X-ray Bursts, Superbursts, and Outbursts from Accreting Neutron Stars: What heats the Interior?

Michigan State University, East Lansing MI

Investigators

Abstract

Neutron stars are the densest objects in the Universe and have fascinated theorists and observers since they were first conjectured. Despite a wealth of observations across the electromagnetic spectrum over the last four decades, much of what happens in their deep interior remains inscrutable. Owing to X-ray telescopes such as Chandra, laboratory nuclear experiments, and perhaps in the near future gravitational wave detectors such as the Laser Interferometer Gravitational Wave Observatory (LIGO), knowledge of neutron star physics is growing rapidly. Indeed, the National Academy report, "Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century" identified the nature of matter at exceedingly high density and temperature as one of the eleven critical science questions for the twenty-first century. Moreover, the recent 2010 Decadal Survey "New Worlds, New Horizons" identifies as one of its Frontier questions for Stars and Stellar Evolution, "What controls the mass, radius, and spin of compact stellar remnants?" The models developed in this project will enable existing and future observations to further our knowledge of the physics of dense matter and stellar evolution. This project includes activities to recruit the next generation of scientists and to inform the public and provides support of a graduate student for three years. The project supports an undergraduate to assist with running the codes used in the project, and to assist with testing, maintaining, and posting educational resources using the open-source MESA stellar evolution code. The PI with assistance from the undergraduate will develop problem sets, plots, and animations using MESA. Michigan State is a large public institution with a diverse student population and will serve as a testbed for development of these resources. In particular, the PI will develop problem "templates" so that students can more readily use MESA to explore stellar physics. These tools will be posted on the MESA web forum http://mesastar.org/. Posting these tools will make it easier for other instructors to adopt MESA for classroom instruction and is expected to motivate others to post their results as well. For public outreach, the PI will incorporate discoveries made during the tenure of this award into public talks at Michigan State's Abrams Planetarium. More technically, for neutron stars in low-mass X-ray binaries, in which the neutron star accretes hydrogen- or helium-rich material, there is a wealth of time-dependent phenomena spanning a wide range of timescales. During accretion, runaway thermonuclear burning of the accreted matter is observable as a Type I X-ray burst. The thousands of observed X-ray bursts, the rarer and more energetic superbursts, and the cooling transients all encode information about the physics of dense matter, in particular the sub-nuclear-density "crust" of the neutron star. For over a decade the model of the neutron star's crust has been that the three layers influence each other: the short bursts produce the fuel for superbursts, superburst ashes form the outer crust, and nuclear reactions in the crust heat the envelope. In the last several years, however, evidence has steadily mounted that this picture is at best incomplete, and that additional, strong, shallow---and as yet unidentified---heat sources are present. This project will generate global models of the neutron star crust with the latest nuclear reaction physics, including a newly discovered neutrino cooling mechanism, and use existing and future observations of cooling transients, superbursts, and X-ray bursts to constrain the depth and strength of additional heating in the crust. In addition, they will make predictions for the uniqueness of the crust composition as a function of density, which is important for determining the neutron star's thermal and magnetic evolution as well as its ability to sustain a mass quadrupole.