• X-ray Binary Systems
• Cygnus X-1 (Black Hole)
• Hercules X-1 (Neutron Star)
• Listen to a Black Hole (GRS1915+105)
• Neutron Stars or Black Holes

    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.

Cygnus X-1 (A Black Hole)

    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. 

Hercules X-1 (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. 

Listen to a Black Hole (GRS1915+105)

    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.

• Windows Audio File (.wav)
• UNIX Audio File (.au)

• Static-like sounds are background X-rays.
• "Whipping" sounds are X-ray/infrared jets escaping from the black hole. 
• the high pitched whistling sounds are the quasi-periodic oscillations (QPOs) or slightly variating pulses of the escaping X-rays.

    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 Greater
In a binary system, if the solar mass of the compact star is greater than three solar masses it is a black hole, if less, a neutron star. Neutron stars have a maximum mass no greater than three solar masses.

X-Ray or Gamma Radiation
Stellar Black Holes have a deeper gravitational well than neutron stars, therefore its accretion disk would heat matter to superhot temperatures, releasing high energy X-rays or even gamma rays.

X-Ray Flux
Neutron stars generally flicker at a tenth of a second, however black holes flicker at 1/1000 of a second from within the accretion disk.