Black Holes With Companions


The black hole that you will be studying is part of a "low mass X-ray binary" system called 4U 1630-472. What exactly is a low mass X-ray binary?

  • In a binary star system, two stars orbit one another. So the black hole is in a binary system with a star.
  • The companion of the black hole is "low mass", which is defined here as a star will a mass less than that of our Sun.
  • Finally, astronomers see X-rays from this binary system, so they call it a "low mass X-ray binary".

The X-rays you see are from matter falling onto the black hole. The black hole and its companion orbit close enough that the gravitational pull of the black hole distorts the companion star and matter streams off of it onto the black hole.

Artist's conception of an accreting binary system

Illustration of an X-ray binary system showing how the accretion disk forms as material is pulled from the companion star an swirls into the black hole.

Image credit: Margaret Masetti (NASA/GSFC)

When the matter falls toward the black hole, it will invariably spiral inward. Why? Because the matter was originally orbiting the companion star, so it will have angular momentum, which must be conserved. Therefore, the matter spirals in toward the black hole.

As the matter spirals in, it piles up in a dense spinning disk orbiting the black hole (called an accretion disk). Matter in the accretion disk heats up due to friction and begins to lose angular momentum. Eventually, it will fall into the black hole. As it falls, the matter is accelerated and radiates energy in the form of X-rays.

Therefore, the X-rays you are about to look at for the black hole system 4U 1630-472 are produced by matter as it falls into the black hole. In other words, you will be looking at matter that is about to be "eaten" by a black hole!

Questions you might be able to answer with the X-ray data are: how fast does the matter fall into the black hole? what kind of matter is there falling into the black hole?