs in IMXBs （similar to M82 X-2） with a - 5.2 M companion and orbital period 2.53 d. By adopting a mildly super-Eddington rate M = 6 × 10-8 M yr-1 for the early Case B Roche-lobe overflow （RLOF） accretion, we find that only in accreting NSs with quite elastic crusts （slippage factor s = 0.05） can the toroidal magnetic fields be amplified within 1 Myr, which is assumed to be the longest duration of the RLOF. These IMXBs will evolve into NS＋white dwarf （WD） binaries if they are dynamically stable. However, before the formation of NS＋WD binaries, the high stellar density in the GC will probably lead to frequent encounters between the NS＋evolved star binaries （in post-early Case B mass transfer phase） and NSs or exchange encounters with other stars, which may produce single NSs. These NSs will evolve into magnetars when the amplified poloidal magnetic fields diffuse out to the NS surfaces. Consequently, our results provide a possible expianation for the origin of the GC magnetar SGR 1745-2900. Moreover, the accreting NSs with s 〉 0.05 will evolve into millisecond pulsars （MSPs）. Therefore, our model reveals that the GC magnetars and MSPs could both originate from a special kind of IMXB.