Isolated Neutron Stars: The Impact of Magnetic Field Decay

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Neutron stars are Nature's laboratory for extreme conditions: the high central density probes particle physics in a theoretically intractable regime, and the extreme magnetic fields require the inclusion of quantum electrodynamics. For over two decades, astronomers have attempted to use X-ray observations of neutron star surfaces to measure their radii and cooling rates, as these can strongly constrain the interior structure and properties of the stars. However, the vast majority of the known neutron stars are not ideal for these measurements, as their surfaces are obscured by emission from their magnetic fields (in the cases of rotation-powered pulsars) or by matter orbiting and falling onto their surfaces (in the cases of accretion-powered X-ray binaries). The Isolated Neutron Stars (INSs) are a particular sample of young ( 1 Myr), nearby ( 1 kpc) cooling neutron stars discovered via their soft X-ray emission. They differ from most pulsars in that they have long periods ( 3 s), no radio emission, and X-ray emission that appear to come directly from their surfaces. They are interesting both because of their apparent abundance (comparable to normal pulsars), and because of the promise of inferring neutron star parameters from their thermal emission. However, no consistent interpretation of the thermal emission has emerged, with the difficulty mostly related to their complicated, highly-magnetized atmospheres. Moreover, recent evidence suggests that the magnetic fields we measure now are in fact the results of complicated, non-linear magnetic field decay. Such behavior has been long suspected in neutron stars but never previously observed at this level, and it presents a number of challenges and opportunities. This project will address three broad questions: (1) What is the composition of INS atmospheres, and how can we measure their radii? (2) How important is magnetic field decay to the population of neutron stars inferred from X-ray surveys, and how do their properties compare with those of the population as a whole? (3) Can we constrain the surface magnetic fields (which affect X-ray emission), independent of the dipole fields (which affect timing)? The observations will comprise related radio, optical, and X-ray observations of the INSs and radio pulsars with similar properties. In particular, studies of radio pulsars allow detailed constraints from radio astrometry and polarimetry that are not available for the INSs but will uniquely help us understand their thermal X-ray emission. This work will provide vital clues to a number of outstanding questions in neutron star astrophysics. Only by obtaining consistent results for a number of disparate sources (each with its own peculiarities and systematic uncertainties) will we have robust astrophysical results that can inform the broader physics community.The PI will also continue work with the astronomy club, broadening the range of topics and engaging students in analysis of recent results; recruit undergraduate students for summer projects focusing on specific objects or data-sets, where they will participate in the end-to-end research process; broaden the awareness of and participation in these opportunities through outreach to introductory astronomy classes and other likely interested groups; and assist with the development of shows at the Manfred Olson Planetarium at UWM, which has nearly 10,000 visitors a year. All of these efforts are part of a larger plan to increase the presence of astronomy at UWM through curriculum development, outreach, and mentoring of students from under-represented populations. The PI will also engage in outreach activities with high school students and the general public.