Abstract

Neutron stars are natural laboratories to test the properties and behaviour of matter under unique conditions, such as extremely high densities, fast rotation and strong magnetic fields. Pulsars and magnetars are observable manifestations of neutron stars and through timing techniques it is possible to track their rotation with high accuracy. This allows us to study small irregularities in their rotation, which are produced by dynamical processes occurring inside or outside the neutron star. The origin and precise physical mechanisms of the two main observed irregularities, namely glitches and timing noise, have remained elusive over decades and our progress towards a better understanding has been relatively slow. Glitches are sudden spin ups in the rotation and timing noise is a slow process in which the rotation wanders around the predictions of a simple slowdown model. In a previous work, my colleagues and I showed that there is a minimum glitch size for the Crab pulsar and that all smaller irregularities could be considered timing noise. This was an unexpected result which defied the predictions of all glitch models. In a following work, we showed that the times of the glitches in the Crab pulsar are not purely random, but more complex. One key aspect that allowed these results is the stable signature of glitches, always indicative of positive frequency steps. However, a recent analysis of one magnetar’s rotation showed the presence of an anti-glitch, i.e. a negative step in the rotation frequency. So far, this is the only time something like this has been reported. Magnetars are known for exhibiting high levels of timing noise and some of their detected glitches have coincided with high-energy bursts. This project aims at characterising in more detail the full extent of the glitch activity in pulsars and magnetars and obtain new constraints for the glitch models. To achieve our goals we propose four studies, which cover different aspects of the glitch phenomenon and are motivated by the observations described above: a) use the method previously used for the Crab pulsar to analyse a dense and precise dataset for the Vela pulsar, which should allow us to characterise at a great level of detail its rotation irregularities; b) upgrade and adapt this method to perform a systematic search for anti-glitch events in a large pulsar timing dataset, with the purpose of establishing, first, whether they exist and, if they do, their nature and connection to standard glitches; c) use X-ray timing data for the more frequently observed magnetars and establish our capability to detect glitches in their noisy rotation. The same data can be used to analyse their glitches and compare them with standard pulsar glitches; d) use time series statistical techniques to analyse the time distribution of the glitches in the most active glitching pulsars. For the Vela pulsar we expect to be able to firmly determine whether there is a minimum size for its glitches and uncover its glitch size distribution down to very small sizes. We will also be able to determine, with well defined detection limits, the existence or absence of an anti-glitch population of events across more than 700 pulsars. These findings will either expand the reaches of the glitch mechanism, creating the need for new models, or add a new phenomenon to the diverse behaviour of timing noise. A simple analysis of the magnetar timing data will immediately tell us the smallest glitches we can detect, thereby defining what should be considered as noise. A more careful analysis could shed light on how to connect magnetar timing irregularities with those of pulsars and establish whether there is timing evidence for magnetospheric glitches. Finally, a better comprehension of the time distribution of glitches will also help constraining the glitch models and help deciding whether there is one or more glitch mechanisms. The above studies will provide new and well defined results that will offer a more complete description of the glitch phenomenon and the effects that strong magnetic field magnetospheres could have on the rotation. As a secondary result, we will also obtain characterisation of certain forms of timing noise. The identification and characterisation of mechanisms able to exert torques on neutron stars will contribute to our understanding of the dynamical behaviour of superfluids and plasmas at the extreme conditions present inside and outside neutron stars.