Thermonuclear X-ray bursts from neutron stars: a double-photospheric-radius expansion

Thermonuclear X-ray bursts observed from neutron stars originate from intermittent unstable nuclear burning of accreted matter on these stars. Such bursts, which significantly outshine the accretion-powered emission, are an excellent tool for probing the strong gravity and superdense matter, as well as for studying nuclear fusion, the flow of matter, and other processes in intense radiation, gravity, and magnetic fields. The strongest ones among such bursts reach the Eddington luminosity and cause a temporary photospheric radius expansion (PRE) due to radiation pressure. We will discuss various aspects of thermonuclear bursts and the first report of two consecutive PRE events within a few seconds during the same burst, which challenges the current theoretical understanding.