When matter falls into a black hole it must lose an enormous amount of gravitational potential energy. The energy lost heats up the matter which then radiates it as X-rays.
To find black holes more easily, therefore, astronomers search for binaries which emit X-rays. These are called X-ray binaries. Below is a representation of what astronomers believe an X-ray binary looks like. Click on the figure below to learn about the various features of an X-ray binary.
In an X-ray binary a neutron star or black hole (also known as compact objects) is accreting matter from the visible companion star orbiting around it. Notice how the companion star is distorted into a tear-drop shape. This is due to its Roche lobe. We will learn about this later. Once a star has reached its Roche lobe, matter is able to transfer from the visible star towards the compact object. Before reaching it, the matter must lose angular momentum. Consequently a disk of material is formed. The matter travels around the disk, spiriling inwards until it is very close to the compact object. It begins emitting X-rays. Then it is accreted by the compact object.
To discern between a neutron star and black hole we must measure the period, mass ratio, binary inclination and K2 derived from data of the visible star and use the mass function equation to calculate the mass of the compact object. If it is greater than 3 solar masses it is a black hole.
We are now going to see how data from binaries gives us the parameters we need to measure the compact object mass.