A blind-search survey for gamma-ray pulsars

It has long been known that young pulsars can emit strongly in gamma rays. However, it is only recently, using observations by the Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope which launched in 2008, that it has been possible to study a large number of pulsars, including millisecond pulsars, through their gamma-ray pulsations. Many of the pulsars discovered by the LAT have not been detected in subsequent radio searches. Blind searches, which directly search for pulsations at unknown frequencies in gamma-ray data, are necessary to find such
“radio-quiet” pulsars, and to provide a more complete view of the local neutron star population. This thesis is a collection of works describing the development of and results from the latest blind gamma-ray pulsar searches. Firstly, techniques were developed to enhance the sensitivity of existing search methods. These
include a revised parameter-space metric approximation which enables more efficient search grids to be constructed; interpolation schemes to mitigate potential signal losses; and improved automatic follow-up and refinement stages. By simulating and searching for thousands of weak pulsar signals, these improved methods were shown to result in searches that were almost 50% more sensitive than those performed with previous methods, without requiring additional computational power. A blind-search survey was performed on the distributed volunteer computing system Einstein@Home using these new techniques to search for pulsations in unidentified LAT sources. This
resulted in the discovery of nineteen new isolated gamma-ray pulsars; almost one-third of all pulsars that have been discovered through their gamma-ray pulsations. Using the improved understanding of the survey’s sensitivity to pulsations, upper limits were estimated for the fraction of pulsed gamma-ray flux from sources which remain unidentified. The survey was estimated to be sensitive to pulsations from unidentified LAT sources with a point-source significance above 10σ. The later
detection of a faint pulsar, PSR J1817−1742, with a similar point-source significance lends credence to this estimated threshold. One of the first detections made by this survey, of the young, glitching PSR J1906+0722, explained the nature of a highly significant unidentified gamma-ray source, and revealed additional gamma-ray emission, likely from a nearby unrelated supernova remnant. Another pulsar detected by this survey, PSR J1208−6238, is the youngest pulsar to be found through its gamma-ray pulsations, with an estimated age of just 2,700 years. Its youth, and high magnetic field strength allowed its braking index to be measured; a first for a radio-quiet gamma-ray pulsar. This survey also discovered two isolated gamma-ray millisecond pulsars, both of which have remained undetected in radio observations despite extensive searches. These pulsars could possibly be the first two members of a long-awaited pulsar class: radio-quiet millisecond pulsars.