Research On The Pulsar Nulling Phenomenon And The Emission Beam Structure Of Pulsar Poles

Nulling is a phenomenon that the radio emission of pulsars disappear in a period of time.Nulling can somehow be recognised as an extreme class of mode changing of pulsars.It is possible that during the nulling the radio emission is completely quenched or declines to an extremely low state.Wether the weak signal can be detected or not is quite dependent on the sensitivity of the telescope.We can not detect a signal when its intensity is below the sensitivity of the telescope.But a weak pulsed signal which is not seen before can be detected by integration for a time long enough.Endeavors had been made to add the null pulses to investigate whether there is any signal in the nulling state of a few pulsars,among which PSR B0826-34 was discovered to have very weak emission during its nulling.The first work of this thesis is to investigate this issue for more pulsars.By searching the public sub-integration data of the nulling pulsars archived in the Parkes Observatory,four pulsars(PSR J1107-5907,PSR J1820-0509,PSR J1727-2739 and PSR J1049-5833)with relatively more nulling sequences were selected.The nulling sequences with total length close to 1 hour were added.No apparent emission profiles were found,and the upper limit of the potential weak emission during nulling were estimated.The nulling fraction(NF)is a useful and important parameter defined as the ratio between the nulling length and the total observation time.Being affected by the time length of observation and the nulling length.the NF of the same pulsar may differ from observation to observation.The second work of this thesis is to develop a method to assess the uncertainty of the published NFs of pulsars.Since most of the pulsars with known statistics of the burst and nulling lengths follow exponen-tial distributions,in this thesis the following analysis is based on the assumption of exponential distribution.The relationship between the uncertainty of NF and the observation time was found by the simulation of burst and nulling sequences con-structed given the NF and the mean nulling length(or runs of nulling sequences).The uncertainty of the published NF was estimated for 15 pulsars of which the runs can be found in literature.This method is useful in grading the published NFs based on their uncertainties and will improve the reliability of the statistics by using the graded NFs.The nulling only reflect the state change in one magnetic pole of pulsar magne-tosphere.Is it possible that the emission states are different in two separated poles?Our line of sight only sweeps the emission beam in one dimension.Even though the precession of a couple of pulsars enable us to view a small fraction of a whole emssion beam,it is from one pole for most of the pulsars.These limitations leave the problem unexplored,and it is widely believed that the emission states should be the same(symmetrical)on both of the poles.The third work of this thesis is to investigate this issue by using the published polarization pulse profiles of a uniquely precessional pulsar PSR J1906+0746,which has the inter-pulse believed to come from the oppo-site pole from that of the main pulse.We corrected an error of the beam orientation in literature and reconstructed the two-dimensional beam structure.It was found that both the beam width and the intensity of the beams from the two poles are sig-nificantly different,therefore the asymmetry in the beam structure from the opposite poles was revealed for the first time.This discovery has important implication to the study on the electrodynamics of pulsar magnetosphere and the emission mechanisms.Combing with the above studies on nulling and asymmetric beams from double poles,and the other mode switching phenomena,the diversity of the emission prop-erty not only exhibit various forms but also may exist in the whole magnetosphere.