The gravitational capture of compact objects by massive black holes
  The gravitational capture of a stellar-mass compact object (CO) by a
supermassive black hole is a unique probe of gravity in the strong field
regime. Because of the large mass ratio, we call these sources extreme-mass
ratio inspirals (EMRIs). In a similar manner, COs can be captured by
intermediate-mass black holes in globular clusters or dwarf galaxies. The mass
ratio in this case is lower, and hence we refer to the system as an
intermediate-mass ratio inspiral (IMRI). Also, sub-stellar objects such as a
brown dwarf, with masses much lighter than our Sun, can inspiral into
supermassive black holes such as Sgr A* at our Galactic centre. In this case,
the mass ratio is extremely large and, hence, we call this system ab
extremely-large mass ratio inspirals (XMRIs). All of these sources of
gravitational waves will provide us with a collection of snapshots of spacetime
around a supermassive black hole that will allow us to do a direct mapping of
warped spacetime around the supermassive black hole, a live cartography of
gravity in this extreme gravity regime. E/I/XMRIs will be detected by the
future space-borne observatories like LISA. There has not been any other probe
conceived, planned or even thought of ever that can do the science that we can
do with these inspirals. We will discuss them from a viewpoint of relativistic
astrophysics.
