This award will support an effort to understand the properties of pulsating neutron stars across the electromagnetic spectrum. The research team will create a population synthesis code that will be used to characterize the sample of neutron stars detected by the Fermi satellite and in other surveys. The codes will incorporate treatments of the acceleration/emission geometry, operating radiation mechanisms and population characteristics. The principal goal of this coding effort is to determine how the geometries of the radio and gamma-ray emission are defined and how they evolve with the age of the pulsar.
The project will also provide research opportunities for undergraduate and graduate students. Undergraduates from Hope College will engage in collaborative visits at NASA's Goddard Space Flight Center and make presentations at meetings. The team also outlines plans for outreach and public education. In addition, they will make their codes available to the community via a website that will facilitate comparison between their work and that of other groups.
This project developed a study of the nature of X-ray and gamma-ray radiation in two classes of neutron stars, some ofthe most interesting compact objects in the Universe. There are perhaps as many as 2400 presently identified, with many of them exhibiting powerful emission in X-rays and gamma-rays. They are borne as remnants of stellar explosions, supernovae, and can live billions of years. Our research program addressed the two classes of gamma-ray pulsars and magnetars, whose fields range from a billion times larger than the Earth's magnetic field to over 500 trillion times larger! Neutron stars present a unique forum for testing fundamental quantum electrodynamics (QED) physics that is not presently accessible in territorial laboratories. Our theoretical modeling addressed data obtained by different satellite experiments, such as NASA's Fermi Gamma-Ray SpaceTelescope and Swift Telescope. Several outstanding questions pertaining to neutron stars are at the forefront of scientific interest at the moment. Among these, this project provided insights into (i) whether or not magnetars are inherently different from normal gamma-ray pulsars, or is their emission driven by similar physics operating in different portions of neutronstar parameter space, (ii) what causes the vast majority of the 150+pulsars in the Fermi-LAT (Large Area Telescope) database to exhibit spectral turnovers at maximum energies of 1-7 GeV, (iii) what causes the maximum energies of magnetar quiescent emission to be generally calibrated to around 100-300 keV; and (iv) where exactly is the site of energy injection in the magnetosphere surrounding the star that seeds non-thermal radiation in both these classes of neutron stars, i.e. how proximate is this locale to the stellar surface and/or the magnetic equator? The outcomes of this study are now outlined. Our analysis computed emission spectra in magnetars mediated by relativistic electron scattering of photons to high energies, and multiple integral determinations of absorption of gamma-rays due to the exotic QED process of magnetic pair creation in pulsars. The Compton scattering physics had to be reformulated to improve its precision in resonant domains where the electrons access natural gyrational states in strong magnetic fields. The analysis of the magnetic Compton physics using a special electron spin-dependent formulation thereby provided improvements upon older treatments, which were numerically inaccurate by factors of the order of 1.3-2. Computations of emergent Compton upscattering spectra from magnetar magnetospheres indicate that in most cases, the observer views at significant angles to a given field loop, and so the spectra are quite soft, below around 1 MeV in energy. Their slope and cutoff energy approximately matches data from the magnetars exhibiting hard X-ray tails to their quiescent (steady) signals. This model predicts strong photon polarization, and offers the prospects of one day leading to observational diagnostics on the global magnetic field geometry of magnetars, as well as a second demonstration of the intensity of their fields. For gamma-ray pulsars, the pair opacity analysis defined altitude lower bounds for Fermi-LAT pulsars using the criterion of magnetospheric transparency of GeV-band photons to magnetic pair creation. These generally fall in the range of 2-5 stellar radii above the surface. A notable exception is the Crab pulsar, whose >120 GeV emission seen by the VERITAS and MAGIC telescopes must originate above at least 30 stellar radii, i.e. above around 20% of the light cylinder radius, the intrinsic size of a neutron star magnetosphere. Relativistic rotational aberration influences impacting photon directions were found to have only a small impact on pair transparency criteria. The project supported two graduate students, both of whom finished their PhD theses in the last year of the program. One of these students is presently working as a Postdoctoral Fellow in high energy astrophysics at North-West University in South Africa. The other is currently employed as a spacecraft operation/data analysis scientist in Earth Sciences with a specialist technology company in Greenbelt, MD. Training STEM-discipline scientists for broader workforce tasks is a core goal of astrophysics PhD research programs in the U.S. This project has delivered important and diverse skill sets to these students, including the ability to critically assess open-ended problems with no textbook solutions. The research results were disseminated in several publications in peer-reviewed journals, and the students were trained inreport writing and communicating their findings, both to their peers at Rice University in seminars, and also to the broader astrophysics community at international conferences. This training lays an essential foundation for building a successful career in science, engineering, and technology fields, either in industry or in research or educational sectors. The students also occasionally taught classes to Rice undergraduates, and moreover participated in astronomy outreach to Houston area K-12 schools, giving back to the community, and exposing the broader public to the beauties of the Universe. The Principal Investigator thanks the National Science Foundation for its support in helping build a bright future for the national STEM interests.
