Simulation Study On The Profile-variation Of Millisecond Pulsars

Pulsars are high-speed rotating neutron stars with strong magnetic fields,which periodically radiate pulse signals to the surroundings.Because the single-pulse signal observed by the telescope is very weak,and the profile changes irregularly,the change of polarization is also more complicated.Therefore,we usually add hundreds of pulses or even more after dispersion and synchronously add them in cycles to form a cumulative pulse profile with a high signal-to-noise ratio,also known as the average pulse profile.The average pulse profile of a pulsar is very stable and is considered to be the pulsar's radiation window.From the average pulse profile,we can directly observe the pulsar radiation area,radiation width,and linear polarization,circular polarization and other information.Therefore,these average pulse profiles from the radio waveband to the high-energy gamma ray band are important tools for studying the three-dimensional structure,radiation mechanism,and polarization behavior of the pulsar radiation region.In recent years,astronomers have discovered through long-term and extensive observations that there are certain changes in the average pulse profile of a few pulsars.The two typical changes are the mode shift phenomenon and the pulse nulling phenomenon.The mode change phenomenon is that some pulsars have two completely different average pulse profiles,and the two profiles can be frequently switched to each other.Pulse nulling is a phenomenon in which the pulsar's pulse signal suddenly disappears or falls below 1% of the average radiation flow rate and resumes normal after many cycles.Both of these phenomena usually occur on normal pulsars,and the average pulse profile of a millisecond pulsar is very stable.The average pulse profile of a pulsar will change due to certain factors.It is generally believed that the profile will change as the frequency of observation changes and the flow density at different frequencies due to interstellar scintillation changes.The average pulse profile of a millisecond pulsar is usually more stable than the profile of a normal pulsar,but the average pulse profile of the two millisecond pulsars PSR J1022+1001 and PSR J1730-2304 is observed to change.The peak ratios of the two peaks that dominate the average pulse profile of a pulsar will vary between less than 1 and greater than 1.Therefore,this paper selects the observation data of the two millisecond pulsars to carry out a simulation study to find out which factors cause the change of the profile.First,we superimposed all the observations of the two millisecond pulsars together to form two extremely high SNR data files,thereby eliminating the effects of interstellar scintillation and dividing its 256 MHz total bandwidth into 8 bandwidths.With a subbandwidth of 32 MHz,an average pulse profile with a higher signal-to-noise ratio at this frequency can be obtained for each sub-bandwidth.Then,we use the Von Mises equation to fit the analysis profiles at the corresponding frequencies according to the pulse profiles on the eight sub-bandwidths respectively.This results in eight standard models.Afterwards,according to the difference of the actual observed flow,the eight standard models are zoomed in or out to fit the pulse profiles at the corresponding frequencies on the eight sub-bandwidths,and then the weighted sum of the eight analyzed profiles after scaling is added to get an overall sum.The profile is analyzed and compared with the original average pulse profile.For most of the average pulse profiles with a peak ratio less than 1,this method can fit the pulse profile well,indicating that the profile of each sub-bandwidth will change with the observation frequency and the influence of interstellar scintillation.For a very small number of average pulse profiles with a peak ratio greater than 1,the analysis profile obtained by this method does not fit the pulse profile well.The reason is that the pulse profile had changed.Through in-depth research,it was found that the peak ratio of the average pulse profile over eight sub-bandwidths is always less than 1 for both millisecond pulsars,and PSR J1022+1001 is still more special.It has a case that peak ratio is always greater than 1.And using this method can not be properly fitted,through the analysis that is at the same sub-frequency,the average profile of the pulsar has changed,so the instability may be due to pulsar or other factors.