Research
Fast Radio Burst
Fast radio bursts (FRBs) might be one of the most important discoveries in radio astronomy in the past decade or so. Their uniqueness is reflected in their short duration, cosmological distance, and enormous isotropic energy bursts. The mystery lies in the fact that we still do not know the exact origin and mechanism of FRBs and their populations. This provides us with a great opportunity to develop new methods to uncover parts of the unknown universe. These aspects have strongly interested me in using statistical and theoretical methods to explain the origin and mechanisms of FRBs.
Recent Work
Statistical analysis is one of the key methods to uncover the mysteries of FRBs. Ensuring statistical completeness is essential for achieving reasonable results and their physical interpretations. In a recent work, we built a statistical model that considers the correlation between various FRB properties, as well as selection and beam effects, based on the first CHIME/FRB catalog and its injection system. By taking multi-dimensional parameters into the model, we can obtain a multi-weight function. Then, the intrinsic distribution of each property can be determined using these multi-weights, and the FRB spectra are further re-weighted. According to the intrinsic distributions from CHIME, we can compare them with ASKAP and FAST data to analyze their similarities and differences. More results are coming…
Statistical Analysis
In the statistical aspect, we attempt to analyze the statistical characteristics of FRBs using statistical methods to uncover their population distribution. In this way, we may be able to provide some clues about the origin and mechanisms of FRBs.
In the earlier work, we analyzed both repeating and non-repeating FRBs based on the data available at that time (in mid-2020) and found that they exhibit different statistical distributions in terms of their intrinsic pulse widths and isotropic radio luminosities. During this process, we applied statistical methods to small samples for the first time, enhancing the completeness of our statistics. This led to an earlier perspective of the potential diversity of FRBs, suggesting that they might have multiple origins or mechanisms. For detailed information, please read the paper.
Later, we conducted a statistical analysis of the isotropic luminosity distribution using the data of CHIME/FRB Catalog 1. To account for the influence of DM errors on distance, we introduced different distributions and scales of DM-z conversion errors when estimating distances using the Macquart relation. We found that when the errors were less than 40%, the log-normal characteristics of the distribution were more pronounced compared to the power-law distribution. However, beyond this point, the log-normal characteristics gradually decreased. This result further indicated the crucial importance of an accurate DM-z relation in obtaining the intrinsic population of FRBs. For detailed information, please read the paper.
Recently, we have further focused on the CHIME/FRB data. By considering a more comprehensive statistical model, the intrinsic population and the total spectrum have been re-built. Please see recent work.
Mechanism Model
The mechanism of FRBs is another significant question in this field. Currently, there are numerous models, such as propagation mechanism models and radiation mechanism models, proposed to explain FRBs. Here, we are particularly interested in how FRBs emit radiation. While there is still some disagreement regarding this issue like inside or outside the magnetosphere, coherent radiation is generally acknowledged as a key aspect of FRB emission. Therefore, we built our model with this perspective.
The compressed bunch model was constructed within the framework of coherent curvature radiation and magnetized neutron stars. In this kind of model, it is generally believed that coherent curvature radiation is produced by bunches moving along curved magnetic field lines, and thus the generation of these bunches is one of the fundamental issues in such models. Therefore, the core of our model focuses on how these bunches are formed before the emission of FRBs. Here, we proposed a new approach (different from two-stream instability) that outflowing particles undergo an energy loss dominated by inverse Compton scattering. This process leads to a compression effect in the particle flow (imagine a toll booth on a high-speed highway), which could potentially facilitate the formation of bunches, which provided a comprehensive physical picture of the FRB radiation process. For detailed information, please read the paper.
Pulsar
Pulsars, which are believed to be rapidly rotating neutron stars, have been discovered for over half a century since 1976. Despite extensive research over the years, the population distribution of pulsars and the evolution of magnetic field and spin period still remain subjects of great interest.
Statistical Analysis
In the statistical work, we analyzed the period and characteristic magnetic field distributions of young normal radio pulsars. We then classified the data into two sets based on whether they contained supernova remnants (SNR): those with remnants and those without. We found that they have different statistical distributions, which may suggest that they have different birth processes. Moreover, based on the spin-down age, we compared the cumulative number distribution of the two groups and found that they diverge around 10k to 15k years. This indicated that the lifetime boundary between the two types of SNRs is approximately 10k to 15k years. Subsequently, we also analyzed the progenitor star properties based on the initial mass function. These results further support the hypothesis of multiple origins for pulsars and provide quantitative statistical analysis to substantiate it. For detailed information, please read the paper.
Evolutionary Model
The magnetic dipole radiation (MDR) model is widely discussed and applied to the evolution of pulsars. However, the MDR for pulsar evolution has its challenges. One of the most prominent issues is that the model predicts a braking index of 3, yet no pulsar has a precisely measured braking index of 3, and they all fall within a range between 1 and 3. This difference between theory and observation is a topic of ongoing research and debate in the field of pulsar astronomy.
Our evolutionary model, which combined the MDR (magnetic dipole radiation) and particle outflow wind, provided an analysis of this problem and offered precise analytical solutions to address it. By applying this model to the Crab pulsar, we obtained three conclusions: (1) the initial period of the Crab pulsar is approximately 18.3 ms; (2) its characteristic magnetic field (also called the $P \dot{P}$ magnetic field) increases as it evolves, assuming that the actual magnetic field does not change; (3) there is an upper limit to the characteristic age, which is around 10k years. The most intriguing conclusions were the last two regarding the characteristic magnetic field and characteristic age. This implies that we cannot solely rely on the $P \dot{P}$ magnetic field to determine if a magnetar has an ultra-strong magnetic field. Additional evidence, such as high-energy bursts and other indicators, is needed for confirmation. Meanwhile, it further indicates that the characteristic age cannot represent the real age of pulsars. For detailed information, please read the paper.