AbstractsAstronomy & Space Science

Light passing near a massive object (star) will take longer to arrive at the Earth than it would if the object was not present. This additional time is called the Shapiro delay. In globular clusters, where there are millions of stars, the cumulative effect of the Shapiro delay from these stars will affect pulsar timings by introducing an additional noise term. This effect has been previously assumed to be small, yet no definite investigation has been done to determine its magnitude. In this thesis a model of the globular cluster 47 Tucanae was created in order to determine the effect of the change in Shapiro delay (called the Shapiro noise) for an observed duration of 3600 days  – the current longest observation period for pulsar timing. This noise was then added to the pulsar time of arrival (TOA) as the only noise source in pulsar timing. A polynomial fit was then used to subtract the first two orders from the pulse arrival time (the f and \dot{f} terms) to determine the timing residuals. This model was then realised 100 times to obtain the average root mean square (RMS) timing residual for every pulsar. The model showed that the Shapiro noise has a significant, and observable effect on pulsar timing, especially for pulsars situated close to the core of the globular cluster. From the model the average RMS timing residuals were of the order of 10^{-5} to 10^{-7} seconds and the variance of the RMS timing residuals were significantly larger in magnitude, ranging from 10^{-4} to 10^{-7} seconds for every pulsar. The importance of this result motivated further investigation of the stellar distribution of the globular cluster. In addition an investigation on how the effect of gravitational acceleration (produced by stars situated close to the pulsar) affects pulsar timing residual was also done. While the acceleration has an effect, the effect is smaller than that of the Shapiro noise. From the timing residuals produced by the Shapiro noise, it was then discussed whether any star close to the LOS would have an affect on the pulsar timing residuals. From additional simulations it was determined that stars anywhere along the LOS will have an affect on pulsar timing, however the stellar density of such a region would have to be greater than \rho_{min} > 10^{5} M_{\sun} pc^{-3}. The implications of this result for other pulsars in (other) globular clusters is discussed.