image: Polarimetric results of a selected sample of bursts with high degrees of circular polarization or abrupt jumps in the linear polarization position angle.
Credit: ©Science China Press
Fast radio bursts (FRBs) are a kind of energetic cosmic radio burst. Though their duration is usually at millisecond scale, their energy (estimated isotropically) reaches 10^36 ～10^41 erg, equivalent to the energy release of the sun in minutes to months. Some FRB sources emit bursts repeatedly, releasing even more energy in total. Since the discovery in 2007, the physical mechanism for such bright radio emission has become atrending topic in astrophysics. The polarization of electromagnetic wave is usually thought to carry key information of the astrophysical sources, e.g. the linear polarization usually traces the geometric configuration of magnetic fields in the radiation zone and circular polarization probes both intrinsic radiation mechanisms and propagation effects. The “China Sky Eye” Five hundred meter Aperture Spherical radio Telescope (FAST) is the largest single dish radio telescope in the world. With its superb sensitivity and polarimetry precision, FAST has many important discoveries in the field of FRB observation.
FRB 20201124A is an FRB repeater discovered on 2020 November 24 by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope. During its first active episode in 2021 March-May, radio telescopes around the world including FAST achieved rich observational results. This FRB repeater entered another short but fierce active episode in late September 2021, during which the highest event rate recorded by FAST exceeded 500 bursts per hour.
Recently, the FAST key science project fast radio burst searches and multi-wavelength observations analyze the FAST data of the second active episode of FRB 20201124A in 2021 September, in which the scientists find a fast radio burst with 90% circular polarization. Such high degree of circular polarization is both unprecedented in FRB observations, and also unpredicted in theoretical models. In addition, rapid variation and jumps are also found in the observational data. These findings may provide new constraints and clues for the theoretical research on the radiative mechanism of FRBs. The research team have measured the polarization of more than 500 fast radio bursts in four FAST observation session between 2021 September 25 and 28. Among them, the average or peak circular polarization fraction of 32 bursts exceed 50%. The highest degree of circular polarization reaches 90.9%, which is the new record in FRB observation. Rapid variation in polarization fraction and position angle, sign reversal of circular polarization, and orthogonal jump of position angle due to orthogonal polarization modes (OPMs) are also found in the observation. These findings are recently published in National Science Review, with title “Ninety percent circular polarization detected in a repeating fast radio burst”. The corresponding authors are Prof Kejia Lee from Peking University, Prof. Weiwei Zhu from National Astronomical Observatories, Chinese Academy of Sciences (NAOC, CAS) and Prof. Bing Zhang from University of Nevada, Las Vegas (UNLV). Jinchen Jiang and Jiarui Niu of NAOC and Jiangwei Xu of Peking university are the first authors.
There are currently two categories of emission mechanisms for FRB repeaters. The gamma-ray burst (GRB) like models presume that relativistic shocks from a compact central engine stimulate masers in the outer medium, while in the pulsar-like models the FRBs are generated in the pulsar magnetosphere. In the early days, the FRB observations
generally show linearly polarization with constant direction, which can be explained by both kinds of models. Thus, the two types of models cannot be tested. The polarization observational results of FRB 20201124A challenges both kinds of models. For GRB-like models, the high degree of circular polarization can only be observed at the edge of the emission beam generated by relativistic particles, where the emission should be fainter compared with the beam center. However, such brightness difference is not significant in observation. The rapid variation of linear polarization is also difficult to explain under GRB-like models. It’s more convenient to explain the observation using pulsar-like models, though the observed circular polarization fraction curves also limit the geometric parameters of the magnetosphere.