Using Long-Term Millisecond Pulsar Timing to Model Pulsar Companions and their Host Star Clusters

Since their discovery in 1967, neutron stars have been of great interest to the scientific community for their extreme physical characteristics. With supra-nuclear core densities and magnetic field strengths anywhere from 10^9 to 10^15 Gauss, these objects exist on the boundaries of science as we know it. Of particular interest are those neutron stars with extraordinarily stable rotational periods of a few milliseconds. These millisecond pulsars are thought to be the end result of a neutron star that has accreted mass from a companion star, giving it the angular momentum needed to spin rapidly.

While we are beginning to find more pulsars in the process of turning into millisecond pulsar in the spiral disk of the Milky Way, we have traditionally found more systems close to this evolutionary stage in massive star clusters known as globular clusters. The large stellar interaction rate within globular clusters effectively increases the chances that a pulsar will have a nearby companion to interact with. More than 15 years of observations have been accumulated of millisecond pulsars in globular clusters, with some systems containing as many as 37 known pulsars within their core (with evidence of many more yet to be discovered).

My thesis is broken into three broad areas of study. The first is designed to take the more than fifteen years of pulsar data available for the globular cluster Terzan 5 and use it to model the physical characteristics of the system. Terzan 5 is a unique globular cluster in that it is thought to be the remnant of a smaller galaxy that was absorbed by the Milky Way, yet studies of the system have been made difficult due to strong foreground contamination. Using pulsars as accelerometers to map out the density of the cluster I have derived the mass and physical extent of the cluster, which can better inform our understanding of the origin of Terzan 5. We have also used pulsar data to find an upper limit on the mass of any potential intermediate mass black holes in the core of the system, something which has not been possible in previous studies of the system.

The second goal of my thesis is to model the interior of stars orbiting a type of pulsar called a redback. Redbacks are those pulsars that have millisecond rotational periods yet are still interacting strongly with a low-mass (M$_c\ge$0.08 M$_\odot$, where M$_\odot$ is a solar mass) non-degenerate companion star. Contained within the globular cluster data I am analyzing are six known redback systems that have not received much attention from the scientific community to date. My work on these six systems will approximately double the number of redback sources studied in available literature. By looking at the long-term orbital period evolution I have measured changes in the orbital period of the pulsars and their companions and related it to changes in the gravitational field between the two stars. As pulsars are highly degenerate forms of matter, this change must arise due to changes in the companion star's interior density. These density perturbations allow me to provide better constraints on models of similar stars; these results also help to rule out the existence of large deformations in the companion star due to magnetic field realignments.

My final goal for my thesis is to study eclipses in redbacks in an attempt to identify average properties of the unbound plasma coming off of the companion star. In redback systems, outer layers of gas may be stripped from the companion star due to stellar winds or interactions with the pulsar. This unbound gas can cause eclipses that obscure radio emission from the pulsar at both regular and irregular intervals of its orbit. Using four of the six redbacks previously mentioned, I have analyzed hundreds of eclipses to produce average eclipse properties for my sample of pulsars. My goal is to use these average eclipse properties to produce better estimates on the typical amount of unbound gas in these systems, as well as predict the distribution of the gas in each system. If possible, we will also determine the the most likely physical mechanism that is actually obscuring the emission from the pulsar.