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Title: Observing pulsars with LOFAR
Author: Hassall, Tom
ISNI:       0000 0004 2719 2831
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2012
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The LOw Frequency ARray (LOFAR) is the first of a new generation of telescopes which, instead of using a single large dish, will combine the signals from thousands of antennas. Combining these signals creates a virtual beam, which can be 'pointed' like traditional (dish-like) telescopes, but with significantly improved angular resolution and sensitivity at low frequencies. This unique configuration also allows LOFAR to observe in several modes which are not possible with other telescopes, including forming multiple beams, and simultaneously taking imaging and beam-formed data. In this thesis we present some of the early science and commissioning work which has been undertaken with LOFAR. Wide-band simultaneous observations of four pulsars using LOFAR, the Lovell Telescope and the Effelsberg 100-m Telescope are used to constrain emission heights and examine the evolution of the average pulse pulse profile between 50~MHz and 8~GHz. We find that the radio emission from all of the pulsars observed comes from a region not larger than 400~km in size, and also see pulse profile evolution that deviates significantly from what is expected from radius-to-frequency mapping. The same observations are also used to probe the interstellar medium (ISM). The ISM imparts a frequency-dependent dispersive delay on pulses propagating through it. This delay is well understood, and well measured but there are many second-order effects of the ISM, like refraction and anomalous dispersion, which have not yet been detected, and which are expected to scale steeply with frequency. These effects are potentially only observable at low frequencies, and if measured, can be used to determine the distribution of electrons and other charged particles along the line-of-sight. We find no evidence of these effects, instead finding that that the cold dispersion law is accurate to better than 1 part in 100,000 between ~40 and ~180 MHz. The absence of any of these effects in our data was used to place upper limits on some of the properties of the ISM. From these upper limits, we also show that delays from the ISM are <50 ns at normal pulsar timing frequencies (1400 MHz).We also present an investigation in to the behaviour of the single pulses of PSR B0809+74 at low-frequencies. We show that the drift rate of PSR B0809+74 is extremely constant on timescales of a few hours and a few years and compare the shape of driftbands at low frequencies to their high frequency counterparts. We identify a slowly evolving microstructure, which is persistent for several pulse periods, allowing us to solve the aliasing problem and determine the speed of the carousel. We also find evidence for 'partial nulls', in which one subpulse is extinguished without interrupting the emission of the others.
Supervisor: Stappers, Benjamin Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available