Study On Statistical Characteristics Of Millsecond Pulsars

Pulsars are highly rotating neutron stars with strong magnetic field.Pulsars are important Galactic gamma ray sources,which can provide extreme physical conditions that cannot be achieved on the earth,such as strong electromagnetic field,extremely high density,strong gravitational field,etc.There are mainly two kinds of pulsars,young pulsars and millisecond pulsars.Since the 1970s,the high-energy radiation from young pulsars such as Crab,Vela and Geminga have been detected.Whether millisecond pulsars can produce high-energy radiation is an open question.The Fermi gamma ray telescope was launched in 2008 and soon detected high-energy radiation from millisecond pulsars.At present,about 150 gamma ray millisecond pulsars have been detected,accounting for about half of the total gamma ray pulsars.The increasing observational data provides very useful information for the further sdudy of high-energy physical processes of millisecond pulsars.In this thesis,we collected 107 gamma ray millisecond pulsars with known period,period derivative and distance.Using the outer gap model,we studied the high-energy radiation and statistical properties of millisecond pulsars.In the outer gap model,the particles are accelerated to relativistic energies in the outer gap and produce high-energy radiation through curvature radiation.The model takes into account the effects of magnetic inclination(α)and magnetic field geometry.It is assumed that there is a strong multipole magnetic field near the surface of millisecond pulsars,which is close to the magnetic field(～1011-1012 G)of young pulsars.In the outer gap,half of particles will flow back toward the neutron star,and the backflowing particles from outer gap can produce soft X-ray photons through a cascade process in the strong magnetic field.These photons collide with gamma ray photons to produce positron and electron pairs,which maintain the outer gap.The fractional size of the outer gap is not only a function of the pulsar’s period and magnetic field strength,but also a function of the magnetic inclination and the radial distance to the neutron star.In the outer gap,it is assumed that the parallel electric field satisfy Gaussian distribution along the perpendicular direction,and the gamma ray emission is expressed by the average radiation,distance along the central field line of the outer gap.Using this model,we calculated the phase averaged gamma ray spectra of 107 pulsars,and obtained the following results:(1)the high-energy radiation of most millisecond pulsars can be explained by curvature radiation.We obtained the typical position,fractional size of outer gap,magnetic inclination angle,radiation solid angle and other parameters of pulsar radiation,and found that millisecond pulsars that detected gamma ray radiation had a large magnetic inclination angle(α～65°-85°);(2)The sepctra of some millisecond pulsars,such as PSRs J0218+4232 and J1227-4853,can not be well explained,which means that the curvature radiation process do not dominate the radiation,and these pulsar may have similar origin like Crab pulsar,and needs to consider the synchrotron self Compton scattering process;(3)From the fitting results,the spectra of most of pulsars cutoff at about several GeV,indicated that the inverse Compton scattering process should be taken into consider in order to explain the observations with energies 10-30 GeV and above.