Burst Oscillations to Probe Neutron Star Structure
Constellation-X will also be able to probe neutron star structure using the spin modulation of a non-uniform brightness pattern generated on the neutron star surface by thermonuclear burning. Both the amplitude and shape of these pulsations encodes mass and radius information. For example, the modulation amplitude is influenced by gravitational light deflection in the strong gravitational field of the neutron star, which depends directly on the compactness. Fitting of the observed pulses to a physical model of surface emission from a rotating neutron star can provide constraints on the stellar mass and radius (Nath, Strohmayer & Swank 2001; Muno, Ozel & Chakrabarty 2002; Bhattacharyya et al. 2005).
The shapes of surface absorption (or emission) lines from neutron stars may also carry information about fundamental aspects of Einstein's theory of General Relativity. A prediction of Einstein's theory is that a rotating star will drag the local space-time frame of reference with it, a phenomenon known as frame dragging, which affects the motion of objects and photons near the star. It is known from observations of burst oscillations that some of these accreting neutron stars produce an X-ray hot spot during some X-ray bursts. An absorption line observed from such a rotating hot spot will have a double-horned profile (not unlike the relativistic Fe fluorescence line seen from black hole accretion disks). The red-shifted portion is produced as the hot spot recedes, and the blue-shifted component when the spot is approaching. Frame dragging alters the relative strengths of the red- and blue-shifted horns (Bhattacharyya et al. 2006). By observing such line features Constellation-X may be able to measure the amount of frame dragging, and thus test Einstein's theory.
