Study On Magnetothermal Evolution Of Young Neutron Stars With Strong Magnetization And Optical Measurement By Muon Detector

Young pulsars are considered to be highly magnetized neutron stars,playing a unique role in pastronomy and astrophysics.They are also a natural cosmic laboratory for testing extreme physics theories.Although abundant achievements have been made in theoretical research and observation of pulsars,there are still some unsolved issues.In particular,there are some issues in the magnetic field evolution of NSs,such as the origin of super-strong magnetic field,the initial magnetic field strength,the way and timescale the magnetic fields evolve;there are some issues in the thermal evolution of NSs,such as conductivity,magnetization parameters and energy conversion efficiency.In addition,there are some issues concerning the origin of rotation instability and the determination of braking index in the evolution of NS spin-down.These issues provide a good opportunity for the research of this thesis.The magnetization parameter of a NS reflects the relative weight of the Hall drift timscale and Ohmic dissipation timescale of its crustal magnetic field,and also determines the degree of anisotropy of heat conduction inside the star.At first,we adopted the Hartree-Fock-Bogoliubov method to calculate a unified equation of state for cold non-accretion NS nuclear matter in this thesis.Then,combined with the ranges of the magnetic field,matter density,temperature,impurity and other parameters in the inner crust of pulsars,we calculate the electrical conductivity of the inner crust of the stars,and gave a general expression of the magnetization parameter and the initial magnetic field strength,which is suitable for young pulsars,through numerical simulating.PSR J1640-4631 is the first pulsar having a braking index greater than 3,which is expected for pure magneto-dipole radiation model.The braking index provides a new window for the study of the radiation mechanism of young pulsars.We applied the general expression of magnetization parameter and initial magnetic field strength to high-n pulsar PSR J1640-4631,and estimated the initial rotation period P₀,initial dipole magnetic field strength B₀,the Ohmic decay timescaleτ_Ohmand Hall drift timescaleτ_Hall of the pulsar.Using the method of numerical simulation,we studied the evolutions of the magnetic field and the braking index of PSR J1640-4631,and the theoretical results are in good agreement with the observations.It is expected that this research will be applicable to more young pulsars and be verified by future pulsar observations.Secondly,in order to study the influence of the magnetic energy dissipation on the thermal radiation of pulsars,we considered the effect of general relativity and derived the characteristic equation of the Ohmic decay of the toroidal magnetic field in a curved space-time,which belongs to nations found.In the two cases of pure Ohmic decay and the combination of Ohmic decay and Hall drift,we studied the magnetic field energy dissipation rates.The latter case is called the enhanced Ohmic decay.In both cases of magnetic field deacy,the dissipation rates of magnetic-field energy are lower than an isotropic soft X-ray luminosity observed by Chandra+Nu STAR telescopes.The braking index of PSR J1640-4631 was recalculated,the calculated value of n was lower than its observed value n=3.15（3）,indicating that there could be other factors influencing the evolution of the pulsar’s slow-down,such as a decrease in magnetic inclination angle,gravitational wave radiation and etc.We attributed the higher surface temperature of PSR J1640-4631 to the"magnetic spots"formation or to the thermo-plastic flow wave（TPW）heating caused by the decay of toroidal field in polar gaps of the star.When calculating thes urface luminosity of soft X-rays of the star,we simultaneously considered the decay of the first-order and second-order components of toroidal magnetic field,and calculated the heating rate of TPW.Theoretically,we gave expected values of the effective X-ray flux density and the pola-cap temperature T_pof PSR J1640-4631,and these expected values are consistent with the observations of the star.In addition,pulsars are a kind of pulsars powered mainly by superhigh magnetic fields.The study of magnetars is an important hot pot in the field of pulsars.AXP 1E 2259+586 is in association with the supernova remnant（SNR）CTB 109,and it is a young and canonical magnetar.The recent observation of"anti-glitch"provides a new platform for the study of magnetars.The thesis also studies the effects of super-strong magnetic fields on the magnetization parameters and heat transfer coefficients of the magnetar crust.At the same time,considering the general relativity effect,we calculated the decay rate of the toroidal magnetic field,the dissipation rate of magnetic-field energy and the soft X-ray luminosity of AXP 1E2259+586.The results confirmed that the surface thermal radiation of the AXP came from Joule heating caused by the decay of its crustal magnetic field.Recently,high-energy X-ray satellite data also confirmed that three other high-magnetic-field pulsars,PSR J1846-0258,PSR J1734-3333,and PSR J1119-6127,have experienced X-ray bursts（similar to magnetar outbursts）.These outbursts could be caused by the decay of their initial toroidal fields.It is possible that the multipole magnetic fields in these pulsars are experiencing a merger through crustal tectonics to form a dipolar poloidal field,which leads to the low braking indices and high soft X-ray fluxes.These hypotheses will be confirmed by future pulsar multi-band observations.Finally,we designed a series of experimental methods to measure the optical properties of the muon detector.The observations of the high-energy gamma-ray radiation of pulsars play a decisive role in understanding the radiation mechanism and magnetic field evolution of pulsars.The new generation of ground shower detector array LHAASO is a hybrid detector array with measurement advantages,such as high sensitivity,and wide energ-spectrum range.In the case ofγ-rays with energy higher than several Te V,one-year integrated sensitivity of LHAASO is better than the sensitivity of MAGIC or HESS within50 hours,so it can measure the high-energyγ-ray pulses of Crab pulsar.The muon detector in the array is a water Cherenkov detector,and its optical characteristics are crucial to the construction and understanding of the detector.A series of measurement methods and experiments were carried out to measure the water optical length and inner reflectance optical parameters of the muon detector,our results are consistent with each other.Our measurement methods,as well as results,are proved to be reliable,which has already greatly increased our knowledge about the muon detector.