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Imgrund, Maximilian (2016): On pulsar radio emission. Dissertation, LMU München: Fakultät für Physik



This work intends to contribute to the understanding of the radio emission of pulsars. Pulsars are neutron stars with a radius of about 10^6 cm and a mass of about one to three solar masses, that rotate with a period between seconds and milliseconds. They exhibit tremendous magnetic fields of 10^8 to 10^13 Gauss. These fields facilitate the conversion of rotational energy to mainly dipole radiation, x-ray emission and the pulsar wind. Less than a thousandth of the total energy loss is being emitted as radio emission. This contribution however is generated by a collective plasma radiation process that acts coherently on a time scale of nanoseconds and below. Since the topic has been an active field of research for nearly half a century, we introduce the resulting theoretical concepts and ideas for an emission process and the appearance of the so called “magnetosphere”, the plasma filled volume around a pulsar, in Chapter 1. We show that many basic questions have been answered satisfactorily. Questions concerning the emission process, however, suffer some uncertainty. Especially the exact energy source of the radio emission remains unclear. The early works of Goldreich and Julian [1969] and Ruderman and Sutherland [1975] predict high electric fields to arise that are capable of driving a strong electric current. To supplement the energy to power the radio emission, rather mildly relativistic particle energies and a moderate current are favourable. How the system converts current into flow is unclear. In fact, the earlier theories are opposed by recent simulations that also do not predict a relativistic flow near the pulsar. We examine the observed radiation and its form, especially in light of the illustrated models in Chapter 2. We notice that the radio emission is generated in extremely short time scales, that are comparable to the inverse of the Plasma frequency. We elaborate why this places high demands on the theoretical models leaving in fact only one viable candidate process. We conclude that profound questions of energy flow and energy source remain unanswered by current theory. Furthermore, the compression of available energy in space and time to a few centimetres and nanoseconds remains unclear, especially when facing the fact that only a small fraction of the theoretically available energy is being converted. Since the fluctuations relevant for the compression of the energy take place on an intermediate scale of nanoseconds to micro- and milliseconds, it should be possible to detect these observationally. To facilitate this, we decide to analyse the statistics of the Receiver equation of radio radiation in Chapter 3, also since this is relevant to other topics of Pulsar research. The results presented in Chapter 4 show that the developed Bayesian method excels conventional methods to extract parameters from observation data in both precision and accuracy. The method for example weights rotation phase measurements differently than conventional techniques and assigns a more accurate error estimation to single measurements. This is of great relevance to gravitational wave search with so called “pulsar timing array”, as the validity of the total measurement is substantially dependent on the understanding of the accuracy assigned to the single observations. However, the work on single observation data with Bayesian techniques also exemplifies the numerical limits of this method. It is desirable to enable algorithms to include single observation data in the analysis. Therefore we developed a runtime library that writes out currently unneeded data to hard disk, being capable to manage huge data sets (substantial fractions of the hard disk space, not the main memory) in Chapter 5. This library has been written in a generic form so that it can be also used in other data-intensive areas of research. While we thereby lay the foundations to evaluate fluctuation models by observational data, we approach the problem from theoretical grounds in Chapter 6. We propose that the energetic coupling of radio emission could be of magnetic origin, as this is also a relevant mechanism in solar flare physics. We argue in a general way that the rotation of the pulsar pumps energy into the magnetic field, due to topological reasons. This energy can be released again by current decay. We show that already the annihilation of electrons and positrons may suffice to generate radio emission on non-negligible energy scales. This mechanism is not dependent on relativistic flow and thus does not suffer from the problem of requiring high kinetic particle energies. We conclude that the existing gaps in the theory of the radio emission process could possibly be closed in the future, if we analyse observational data statistically more accurate and especially if we put more effort into understanding the problem of energy transport. This thesis serves as an example that scientific investigation of a very theoretical question such as the origin of radio emission can lead to results that may be used directly in other Areas of research.