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TIMING NOISE OF PSR B0329+54 AND ITS FREQUENT MODE CHANGING

Kwofie, Joseph Amihere

[Thesis]. Manchester, UK: The University of Manchester; 2018.

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Abstract

Pulsars are famed for their highly precise frequency of rotation and periodic pulses. The time of arrival of their periodic pulses can generally be predicted to a high precision, but not perfectly. Deviation of the times of arrival (TOA) of the pulses from prediction is common to many known pulsars. One type of the deviation known as timing noise is characterised by unexplained wandering of pulse phases relative to a simple model. It is believed that this timing noise holds clues to the physics of the stellar interior. Also, if the timing noise is properly modelled, the times of arrival can be improved to allow pulsars to be used as precise clocks. Observations of some pulsars have shown that the spin-down varies with time and it is also linked with perturbations of the conditions in the pulsarâ€™s magnetosphere. This link, in the case of PSR B0329+54, was investigated in this work. PSR B0329+54 is known to undergo mode changes, in which a sequence of pulses has one of the two stable shapes. Firstly, it was demonstrated that the systematic offset of the abnormal mode TOAs from normal mode TOAs is associated with magnetospheric mode changing, rather than the effect of template matching. Secondly, the shape of each pulse profile was quantified using the fractional occurrence of one of the stable pulse shapes and the variations of the spin-down rate was obtained by modelling the timing noise with Gaussian process regression. There was a strong correlation between the fractional occurrence and the timing noise during the section of periodic modulation for which a correlation coefficient of 0.6 was found. There was also a strong correlation between the fractional occurrence and the spin-down variation during the periodic modulation. These results show that the rate of change between these pulse shapes is itself changing with time, and corresponds to variations in the spin-down rate.