These burst sources are thought to involve transfer of matter through an accretion disk onto the neutron star, as described in the left panel. The accretion triggers periodic thermonuclear runaways that produce strong bursts of X-rays. The more steady X-ray emission is probably due to heating of matter in the accretion disk. Flickering is sometimes observed for the more steady emission. It is probably caused by instabilities in the accretion disk.