|Discovering Black Holes
in Binary Systems
A binary system is a star system with two stars that orbit around each other. An X-ray binary system is made up of a companion star and a collapsed star such as a white dwarf, neutron star or black hole which pulls plasma and gas from the companion star. As the matter is pulled toward the collapsed star, it forms an accretion disk around the collapsed star. Gas particles in the accretion disk are accelerated and heated to superhot temperatures releasing X-rays.
In 1972, an invisible X-ray source from the constellation Cygnus was first detected by X-ray observatory Uhuru (Swahili for "freedom") and named Cygnus X-1. The X-ray source was found to orbit every 5.6 days around its companion HDE 226868, a blue supergiant with 30 solar masses.
Why was Cygnus X-1 considered a black hole? For starters, Cygnus X-1 flickers at less than a thousandth of a second bursts. For an object to flicker, light must travel all the way across its surface. So if light travels 300 kilometers per thousandth of a second, that must mean that Cygnus X-1 is much smaller than our planet.
Second, HDE 226868's spectral lines wobble because of the gravitational pull of Cygnus X-1. In order for that to be possible, scientists estimate that Cygnus X-1 must have at least 7 or more solar masses, putting its mass way beyond the Oppenheimer-Volkoff Limit for a neutron star.
In 1972, an X-ray source was identified in the constellation Hercules and named Hercules X-1. A star called HZ Herculis with 2 solar masses, is in orbit with a pulsating neutron star with 1.3 solar masses. The two stars orbit around each other every 1.7 days.
Comparing Cygnus X-1 to Hercules X-1: below is a 15 second graph of raw X-ray data taken from an early rocket experiment at NASA's Goddard Space Flight Center:
As you can see, the graphed behavior of a pulsating neutron star, Hercules X-1, is rather orderly, as opposed to black hole candidate Cygnus X-1's chaotic graph.
Ever wonder what X-rays from a black hole sounds like? X-ray data from GRS1915+105, a binary system with a jet, has been converted into audio clips so you can actually hear the distortions.
Even though both a neutron star and a black hole can have an accretion disk and orbit a companion star in a binary system, we determine whether it is a neutron star or a black hole by several different factors.
3x Solar Mass or GreaterBut all black holes do not exist in binary systems. Black holes that are a million or even a billion times larger reside in the center of galaxies.
Continue to Black Holes in Centers of Galaxies
NASA (Exosat) - Cygnus X-1.
Copyright © 1999 ThinkQuest Team EH - 25715.