Gravitational Waves are ripples in the curvature of spacetime as predicted by Einstein in his theory of General Relativity. Sources include compact binaries, spinning non-axisymmetric objects and supernovae.
A major area of research in X-ray astronomy is the accretion of matter on to black holes and neutron stars in the context of AGN and galactic X-ray binaries. Accretion is an energetic process and can give high luminosities.
Black holes are among the most fascinating phenomena thought to exist in the Universe. A black hole is a region of space in which the gravitational field is so powerful that nothing, not even light, can normally escape.