The pulsar radio emission and polarization

Abstract

The thesis is mainly focused on the modeling of pulsar radio emission and polarization. The radio emission mechanism of pulsar is not fully understood, because of the lack of understanding about the kinematical processes involved in pulsar magnetosphere, radio emission geometry and magnetic field topology obviously. Observation strongly suggest that pulsar radio emission is a coherent process, as very high energetic emission with corresponding brightness temperature 10^25 K is taken place from a source with size around 10 km. From an astrophysical point of view, pulsars are often characterized as quantum degenerate stars i.e., neutron star, which is highly magnetized (B = 10^8 − 10^13 G), highly compact rotating object. There has been attributed specific locations or magnetospheric gaps, for example polar gap, outer gap etc. in literatures, which are demarcated as the locations exclusively for originating radio emission and X-ray emission respectively. Polar cap boundary is defined as the footprint of last open magnetic field lines. On the other hand outer gap is located in a region close to neutron star’s surface and in between open and closed magnetic field lines. As pulsar is spinning rapidly with huge magnetic field, it induces large electric field across polar cap region, which is around 10^12 volt per cm. This huge electric field exceeds the vacuum breakdown condition and as a result the region above polar cap is filled with charged particles. Due to huge background magnetic field, the momentum of particles, perpendicular to magnetic field is dissipated away very quickly and thereafter they continue to accelerate along the open magnetic field line. As a result of charged particle acceleration due to movement along curved magnetic field line, curvature radiation photons (gammarays) are generated. These curvature photons subsequently decay into electron and positron pairs via cascade process which produce a cloud of secondary charged particles with Lorentz factor 100-1000. This secondary charged particles are believed to be responsible for pulsar radio emission. There have been suggested three processes to explain pulsar radio emission mechanism, (i) Antenna mechanism, (ii) Relativistic plasma instabilities and (iii) Maser emission mechanism, whose elaborate descriptions are provided with sufficient details in chapter 1 and chapter 2. Masers are in fact closely related to plasma instabilities. Abinitio a generic introduction to radio pulsar is given in chapter 1 with diferent polarization related properties. Nevertheless the main aim of this thesis is to make a model by considering collective plasma process and suitably chosen geometry to estimate radio pulsar brightness temperature theoretically, which is shown in 3rd chapter. We develop a mechanism for pulsar radio emission which takes into account of the detailed viewing geometry of pulsars and dipolar magnetic field configuration. By using a suitably chosen geometry and plasma parameters we derive analytical expressions for the Stokes parameters of the radiation field in the neutron star centered frame. We have simulated the pulse profiles based on our analytical formulation. It seems we can explain the enhanced radiation and most of the diverse polarization properties of radio pulsars. We have estimated the brightness temperature, which seems to agree with the observations. The polarization angle predicted by the model is in good agreement with the rotating vector model. The brightness temperature shows a reasonably good agreement between theoretically computed values and observation. In 4th chapter, I have discussed about the power spectra modeling of radio pulsars by introducing plasma processes such as Stimulated Raman scattering (SRS) and Stimulated Compton Scattering (SCS). Usually flux of pulsars in radio regime falls inversely with frequency, with mean value of index lying between -3 to -1. There has been proposed diferent types of power spectra like simple power law, two segmented broken power law, log-parabolic etc. It is evident that, two diferent distinct plasma processes are responsible for generating two segmented broken power law. So in chapter 4 I have used SRS and SCS as potential tools to reproduce power spectra of a few pulsars theoretically. It seems like about 80% of pulsars exhibit single power law and rest of the 20% show two segmented broken power law and other types. In chapter 4, I have basically shown how non-linear plasma processes such as SRS and SCS can be applied to pulsars and reproduce their power spectra in radio regimes theoretically. In chapter 5, I have investigated the role of SRS on the polarization state of pulsar radio emission. It is shown that rapid temporal variability of intensity and switching of polarization state from elliptical to circular, linear and vice-versa can be incorporated by implementing SRS, under diferent plasma conditions. Apart from that, it is shown that the efect of SRS is more pronounced in the typical pulsar magnetosphere than other efects such as Faraday rotation. In chapter 6, all the chapters are summarized together and merits of the theories in connection to observations are justified with future implication. My future plans are to: (1) Make observations of pulsars and their power-law spectra, (2) Estimate the aberration-retardation (A/R) efects on pulsar spectra, (3) Study the orthogonal polarization mode (OPM) phenomena. In addition to pulsars, my other future plans are to study Fast Radio Bursts (FRB).