What has stymied theorists for so long is how nuclear burning appears almost simultaneously at both poles of a neutron star. The simulation shows that a supersonic detonation, not a slow-moving flame front, ignites everything in its path. An example of a slow-moving flame front is a trail of gasoline lit at one end. The flame spreads down the trail more slowly than the speed of sound. A detonation is what happens when a flame encounters a closed barrel of gasoline.
Pressure builds in the barrel until the barrel ruptures.
The burning front now travels faster than the speed of sound, and all the fuel explodes seemingly simultaneously. Back on a neutron star, accretion dumps a steady flow of matter at one-third light speed onto the surface, and gravity quickly spreads it evenly across the sphere. The pressure at the base of this accreted material builds until it is high enough to start a nuclear reaction. The nuclear fusion creates a chain reaction, detonating across a surface of evenly spread accreted gas. Within milliseconds, the polar regions erupt in X-ray light.