Pulsar timing and gravitational waves

Abstract

Pulsars are magnetized rotating neutron stars that formed from the collapsed core of a massive star following a supernova. Charged particles are accelerated along the magnetic field lines producing electromagnetic radiation. As a result, the beam is confined along the magnetic poles. A misalignment between beam emission axis and rotation axis means that they are observed as a pulsating radio source like a lighthouse. The emitted signals from pulsars undergo delays due to their position, motion, relativistic effects and the effects of the interstellar medium. By analyzing and modelling the arrival times of pulses by account for every rotation of the pulsar, those physical parameters can be measured. This technique is called pulsar timing. Pulsar timing measurements provide various applications to fundamental physics and astronomy. <br/> This thesis investigated 1) low frequency gravitational wave detection via multiple pulsar observations 2) the properties of a pulsar, PSR J1439−5501, through pulsar timing analysis and interstellar scintillation study 3) the potential usage of gravitational wave observations combined with pulsar timing measurements to study pulsar binaries. <br/> Low-frequency gravitational waves are expected to be caused by cosmological phenomena such as super massive black hole binaries (SMBHBs) formed during galaxy mergers. By detecting such gravitational waves, the physics of the early Universe, which are often not possible to access via electromagnetic observations, can be measured. An array of pulsars, called pulsar timing array (PTA), can be used as a detector of such gravitational waves. I led a part of a PTA project to produce datasets of 25 precise pulsars used to search for nano-hertz gravitational waves. We obtained improved timing solutions of the pulsars with long timing baselines, from 14 to 25 years. I led analyses of the parallaxes and orbital period derivatives to measure distances and general relativistic effects in the orbital decays. Based on the produced data, we found the first evidence for a nHz gravitational wave background, potentially caused by SMBHBs from galaxy mergers. <br/> PSR J1439−5501 is a binary system, consisting of a pulsar and a compact companion. Optical studies showed a potential detection of a relativistic effect causing a periodic signal delay in the binary. We detected the relativistic effect in the pulsar through pulsar timing analysis. By analyzing the +0.13 relativistic effect, we could measure the mass of pulsar (1.57 ^{+0.30 −0.26} M_⊙), companion (1.27 ^{+0.13 −0.12} _⊙), and orbital inclination angle (75(1)°, or 105(1)°). An orbital period derivative, likely to be detected within a decade, will improve the overall measurements. Finally, we conducted a scintillation analysis, which uses the intensity variation in time and frequency, caused by the relative motion of the pulsar, the Earth and the ISM. By investigating the scintillation pattern in terms of the orbital motion of PSR J1439−5501 and annual variation, we estimated the position of the ascending node (16(7)° or −20(6)°), and the location of the screen (260 ± 100 pc). <br/> When launched in 2030s, LISA, a space based gravitational wave observatory will enable us to detect gravitational waves at milli-hertz frequencies. Binary pulsars are one of the expected sources for LISA. Multi-messenger studies using gravitational wave and radio timing observations can be used to investigate physical properties of binary pulsars. We conducted simulations to show how two different observations can be combined to study binary pulsars. Based on the assumed properties of a hypothetical binary pulsar, we ran LISA and timing simulations. We showed that LISA can detect the signal of the assumed source in 3 months from the beginning of the observation. We considered priors from timing analysis for LISA, and showed that using priors can improve the measurements of distance and chirp mass. Finally, we reported that the individual masses of the binary pulsar can be better constrained if the inclination angle obtained from LISA is additionally considered.