Role Of Fan Beam Model In Population Studies Of Isolated Radio Pulsars

Population synthesis is an important tool to study population characteristics and evolution of pulsars,which usually adopt the method of Monte Carlo simulation to create artificial pulsars that satisfy the criteria for detection of existing surveys,to infer underlying distribution of pulsars,and predict for future survey yields.The radio emission beam model plays an important role in population studies,since it directly determines the intrinsic width of the pulse profile,geometric distribution of the radiation intensity,etc.,which has an impact on the signal-to-noise ratio of pulse signal.The previous work generally adopted the traditional conal beam model,but some observa-tional evidence in recent years suggests that an alternative fan beam model extended is much more consistent with observational factsOn the basis of the PsRPopPy software package,we developed an evolution model of popula-tion synthesis for isolated radio pulsars by incorporating the fan beam model for the first time.With proper choice of related models and parameters,we succesfully reproduced the observational distri-butions of Galactic longitude(l)and latitude(b),spin period(P),period derivative(P),dispersion measure(DM)and 1.4-GHz flux density(S1400)for the 1214 isolated pulsars detected by the Parkes multibeam and Swinburne pulsar surveys.The number of underlying population of radio-loud pul-sars is found to be 2.27 × 106,which is larger than the underlying population of radio-loud pulsars simulated with conal beam model.The optimal population can be used to estimate the number of isolated radio pulsars detected by the Galactic-plane pulsar survey with FAST(Five hundred-metre Aperture Spherical Telescope).Approximately 2600 and 230 new isolated pulsars are expected to be discovered in the inner galactic plane(20°<l<90°,|b|<10°)and the outer galactic plane(150°<l<210°,|b|<10°),respectively.These numbers are smaller than those estimated by the evolution model with the conal beam and the snapshot modelRecent observations of precessing pulsar PSR J1906+0746 make it possible to test the fan beam model directly by a single pulsar.In this paper,the properties of the main-pulse and interpulse radio emission beams were studied for PSR J1906+0746 by using the open data.By fitting the integrated pulse profile of each epoch with Gaussian components,we obtained the corresponding 10%peak pulse width and the peak flux density,and then analyzed the relationships between them and the impact angle.The results show that the major visible part of the emission beam of the main pulse is generated by a magnetic flux tube with a certain range of magnetic longitude,while the emission of the interpulse comes from magnetic field lines around the magnetic axis.The properties of the main pulse can be well explained by the existing fan beam models,but the properties of the interpulse are difficult to interpret by the fan beam models,which requires the improvement of the fan beam model.Under the assumption that radio radiation can be generated along the magnetic field lines around the magnetic axis,the observed strip-shaped emission beam can be simulated by reasonable selection of the model parameters.This provides a direction for the subsequent improvement of the fan beam model,and will inevitably have a certain impact on the studies of pulsar population.