It Takes a Supercomputer to Properly Simulate a Neutron Star’s Surface

Neutron stars, the remains of massive stars that have imploded and gone supernova at the end of their life, can still create massive flares. These incredible bursts of energy release X-rays that propagate through space. It is a complex process to simulate but astronomers have turned to a supercomputer to help. Modelling the twisting magnetic fields, the interaction with gas and dust, the surface of flaring neutron stars has been revealed in incredible 3D.

Throughout a stars life, the inward force of gravity is balanced by the outward pushing thermonuclear force. Stars like our Sun will experience the thermonuclear force overcoming the force of gravity. The force of gravity wins over the thermonuclear force in more massive stars as the star’s core collapses, leading to a rebound and supernova explosion. The result is a super dense core where the space between the protons and neutrons are eradicated during collapse. The result, is a great big neutron a few kilometres across.

It is quite possible for neutrons stars to have a companion star and, as the stars orbit, the neutron star strips material off its companion. The material will build up on the neutron star, become compressed under the force of gravity which leads to a thermonuclear explosion and a release of X-rays. Understanding this X-ray release and how it spreads across the neutron star’s surface can tell us a lot about the neutron star and its composition. 

A team of astrophysicists from the State University of New York and the University of California have been attempting to simulate the X-ray bursts in 2D and 3D models. One of the challenges in achieving this is the immense amount of computing power required to achieve the task. To overcome this, the team used the Oak Ridge Leadership Computing Facility’s Summit super computer to analyse and compare models. 

The Summit supercomputer is well suited to the task. Combining high-performance CPU and an accelerated graphics processing unit the team were able to run the simulations. By delegating the task of running the simulations to the graphics processing unit the central processing unit was freed up to compare the models. The researchers were able to restrict the size of the source so that they could calculate the neutron star radius. Typically a neutron star has a mass of up to 2 times the mass of the Sun even though they are usually up to 12km across. Studying the flares means the mass and radius of a neutron star can be deduced due to the way matter behaves under extreme conditions. 

The generated models in 3D were informed from previous 2D models. Using models under different star surface temperature and rotation rate, the flames propagation was explored. the 2D study showed that different physical conditions led to a different rate of flame spread. The 3D simulations looked at the evolution of a flare across the surface of a neutron star with a surface temperature several million times more than the Sun and a rotation rate of 1,000 hertz or 1,000 revolution per second. In these simulations the flame does not remain circular and the resultant ash was used to learn how quickly the burning progressed. 

The results revealed that the 2D model burning was slightly faster than the 3D model but both were similar. If more complex interactions are required such as turbulence then the 3D model will be required. Exciting times are ahead for the time as they continue to strive to be able to model the whole flame spread across the entire star. 

Source : Scientists use Summit supercomputer to explore exotic stellar phenomena