Global Dynamics of Accretion in High-Mass X-Ray Binary Systems

Some of the most spectacular X-ray sources in the sky are High Mass X-ray Binaries (HMXBs). These are systems where a compact object, such as a neutron star or a black hole, rapidly orbits a hot, young, massive star. The gravitational pull of the compact object pulls material from the star which then spirals onto the compact object producing bright X-ray emission. The PI and his team will run computer simulations to better understand how much material falls onto the compact companion vs. how much escapes the gravitational field of the binary and is lost to interstellar space – this quantity is key for understanding how HMXBs evolve. This work will aid the interpretation of current observations of HMXBs by relating observational properties to the mass transfer rate. Much of this research will be done by undergraduates and will be used to introduce a diverse group of young students to astrophysics research. In addition to their research, the students will present their work at the North Carolina Museum of Natural Sciences and work with staff at the Digital Media Lab in the NC State library to create short YouTube videos. The PI will continue to build a series of short (10 minute) video lessons on fluid dynamics and computational physics that help students get started in this kind of research, with more senior students presenting some of the lessons. The investigator will use a new circumbinary gas dynamics code to investigate the global dynamics of mass and angular momentum accretion in HMXBs to connect observations to system parameters relevant to the evolution and classification of these systems. This work will focus both on well-observed systems like Vela X-1 as well as the broad class of systems ranging from Be X-ray binaries to supergiant fast X-ray transients and accreting black hole systems. By coupling together numerical models of the circumstellar gas dynamics (e.g., Roche lobe overflow, acceleration of the tidally enhanced wind from the donor star, X-ray heating and photoionization of the wind) and of the accretion flow (following the gas to small radii while conserving angular momentum), the investigators have developed a circumbinary gas dynamics code that can improve our understanding of the varied properties of HMXBs. This effort will enable simulations of gravitational accretion flows in three dimensions with sufficient detail to qualitatively describe the origin of any dynamical instability and to quantitatively characterize the resulting flux of mass and angular momentum accreted onto the compact companion or lost from the system in a variety of circumstances.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.