Modeling Thermonuclear Explosions on Neutron Stars

David J. Lin
Theoretical Astrophysics Group
Northwestern University

Welcome!

      Here, we are studying X-ray bursts from neutron stars. These are believed to be huge explosions involving the entire outer surface of the neutron star. Many details about these extremely energetic processes have been explained, but many more puzzles remain. For instance, outstanding questions include:

* How fast does such an explosion actually travel across the neutron star?
* Does it propagate supersonically (detonation)?
* Does it propagate subsonically (deflagration)?
* How does convection and turbluence affect the dynamics?
* What are the physical time-scales involved?
* Is this consistent with actual observations of these bursts?

Click on the picture to watch cool movies:

X-ray Binary	X-ray Burster	Gamma-ray Burst

Many challenges exist in trying to answer these questions, including:

* Constructing models which involve realistic physics, such as thermonuclear burning, equation of state, hydrodynamics, radiative diffusion, and strong gravity, to name a few. Magnetic fields and spin are other physical ingredients which must also be considered. Thus, this problem is wonderfully rich in physics.

* Being able to calculate these models in a realistic amount of time. This is not a trivial problem, because given available computational methods and resources, a calculation might take months or even years of calender time to complete, even using parallel processing. Thus, a major challenge is to apply the Low Mach Number Approximation method to astrophysical conditions. Ideally, this method will help decrease the time necessary to complete a given computation by a factor of 10 to 100, depending on the Mach number of the resulting flows.

* Interfacing the Low Mach Number Approximation method into a larger code structure, making it accessible to the larger computaional astrophysics community.

      You can read more details here: here.

      The 2D GIFs and animations are graphs of the models we use to simulate the physical processes which occur on the surface of a neutron star. Energy is being added to a small region in the center of the graph to simulate the ignition of nuclear fuel there. (EGR stands for Energy Generation Region.) As you can see from the graphs, large outflowing behavior due to hydrodynamics results rather quickly. Possibly, these flows are fast enough to propel matter and energy around the star in a very short period of time. So, if a small patch ignites and initiates a limited burst, then the entire star can become involved in the burst very shortly afterwards.

      The charts of velocity, flux and density quantify this hydrodynamic outflow.

      Please choose selections from the links frame at left.