Probing Gravity with High-Precision Pulsar Timing

Pulsars are highly magnetised neutron stars which spin tremendously fast, at periods that are as low as a few milliseconds. Chapter 1 gives an overview of their basic observational properties and applications. Pulsars are observed as sources with periodic, broadband and highly polarised signals, which are thought to be formed by rotating, beamed electromagnetic radiation emitted from their magnetic poles. This simple geometric picture, known as the lighthouse model, is the basis of the pulsar timing technique. Pulsar timing, which is explained in Chapter 2, makes use of the clock-like stability of pulsars’ rotations to create a model of the rotational and orbital (if in a binary) parameters of pulsars, which are compared to the observed pulse times-of-arrival. Modern pulsar-timing instrumentation can record of pulse arrival times with precision as high as a few hundreds of nanoseconds. The comparison of the high-precision pulsar-timing data with the predictions of the equations-of-motion of General Relativity and alternative theories of gravity allows, among others, accurate tests of gravity theories in the strong-field regime of gravity. Pulsars are also employed as high-precision cosmic clocks which can trace space-time perturbations caused by propagating gravitational waves. This application requires the use of data from an ensemble of millisecond pulsars, the fastest and most rotationally stable pulsars, known as Pulsar Timing Array. The Effelsberg 100-m radio telescope in Germany, is part of a network of telescopes conducting regular pulsar-timing observations. In Chapter 3, I first present the reduction and analysis of timing data from recorded at Effelsberg in the period 1996-2013. The chapter then focuses on the combination of the Effelsberg data with the that from the other telescopes that are part of the European Pulsar Timing Array, and the timing analysis of the resulting data set, which includes 42 millisecond pulsars. This work was highlighted by the employment new analysis methods, the first measurements of a significant amount of astrometric and orbital parameters and an in-depth analysis of pulsar distance estimations. Chapter 4 extends the analysis of the 42 millisecond pulsars, focusing on the characterisation of the noise in the individual-pulsar data. The noise levels present in the timing data have a direct impact on the sensitivity of a timing array to gravitational waves, and the detailed characterisation of the noise is necessary prior to any searches for spatially correlated gravitational-wave signals in the timing data. This work marked the first ever comprehensive comparison of two independent methods for characterising the low-frequency, stochastic and achromatic noise component. The study also focused on searching for instrumental or analysis-systematics noise. Finally, the analysis quantifies the impact low-frequency noise on the data set’s sensitivity to gravitational waves. Chapter 5 presents two tests of gravity theories using timing data from individual millisecond pulsars. The first test is based on data from solitary millisecond pulsars to place the best-to-date limits on one of the three post-Newtonian parameters that describe preferred-frame effects, generally predicted by theories that include isotropic violations of local Lorentz invariance of gravity. The test was based on upper limits of variations in the pulsar pulse profiles. Pulse profiles are predicted to change over time due to a precession of their spin axis in the presence of preferred-frame effects. The second test uses data from a binary millisecond pulsars to perform a radiation damping test. These tests focus on the change of orbital parameters due to energy loss by gravitational wave emission from the system. Focusing on the predictions by the physically motivated mono-scalar-tensor theories of gravity, this work places a stringent upper limit on the existence of dipole gravitational waves which is predicted by a variety of alternative theories of gravity. Finally, Chapter 6 concludes the thesis with an overview of the research and the results, and a discussion on further work being made in the framework of these research topics.