THEORETICAL AND PRECISION TIMING STUDIES OF NEUTRON STARS

Abstract

Pulsars are rotating neutron stars with extremely strong gravity, stable periods and high magnetic fields. They are remarkably stable systems which enables us to time them precisely. However, two categories of timing irregularities are exhibited by pulsars: glitches and timing noise. A sudden step change in the rotation rate of pulsar is known as a pulsar glitch. To date, hundreds of glitches have been detected with relative increases in the spin frequency that range from 10􀀀9 to 10􀀀6. The model which is mostly used to explain glitches today is based on pinning of superfluid vortices. Pulsar glitches provide us insights into the interior of neutron stars. Timing noise is the quasi-random wandering of the period of the pulsar. Timing noise may be caused due to various contributions from the interior of the star or changes in the magnetospheric structure. The pulsar signal as it propagates through the interstellar medium, is a ected by the electron distribution in the inter-stellar medium. The pulse is smeared due to variation of the group velocity of the radiation with observation frequency by the electrons in the line of sight, characterised by the integrated column density of electrons (Dispersion Measure). In addition, inhomogeneities in the electron distribution across the line of sight leads to multi-path propagation, which also broadens the pulse. This is known as scatter broadening. Due to extreme stability of millisecond pulsars, they can be used to to detect nanohertz gravitational waves by precision timing of an ensemble of them. A dedicated experiment, pulsar timing array, aims to detect these gravitational waves of low-frequencies. At present, there exist four such established e orts that employ the world’s best radio telescopes along with a few emerging experiments. The data from all these e orts will be pooled together under the umbrella of the International Pulsar Timing Array. The main aim of such e ort is to find a common red noise in the timing residuals of these pulsars. These are signatures of the stochastic gravitational wave background. It is important to make the timing residuals pristine and remove all other factors that could contribute to noises in the residuals. This thesis mainly focuses on various aspects of theoretical and precision timing of pulsar in order to understand a wide range of astrophysical phenomena : neutron star interior, interstellar medium, etc. After a comprehensive introduction to various concepts related to neutron stars in Chapter 1, Chapters 2-4 present the works related to timing of young pulsars to study the interior structure of neutron stars and Chapters 5-7 presents the various aspects of pulsar timing array science, conducted as a part of the Indian Pulsar Timing Array.We have developed an automated pipeline in order to detect glitches in real time. This pipeline has been tested using real glitches and is now implemented at the Ooty Radio Telescope. Such a pipeline will also be very useful in the era of large telescopes like the Square Kilometre Array. The details of this pipeline is presented in Chapter 2. In order to understand the contribution of the superfluid neutrons in the crust towards a glitch, we used some equation of states and estimated the fractional moment of inertia of the crust for both rotating and non-rotating neutron stars. We also tried to study the glitch rise times using equation of states in two fluid formalism. We present this work in Chapter 3. We have been successfully monitoring a sample of pulsars using the Ooty Radio Telescope and the upgraded Giant Metrewave Radio Telescope. The observational details, glitches detected, timing noise analysis and the implications of such programs for the upcoming telescopes have been presented in Chapter 4. We have reported a profile change event in one of the most precisely time millisecond pulsars, PSR J1713+0747 in Chapter 5. It is important to understand the cause of this event in order to continue using this pulsar in the pulsar timing array sample. We tried to understand the e ect of scatter broadening on the estimation of dispersion measures in Chapter 6. In order to remove the e ects of scattering from low frequency observations, we also proposed a new technique. Furthermore, we demonstrated the applicability of this technique using simulations. In Chapter 7, we presented the timing analysis of a sample of millisecond pulsars which were part of the Indian Pulsar Timing Array Data Release 1. All the major results and future plans have been summarised in Chapter 8.