Searching for Pulsars in Unassociated <em>Fermi</em>-LAT sources

Abstract

Pulsars have always been under the γ-ray spotlight since the birth of γ-ray astronomy. They were the first sources that were firmly established as γ-ray sources in the 70s. Since then, pulsars have been the prime suspect of unassociated γ-ray sources in our Galaxy. The Large Area Telescope (LAT) is the game changer of γ-ray astronomy, on-board the Fermi Gamma-ray Space Telescope which launched in 2008, has increase the known population of γ-ray by over a factor of 30 in just 8 years. It enabled the discoveries of pulsars in different types through their γ-ray pulsations, including a large number of millisecond pulsars, which were originally thought to be weak in γ-rays. Many of the young γ-ray pulsars discovered have not yet been detected in subsequent radio searches, like the famous Geminga pulsar. These “radio-quiet” γ-ray pulsars population can only be accessed through blind periodicity searches in γ-rays. Therefore, γ-ray observations is the only way to complete the radio-quiet part of the Galactic pulsar population.<br /> This thesis describe the most recent major blind search survey of γ-ray pulsar – Einstein@Home γ-ray pulsar survey. Firstly, technique using machine learning were used in identifying viable unassociated γ-ray sources for blind searches. This algorithm incorporate spectral, temporal behaviour and γ-ray flux information of individual γ-ray sources to divide them into different clusters. Secondly, LAT photons associated with these sources were weighted according to the spectral result to improve the sensitivity of subsequent blind searches. γ-ray sources selected for the blind-search survey was performed on the distributed volunteer computing system Einstein@Home using the multi-stages semi-coherent technique, which is optimized for the long duration LAT dataset. This resulted in the discovery of seventeen isolated γ-ray pulsars. Follow-up multi-wavelength analysis reveals the new discoveries are mostly similar with the existing γ-ray pulsar population.<br /> However, temporal behaviours of several pulsars discovered in this survey are peculiar. This includes a young glitching pulsar PSR J1906+0722; the youngest radio-quiet γ-ray pulsar ever found PSR J1208−6238, its high magnetic field strength with its young age enable the measurement of its braking index, which is the only measurement for radio-quiet γ-ray pulsar; the discovery of the first two isolated millisecond pulsars PSRs J1035−6720 & J1744−7619 from γ-ray blind search, one of which remained radio quiet despite the deepest searches conducted by the Parkes radio telescope.<br /> Utilizing Effelsberg’s Ultra-Broad-Band receiver, currently under commission, originally designed for precision pulsar timing, 54 unassociated γ-ray sources were observed to explore the possibility of using wide band receiver for simultaneous pulsar searching observation in multiple radio bands. Simulations of the expected discovery yield suggested that less than one pulsar is expected to be found by random chance. Flux densities upper limits were present for all observations and the reasons of non-detection were also discussed.<br /> This thesis is concluded by the summary of the completed blind search survey, current status of ongoing and upcoming blind search γ-ray pulsar projects, and the impact of the new collection of γ-ray pulsars to the current pulsar population.