PROBING NEUTRON STARS PHYSICS USING NASA'S NICER MISSION

Summary: "Neutron stars (NSs) are compact objects. The tiny NSs being no more than 20-30 km in diameter but containing masses of 1 to 3 times the Sun, represent extremes of gravity, pressure and density, making them the only stars where matter burns on the outside. Fortunately, some NS have close companion stars; gas from these stars, attracted by the compact object strong gravity, funnels and spirals towards it, forming an accretion disk. These systems are called NS low-mass X-ray binaries (LMXBs); the most powerful phenomena we observe from them are directly related to these accretion disks, as a large amount of gravitational energy is released when the matter approaches the compact object. This causes the inner accretion disk to reach temperatures as high as 100 million degrees and therefore to emit the bulk of the energy in the X-ray band of the spectrum. It is the flow of this accreting plasma onto the NS which provides one of the very few opportunities to directly probe the properties of the tiny (few km) regions where we can "see" General Relativity effects in action in otherwise inaccessible regimes. During this project, we will make use of high-time and high-energy resolution X-ray data from the Neutron star Interior Composition Explorer (NICER) to understand the physics of millisecond X-ray variations seen only around NS. "